Adding upstream version 1.37.1.upstream/1.37.1upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 1313 insertions, 0 deletions

diff --git a/database/engine/pagecache.c b/database/engine/pagecache.c
new file mode 100644
index 0000000..4f5da70
--- /dev/null
+++ b/database/engine/pagecache.c
@@ -0,0 +1,1313 @@
+// SPDX-License-Identifier: GPL-3.0-or-later
+#define NETDATA_RRD_INTERNALS
+
+#include "rrdengine.h"
+
+ARAL page_descr_aral = {
+    .requested_element_size = sizeof(struct rrdeng_page_descr),
+    .initial_elements = 20000,
+    .filename = "page_descriptors",
+    .cache_dir = &netdata_configured_cache_dir,
+    .use_mmap = false,
+    .internal.initialized = false
+};
+
+void rrdeng_page_descr_aral_go_singlethreaded(void) {
+    page_descr_aral.internal.lockless = true;
+}
+void rrdeng_page_descr_aral_go_multithreaded(void) {
+    page_descr_aral.internal.lockless = false;
+}
+
+struct rrdeng_page_descr *rrdeng_page_descr_mallocz(void) {
+    struct rrdeng_page_descr *descr;
+    descr = arrayalloc_mallocz(&page_descr_aral);
+    return descr;
+}
+
+void rrdeng_page_descr_freez(struct rrdeng_page_descr *descr) {
+    arrayalloc_freez(&page_descr_aral, descr);
+}
+
+void rrdeng_page_descr_use_malloc(void) {
+    if(page_descr_aral.internal.initialized)
+        error("DBENGINE: cannot change ARAL allocation policy after it has been initialized.");
+    else
+        page_descr_aral.use_mmap = false;
+}
+
+void rrdeng_page_descr_use_mmap(void) {
+    if(page_descr_aral.internal.initialized)
+        error("DBENGINE: cannot change ARAL allocation policy after it has been initialized.");
+    else
+        page_descr_aral.use_mmap = true;
+}
+
+bool rrdeng_page_descr_is_mmap(void) {
+    return page_descr_aral.use_mmap;
+}
+
+/* Forward declarations */
+static int pg_cache_try_evict_one_page_unsafe(struct rrdengine_instance *ctx);
+
+/* always inserts into tail */
+static inline void pg_cache_replaceQ_insert_unsafe(struct rrdengine_instance *ctx,
+                                                   struct rrdeng_page_descr *descr)
+{
+    struct page_cache *pg_cache = &ctx->pg_cache;
+    struct page_cache_descr *pg_cache_descr = descr->pg_cache_descr;
+
+    if (likely(NULL != pg_cache->replaceQ.tail)) {
+        pg_cache_descr->prev = pg_cache->replaceQ.tail;
+        pg_cache->replaceQ.tail->next = pg_cache_descr;
+    }
+    if (unlikely(NULL == pg_cache->replaceQ.head)) {
+        pg_cache->replaceQ.head = pg_cache_descr;
+    }
+    pg_cache->replaceQ.tail = pg_cache_descr;
+}
+
+static inline void pg_cache_replaceQ_delete_unsafe(struct rrdengine_instance *ctx,
+                                                   struct rrdeng_page_descr *descr)
+{
+    struct page_cache *pg_cache = &ctx->pg_cache;
+    struct page_cache_descr *pg_cache_descr = descr->pg_cache_descr, *prev, *next;
+
+    prev = pg_cache_descr->prev;
+    next = pg_cache_descr->next;
+
+    if (likely(NULL != prev)) {
+        prev->next = next;
+    }
+    if (likely(NULL != next)) {
+        next->prev = prev;
+    }
+    if (unlikely(pg_cache_descr == pg_cache->replaceQ.head)) {
+        pg_cache->replaceQ.head = next;
+    }
+    if (unlikely(pg_cache_descr == pg_cache->replaceQ.tail)) {
+        pg_cache->replaceQ.tail = prev;
+    }
+    pg_cache_descr->prev = pg_cache_descr->next = NULL;
+}
+
+void pg_cache_replaceQ_insert(struct rrdengine_instance *ctx,
+                              struct rrdeng_page_descr *descr)
+{
+    struct page_cache *pg_cache = &ctx->pg_cache;
+
+    uv_rwlock_wrlock(&pg_cache->replaceQ.lock);
+    pg_cache_replaceQ_insert_unsafe(ctx, descr);
+    uv_rwlock_wrunlock(&pg_cache->replaceQ.lock);
+}
+
+void pg_cache_replaceQ_delete(struct rrdengine_instance *ctx,
+                              struct rrdeng_page_descr *descr)
+{
+    struct page_cache *pg_cache = &ctx->pg_cache;
+
+    uv_rwlock_wrlock(&pg_cache->replaceQ.lock);
+    pg_cache_replaceQ_delete_unsafe(ctx, descr);
+    uv_rwlock_wrunlock(&pg_cache->replaceQ.lock);
+}
+void pg_cache_replaceQ_set_hot(struct rrdengine_instance *ctx,
+                               struct rrdeng_page_descr *descr)
+{
+    struct page_cache *pg_cache = &ctx->pg_cache;
+
+    uv_rwlock_wrlock(&pg_cache->replaceQ.lock);
+    pg_cache_replaceQ_delete_unsafe(ctx, descr);
+    pg_cache_replaceQ_insert_unsafe(ctx, descr);
+    uv_rwlock_wrunlock(&pg_cache->replaceQ.lock);
+}
+
+struct rrdeng_page_descr *pg_cache_create_descr(void)
+{
+    struct rrdeng_page_descr *descr;
+
+    descr = rrdeng_page_descr_mallocz();
+    descr->page_length = 0;
+    descr->start_time_ut = INVALID_TIME;
+    descr->end_time_ut = INVALID_TIME;
+    descr->id = NULL;
+    descr->extent = NULL;
+    descr->pg_cache_descr_state = 0;
+    descr->pg_cache_descr = NULL;
+    descr->update_every_s = 0;
+
+    return descr;
+}
+
+/* The caller must hold page descriptor lock. */
+void pg_cache_wake_up_waiters_unsafe(struct rrdeng_page_descr *descr)
+{
+    struct page_cache_descr *pg_cache_descr = descr->pg_cache_descr;
+    if (pg_cache_descr->waiters)
+        uv_cond_broadcast(&pg_cache_descr->cond);
+}
+
+void pg_cache_wake_up_waiters(struct rrdengine_instance *ctx, struct rrdeng_page_descr *descr)
+{
+    rrdeng_page_descr_mutex_lock(ctx, descr);
+    pg_cache_wake_up_waiters_unsafe(descr);
+    rrdeng_page_descr_mutex_unlock(ctx, descr);
+}
+
+/*
+ * The caller must hold page descriptor lock.
+ * The lock will be released and re-acquired. The descriptor is not guaranteed
+ * to exist after this function returns.
+ */
+void pg_cache_wait_event_unsafe(struct rrdeng_page_descr *descr)
+{
+    struct page_cache_descr *pg_cache_descr = descr->pg_cache_descr;
+
+    ++pg_cache_descr->waiters;
+    uv_cond_wait(&pg_cache_descr->cond, &pg_cache_descr->mutex);
+    --pg_cache_descr->waiters;
+}
+
+/*
+ * The caller must hold page descriptor lock.
+ * The lock will be released and re-acquired. The descriptor is not guaranteed
+ * to exist after this function returns.
+ * Returns UV_ETIMEDOUT if timeout_sec seconds pass.
+ */
+int pg_cache_timedwait_event_unsafe(struct rrdeng_page_descr *descr, uint64_t timeout_sec)
+{
+    int ret;
+    struct page_cache_descr *pg_cache_descr = descr->pg_cache_descr;
+
+    ++pg_cache_descr->waiters;
+    ret = uv_cond_timedwait(&pg_cache_descr->cond, &pg_cache_descr->mutex, timeout_sec * NSEC_PER_SEC);
+    --pg_cache_descr->waiters;
+
+    return ret;
+}
+
+/*
+ * Returns page flags.
+ * The lock will be released and re-acquired. The descriptor is not guaranteed
+ * to exist after this function returns.
+ */
+unsigned long pg_cache_wait_event(struct rrdengine_instance *ctx, struct rrdeng_page_descr *descr)
+{
+    struct page_cache_descr *pg_cache_descr = descr->pg_cache_descr;
+    unsigned long flags;
+
+    rrdeng_page_descr_mutex_lock(ctx, descr);
+    pg_cache_wait_event_unsafe(descr);
+    flags = pg_cache_descr->flags;
+    rrdeng_page_descr_mutex_unlock(ctx, descr);
+
+    return flags;
+}
+
+/*
+ * The caller must hold page descriptor lock.
+ */
+int pg_cache_can_get_unsafe(struct rrdeng_page_descr *descr, int exclusive_access)
+{
+    struct page_cache_descr *pg_cache_descr = descr->pg_cache_descr;
+
+    if ((pg_cache_descr->flags & (RRD_PAGE_LOCKED | RRD_PAGE_READ_PENDING)) ||
+        (exclusive_access && pg_cache_descr->refcnt)) {
+        return 0;
+    }
+
+    return 1;
+}
+
+/*
+ * The caller must hold page descriptor lock.
+ * Gets a reference to the page descriptor.
+ * Returns 1 on success and 0 on failure.
+ */
+int pg_cache_try_get_unsafe(struct rrdeng_page_descr *descr, int exclusive_access)
+{
+    struct page_cache_descr *pg_cache_descr = descr->pg_cache_descr;
+
+    if (!pg_cache_can_get_unsafe(descr, exclusive_access))
+        return 0;
+
+    if (exclusive_access)
+        pg_cache_descr->flags |= RRD_PAGE_LOCKED;
+    ++pg_cache_descr->refcnt;
+
+    return 1;
+}
+
+/*
+ * The caller must hold the page descriptor lock.
+ * This function may block doing cleanup.
+ */
+void pg_cache_put_unsafe(struct rrdeng_page_descr *descr)
+{
+    struct page_cache_descr *pg_cache_descr = descr->pg_cache_descr;
+
+    pg_cache_descr->flags &= ~RRD_PAGE_LOCKED;
+    if (0 == --pg_cache_descr->refcnt) {
+        pg_cache_wake_up_waiters_unsafe(descr);
+    }
+}
+
+/*
+ * This function may block doing cleanup.
+ */
+void pg_cache_put(struct rrdengine_instance *ctx, struct rrdeng_page_descr *descr)
+{
+    rrdeng_page_descr_mutex_lock(ctx, descr);
+    pg_cache_put_unsafe(descr);
+    rrdeng_page_descr_mutex_unlock(ctx, descr);
+}
+
+/* The caller must hold the page cache lock */
+static void pg_cache_release_pages_unsafe(struct rrdengine_instance *ctx, unsigned number)
+{
+    struct page_cache *pg_cache = &ctx->pg_cache;
+
+    pg_cache->populated_pages -= number;
+}
+
+static void pg_cache_release_pages(struct rrdengine_instance *ctx, unsigned number)
+{
+    struct page_cache *pg_cache = &ctx->pg_cache;
+
+    uv_rwlock_wrlock(&pg_cache->pg_cache_rwlock);
+    pg_cache_release_pages_unsafe(ctx, number);
+    uv_rwlock_wrunlock(&pg_cache->pg_cache_rwlock);
+}
+
+/*
+ * This function returns the maximum number of pages allowed in the page cache.
+ */
+unsigned long pg_cache_hard_limit(struct rrdengine_instance *ctx)
+{
+    return ctx->max_cache_pages + (unsigned long)ctx->metric_API_max_producers;
+}
+
+/*
+ * This function returns the low watermark number of pages in the page cache. The page cache should strive to keep the
+ * number of pages below that number.
+ */
+unsigned long pg_cache_soft_limit(struct rrdengine_instance *ctx)
+{
+    return ctx->cache_pages_low_watermark + (unsigned long)ctx->metric_API_max_producers;
+}
+
+/*
+ * This function returns the maximum number of dirty pages that are committed to be written to disk allowed in the page
+ * cache.
+ */
+unsigned long pg_cache_committed_hard_limit(struct rrdengine_instance *ctx)
+{
+    /* We remove the active pages of the producers from the calculation and only allow the extra pinned pages */
+    return ctx->cache_pages_low_watermark + (unsigned long)ctx->metric_API_max_producers;
+}
+
+/*
+ * This function will block until it reserves #number populated pages.
+ * It will trigger evictions or dirty page flushing if the pg_cache_hard_limit() limit is hit.
+ */
+static void pg_cache_reserve_pages(struct rrdengine_instance *ctx, unsigned number)
+{
+    struct page_cache *pg_cache = &ctx->pg_cache;
+    unsigned failures = 0;
+    const unsigned FAILURES_CEILING = 10; /* truncates exponential backoff to (2^FAILURES_CEILING x slot) */
+    unsigned long exp_backoff_slot_usec = USEC_PER_MS * 10;
+
+    assert(number < ctx->max_cache_pages);
+
+    uv_rwlock_wrlock(&pg_cache->pg_cache_rwlock);
+    if (pg_cache->populated_pages + number >= pg_cache_hard_limit(ctx) + 1)
+        debug(D_RRDENGINE, "==Page cache full. Reserving %u pages.==",
+                number);
+    while (pg_cache->populated_pages + number >= pg_cache_hard_limit(ctx) + 1) {
+
+        if (!pg_cache_try_evict_one_page_unsafe(ctx)) {
+            /* failed to evict */
+            struct completion compl;
+            struct rrdeng_cmd cmd;
+
+            ++failures;
+            uv_rwlock_wrunlock(&pg_cache->pg_cache_rwlock);
+
+            completion_init(&compl);
+            cmd.opcode = RRDENG_FLUSH_PAGES;
+            cmd.completion = &compl;
+            rrdeng_enq_cmd(&ctx->worker_config, &cmd);
+            /* wait for some pages to be flushed */
+            debug(D_RRDENGINE, "%s: waiting for pages to be written to disk before evicting.", __func__);
+            completion_wait_for(&compl);
+            completion_destroy(&compl);
+
+            if (unlikely(failures > 1)) {
+                unsigned long slots, usecs_to_sleep;
+                /* exponential backoff */
+                slots = random() % (2LU << MIN(failures, FAILURES_CEILING));
+                usecs_to_sleep = slots * exp_backoff_slot_usec;
+
+                if (usecs_to_sleep >= USEC_PER_SEC)
+                    error("Page cache is full. Sleeping for %llu second(s).", usecs_to_sleep / USEC_PER_SEC);
+
+                (void)sleep_usec(usecs_to_sleep);
+            }
+            uv_rwlock_wrlock(&pg_cache->pg_cache_rwlock);
+        }
+    }
+    pg_cache->populated_pages += number;
+    uv_rwlock_wrunlock(&pg_cache->pg_cache_rwlock);
+}
+
+/*
+ * This function will attempt to reserve #number populated pages.
+ * It may trigger evictions if the pg_cache_soft_limit() limit is hit.
+ * Returns 0 on failure and 1 on success.
+ */
+static int pg_cache_try_reserve_pages(struct rrdengine_instance *ctx, unsigned number)
+{
+    struct page_cache *pg_cache = &ctx->pg_cache;
+    unsigned count = 0;
+    int ret = 0;
+
+    assert(number < ctx->max_cache_pages);
+
+    uv_rwlock_wrlock(&pg_cache->pg_cache_rwlock);
+    if (pg_cache->populated_pages + number >= pg_cache_soft_limit(ctx) + 1) {
+        debug(D_RRDENGINE,
+              "==Page cache full. Trying to reserve %u pages.==",
+              number);
+        do {
+            if (!pg_cache_try_evict_one_page_unsafe(ctx))
+                break;
+            ++count;
+        } while (pg_cache->populated_pages + number >= pg_cache_soft_limit(ctx) + 1);
+        debug(D_RRDENGINE, "Evicted %u pages.", count);
+    }
+
+    if (pg_cache->populated_pages + number < pg_cache_hard_limit(ctx) + 1) {
+        pg_cache->populated_pages += number;
+        ret = 1; /* success */
+    }
+    uv_rwlock_wrunlock(&pg_cache->pg_cache_rwlock);
+
+    return ret;
+}
+
+/* The caller must hold the page cache and the page descriptor locks in that order */
+static void pg_cache_evict_unsafe(struct rrdengine_instance *ctx, struct rrdeng_page_descr *descr)
+{
+    struct page_cache_descr *pg_cache_descr = descr->pg_cache_descr;
+
+    dbengine_page_free(pg_cache_descr->page);
+    pg_cache_descr->page = NULL;
+    pg_cache_descr->flags &= ~RRD_PAGE_POPULATED;
+    pg_cache_release_pages_unsafe(ctx, 1);
+    ++ctx->stats.pg_cache_evictions;
+}
+
+/*
+ * The caller must hold the page cache lock.
+ * Lock order: page cache -> replaceQ -> page descriptor
+ * This function iterates all pages and tries to evict one.
+ * If it fails it sets in_flight_descr to the oldest descriptor that has write-back in progress,
+ * or it sets it to NULL if no write-back is in progress.
+ *
+ * Returns 1 on success and 0 on failure.
+ */
+static int pg_cache_try_evict_one_page_unsafe(struct rrdengine_instance *ctx)
+{
+    struct page_cache *pg_cache = &ctx->pg_cache;
+    unsigned long old_flags;
+    struct rrdeng_page_descr *descr;
+    struct page_cache_descr *pg_cache_descr = NULL;
+
+    uv_rwlock_wrlock(&pg_cache->replaceQ.lock);
+    for (pg_cache_descr = pg_cache->replaceQ.head ; NULL != pg_cache_descr ; pg_cache_descr = pg_cache_descr->next) {
+        descr = pg_cache_descr->descr;
+
+        rrdeng_page_descr_mutex_lock(ctx, descr);
+        old_flags = pg_cache_descr->flags;
+        if ((old_flags & RRD_PAGE_POPULATED) && !(old_flags & RRD_PAGE_DIRTY) && pg_cache_try_get_unsafe(descr, 1)) {
+            /* must evict */
+            pg_cache_evict_unsafe(ctx, descr);
+            pg_cache_put_unsafe(descr);
+            pg_cache_replaceQ_delete_unsafe(ctx, descr);
+
+            rrdeng_page_descr_mutex_unlock(ctx, descr);
+            uv_rwlock_wrunlock(&pg_cache->replaceQ.lock);
+
+            rrdeng_try_deallocate_pg_cache_descr(ctx, descr);
+
+            return 1;
+        }
+        rrdeng_page_descr_mutex_unlock(ctx, descr);
+    }
+    uv_rwlock_wrunlock(&pg_cache->replaceQ.lock);
+
+    /* failed to evict */
+    return 0;
+}
+
+/**
+ * Deletes a page from the database.
+ * Callers of this function need to make sure they're not deleting the same descriptor concurrently.
+ * @param ctx is the database instance.
+ * @param descr is the page descriptor.
+ * @param remove_dirty must be non-zero if the page to be deleted is dirty.
+ * @param is_exclusive_holder must be non-zero if the caller holds an exclusive page reference.
+ * @param metric_id is set to the metric the page belongs to, if it's safe to delete the metric and metric_id is not
+ *        NULL. Otherwise, metric_id is not set.
+ * @return 1 if it's safe to delete the metric, 0 otherwise.
+ */
+uint8_t pg_cache_punch_hole(struct rrdengine_instance *ctx, struct rrdeng_page_descr *descr, uint8_t remove_dirty,
+                         uint8_t is_exclusive_holder, uuid_t *metric_id)
+{
+    struct page_cache *pg_cache = &ctx->pg_cache;
+    struct page_cache_descr *pg_cache_descr = NULL;
+    Pvoid_t *PValue;
+    struct pg_cache_page_index *page_index = NULL;
+    int ret;
+    uint8_t can_delete_metric = 0;
+
+    uv_rwlock_rdlock(&pg_cache->metrics_index.lock);
+    PValue = JudyHSGet(pg_cache->metrics_index.JudyHS_array, descr->id, sizeof(uuid_t));
+    fatal_assert(NULL != PValue);
+    page_index = *PValue;
+    uv_rwlock_rdunlock(&pg_cache->metrics_index.lock);
+
+    uv_rwlock_wrlock(&page_index->lock);
+    ret = JudyLDel(&page_index->JudyL_array, (Word_t)(descr->start_time_ut / USEC_PER_SEC), PJE0);
+    if (unlikely(0 == ret)) {
+        uv_rwlock_wrunlock(&page_index->lock);
+        if (unlikely(debug_flags & D_RRDENGINE)) {
+            print_page_descr(descr);
+        }
+        goto destroy;
+    }
+    --page_index->page_count;
+    if (!page_index->writers && !page_index->page_count) {
+        can_delete_metric = 1;
+        if (metric_id) {
+            memcpy(metric_id, page_index->id, sizeof(uuid_t));
+        }
+    }
+    uv_rwlock_wrunlock(&page_index->lock);
+    fatal_assert(1 == ret);
+
+    uv_rwlock_wrlock(&pg_cache->pg_cache_rwlock);
+    ++ctx->stats.pg_cache_deletions;
+    --pg_cache->page_descriptors;
+    uv_rwlock_wrunlock(&pg_cache->pg_cache_rwlock);
+
+    rrdeng_page_descr_mutex_lock(ctx, descr);
+    pg_cache_descr = descr->pg_cache_descr;
+    if (!is_exclusive_holder) {
+        /* If we don't hold an exclusive page reference get one */
+        while (!pg_cache_try_get_unsafe(descr, 1)) {
+            debug(D_RRDENGINE, "%s: Waiting for locked page:", __func__);
+            if (unlikely(debug_flags & D_RRDENGINE))
+                print_page_cache_descr(descr, "", true);
+            pg_cache_wait_event_unsafe(descr);
+        }
+    }
+    if (remove_dirty) {
+        pg_cache_descr->flags &= ~RRD_PAGE_DIRTY;
+    } else {
+        /* even a locked page could be dirty */
+        while (unlikely(pg_cache_descr->flags & RRD_PAGE_DIRTY)) {
+            debug(D_RRDENGINE, "%s: Found dirty page, waiting for it to be flushed:", __func__);
+            if (unlikely(debug_flags & D_RRDENGINE))
+                print_page_cache_descr(descr, "", true);
+            pg_cache_wait_event_unsafe(descr);
+        }
+    }
+    rrdeng_page_descr_mutex_unlock(ctx, descr);
+
+    while (unlikely(pg_cache_descr->flags & RRD_PAGE_READ_PENDING)) {
+        error_limit_static_global_var(erl, 1, 0);
+        error_limit(&erl, "%s: Found page with READ PENDING, waiting for read to complete", __func__);
+        if (unlikely(debug_flags & D_RRDENGINE))
+            print_page_cache_descr(descr, "", true);
+        pg_cache_wait_event_unsafe(descr);
+    }
+
+    if (pg_cache_descr->flags & RRD_PAGE_POPULATED) {
+        /* only after locking can it be safely deleted from LRU */
+        pg_cache_replaceQ_delete(ctx, descr);
+
+        uv_rwlock_wrlock(&pg_cache->pg_cache_rwlock);
+        pg_cache_evict_unsafe(ctx, descr);
+        uv_rwlock_wrunlock(&pg_cache->pg_cache_rwlock);
+    }
+    pg_cache_put(ctx, descr);
+    rrdeng_try_deallocate_pg_cache_descr(ctx, descr);
+    while (descr->pg_cache_descr_state & PG_CACHE_DESCR_ALLOCATED) {
+        rrdeng_try_deallocate_pg_cache_descr(ctx, descr); /* spin */
+        (void)sleep_usec(1000); /* 1 msec */
+    }
+destroy:
+    rrdeng_page_descr_freez(descr);
+    pg_cache_update_metric_times(page_index);
+
+    return can_delete_metric;
+}
+
+static inline int is_page_in_time_range(struct rrdeng_page_descr *descr, usec_t start_time, usec_t end_time)
+{
+    usec_t pg_start, pg_end;
+
+    pg_start = descr->start_time_ut;
+    pg_end = descr->end_time_ut;
+
+    return (pg_start < start_time && pg_end >= start_time) ||
+           (pg_start >= start_time && pg_start <= end_time);
+}
+
+static inline int is_point_in_time_in_page(struct rrdeng_page_descr *descr, usec_t point_in_time)
+{
+    return (point_in_time >= descr->start_time_ut && point_in_time <= descr->end_time_ut);
+}
+
+/* The caller must hold the page index lock */
+static inline struct rrdeng_page_descr *
+        find_first_page_in_time_range(struct pg_cache_page_index *page_index, usec_t start_time, usec_t end_time)
+{
+    struct rrdeng_page_descr *descr = NULL;
+    Pvoid_t *PValue;
+    Word_t Index;
+
+    Index = (Word_t)(start_time / USEC_PER_SEC);
+    PValue = JudyLLast(page_index->JudyL_array, &Index, PJE0);
+    if (likely(NULL != PValue)) {
+        descr = *PValue;
+        if (is_page_in_time_range(descr, start_time, end_time)) {
+            return descr;
+        }
+    }
+
+    Index = (Word_t)(start_time / USEC_PER_SEC);
+    PValue = JudyLFirst(page_index->JudyL_array, &Index, PJE0);
+    if (likely(NULL != PValue)) {
+        descr = *PValue;
+        if (is_page_in_time_range(descr, start_time, end_time)) {
+            return descr;
+        }
+    }
+
+    return NULL;
+}
+
+/* Update metric oldest and latest timestamps efficiently when adding new values */
+void pg_cache_add_new_metric_time(struct pg_cache_page_index *page_index, struct rrdeng_page_descr *descr)
+{
+    usec_t oldest_time = page_index->oldest_time_ut;
+    usec_t latest_time = page_index->latest_time_ut;
+
+    if (unlikely(oldest_time == INVALID_TIME || descr->start_time_ut < oldest_time)) {
+        page_index->oldest_time_ut = descr->start_time_ut;
+    }
+    if (likely(descr->end_time_ut > latest_time || latest_time == INVALID_TIME)) {
+        page_index->latest_time_ut = descr->end_time_ut;
+    }
+}
+
+/* Update metric oldest and latest timestamps when removing old values */
+void pg_cache_update_metric_times(struct pg_cache_page_index *page_index)
+{
+    Pvoid_t *firstPValue, *lastPValue;
+    Word_t firstIndex, lastIndex;
+    struct rrdeng_page_descr *descr;
+    usec_t oldest_time = INVALID_TIME;
+    usec_t latest_time = INVALID_TIME;
+
+    uv_rwlock_rdlock(&page_index->lock);
+    /* Find first page in range */
+    firstIndex = (Word_t)0;
+    firstPValue = JudyLFirst(page_index->JudyL_array, &firstIndex, PJE0);
+    if (likely(NULL != firstPValue)) {
+        descr = *firstPValue;
+        oldest_time = descr->start_time_ut;
+    }
+    lastIndex = (Word_t)-1;
+    lastPValue = JudyLLast(page_index->JudyL_array, &lastIndex, PJE0);
+    if (likely(NULL != lastPValue)) {
+        descr = *lastPValue;
+        latest_time = descr->end_time_ut;
+    }
+    uv_rwlock_rdunlock(&page_index->lock);
+
+    if (unlikely(NULL == firstPValue)) {
+        fatal_assert(NULL == lastPValue);
+        page_index->oldest_time_ut = page_index->latest_time_ut = INVALID_TIME;
+        return;
+    }
+    page_index->oldest_time_ut = oldest_time;
+    page_index->latest_time_ut = latest_time;
+}
+
+/* If index is NULL lookup by UUID (descr->id) */
+void pg_cache_insert(struct rrdengine_instance *ctx, struct pg_cache_page_index *index,
+                     struct rrdeng_page_descr *descr)
+{
+    struct page_cache *pg_cache = &ctx->pg_cache;
+    Pvoid_t *PValue;
+    struct pg_cache_page_index *page_index;
+    unsigned long pg_cache_descr_state = descr->pg_cache_descr_state;
+
+    if (0 != pg_cache_descr_state) {
+        /* there is page cache descriptor pre-allocated state */
+        struct page_cache_descr *pg_cache_descr = descr->pg_cache_descr;
+
+        fatal_assert(pg_cache_descr_state & PG_CACHE_DESCR_ALLOCATED);
+        if (pg_cache_descr->flags & RRD_PAGE_POPULATED) {
+            pg_cache_reserve_pages(ctx, 1);
+            if (!(pg_cache_descr->flags & RRD_PAGE_DIRTY))
+                pg_cache_replaceQ_insert(ctx, descr);
+        }
+    }
+
+    if (unlikely(NULL == index)) {
+        uv_rwlock_rdlock(&pg_cache->metrics_index.lock);
+        PValue = JudyHSGet(pg_cache->metrics_index.JudyHS_array, descr->id, sizeof(uuid_t));
+        fatal_assert(NULL != PValue);
+        page_index = *PValue;
+        uv_rwlock_rdunlock(&pg_cache->metrics_index.lock);
+    } else {
+        page_index = index;
+    }
+
+    uv_rwlock_wrlock(&page_index->lock);
+    PValue = JudyLIns(&page_index->JudyL_array, (Word_t)(descr->start_time_ut / USEC_PER_SEC), PJE0);
+    *PValue = descr;
+    ++page_index->page_count;
+    pg_cache_add_new_metric_time(page_index, descr);
+    uv_rwlock_wrunlock(&page_index->lock);
+
+    uv_rwlock_wrlock(&pg_cache->pg_cache_rwlock);
+    ++ctx->stats.pg_cache_insertions;
+    ++pg_cache->page_descriptors;
+    uv_rwlock_wrunlock(&pg_cache->pg_cache_rwlock);
+}
+
+usec_t pg_cache_oldest_time_in_range(struct rrdengine_instance *ctx, uuid_t *id, usec_t start_time_ut, usec_t end_time_ut)
+{
+    struct page_cache *pg_cache = &ctx->pg_cache;
+    struct rrdeng_page_descr *descr = NULL;
+    Pvoid_t *PValue;
+    struct pg_cache_page_index *page_index = NULL;
+
+    uv_rwlock_rdlock(&pg_cache->metrics_index.lock);
+    PValue = JudyHSGet(pg_cache->metrics_index.JudyHS_array, id, sizeof(uuid_t));
+    if (likely(NULL != PValue)) {
+        page_index = *PValue;
+    }
+    uv_rwlock_rdunlock(&pg_cache->metrics_index.lock);
+    if (NULL == PValue) {
+        return INVALID_TIME;
+    }
+
+    uv_rwlock_rdlock(&page_index->lock);
+    descr = find_first_page_in_time_range(page_index, start_time_ut, end_time_ut);
+    if (NULL == descr) {
+        uv_rwlock_rdunlock(&page_index->lock);
+        return INVALID_TIME;
+    }
+    uv_rwlock_rdunlock(&page_index->lock);
+    return descr->start_time_ut;
+}
+
+/**
+ * Return page information for the first page before point_in_time that satisfies the filter.
+ * @param ctx DB context
+ * @param page_index page index of a metric
+ * @param point_in_time_ut the pages that are searched must be older than this timestamp
+ * @param filter decides if the page satisfies the caller's criteria
+ * @param page_info the result of the search is set in this pointer
+ */
+void pg_cache_get_filtered_info_prev(struct rrdengine_instance *ctx, struct pg_cache_page_index *page_index,
+                                     usec_t point_in_time_ut, pg_cache_page_info_filter_t *filter,
+                                     struct rrdeng_page_info *page_info)
+{
+    struct page_cache *pg_cache = &ctx->pg_cache;
+    struct rrdeng_page_descr *descr = NULL;
+    Pvoid_t *PValue;
+    Word_t Index;
+
+    (void)pg_cache;
+    fatal_assert(NULL != page_index);
+
+    Index = (Word_t)(point_in_time_ut / USEC_PER_SEC);
+    uv_rwlock_rdlock(&page_index->lock);
+    do {
+        PValue = JudyLPrev(page_index->JudyL_array, &Index, PJE0);
+        descr = unlikely(NULL == PValue) ? NULL : *PValue;
+    } while (descr != NULL && !filter(descr));
+    if (unlikely(NULL == descr)) {
+        page_info->page_length = 0;
+        page_info->start_time_ut = INVALID_TIME;
+        page_info->end_time_ut = INVALID_TIME;
+    } else {
+        page_info->page_length = descr->page_length;
+        page_info->start_time_ut = descr->start_time_ut;
+        page_info->end_time_ut = descr->end_time_ut;
+    }
+    uv_rwlock_rdunlock(&page_index->lock);
+}
+
+/**
+ * Searches for an unallocated page without triggering disk I/O. Attempts to reserve the page and get a reference.
+ * @param ctx DB context
+ * @param id lookup by UUID
+ * @param start_time_ut exact starting time in usec
+ * @param ret_page_indexp Sets the page index pointer (*ret_page_indexp) for the given UUID.
+ * @return the page descriptor or NULL on failure. It can fail if:
+ *         1. The page is already allocated to the page cache.
+ *         2. It did not succeed to get a reference.
+ *         3. It did not succeed to reserve a spot in the page cache.
+ */
+struct rrdeng_page_descr *pg_cache_lookup_unpopulated_and_lock(struct rrdengine_instance *ctx, uuid_t *id,
+                                                               usec_t start_time_ut)
+{
+    struct page_cache *pg_cache = &ctx->pg_cache;
+    struct rrdeng_page_descr *descr = NULL;
+    struct page_cache_descr *pg_cache_descr = NULL;
+    unsigned long flags;
+    Pvoid_t *PValue;
+    struct pg_cache_page_index *page_index = NULL;
+    Word_t Index;
+
+    uv_rwlock_rdlock(&pg_cache->metrics_index.lock);
+    PValue = JudyHSGet(pg_cache->metrics_index.JudyHS_array, id, sizeof(uuid_t));
+    if (likely(NULL != PValue)) {
+        page_index = *PValue;
+    }
+    uv_rwlock_rdunlock(&pg_cache->metrics_index.lock);
+
+    if ((NULL == PValue) || !pg_cache_try_reserve_pages(ctx, 1)) {
+        /* Failed to find page or failed to reserve a spot in the cache */
+        return NULL;
+    }
+
+    uv_rwlock_rdlock(&page_index->lock);
+    Index = (Word_t)(start_time_ut / USEC_PER_SEC);
+    PValue = JudyLGet(page_index->JudyL_array, Index, PJE0);
+    if (likely(NULL != PValue)) {
+        descr = *PValue;
+    }
+    if (NULL == PValue || 0 == descr->page_length) {
+        /* Failed to find non-empty page */
+        uv_rwlock_rdunlock(&page_index->lock);
+
+        pg_cache_release_pages(ctx, 1);
+        return NULL;
+    }
+
+    rrdeng_page_descr_mutex_lock(ctx, descr);
+    pg_cache_descr = descr->pg_cache_descr;
+    flags = pg_cache_descr->flags;
+    uv_rwlock_rdunlock(&page_index->lock);
+
+    if ((flags & RRD_PAGE_POPULATED) || !pg_cache_try_get_unsafe(descr, 1)) {
+        /* Failed to get reference or page is already populated */
+        rrdeng_page_descr_mutex_unlock(ctx, descr);
+
+        pg_cache_release_pages(ctx, 1);
+        return NULL;
+    }
+    /* success */
+    rrdeng_page_descr_mutex_unlock(ctx, descr);
+    rrd_stat_atomic_add(&ctx->stats.pg_cache_misses, 1);
+
+    return descr;
+}
+
+/**
+ * Searches for pages in a time range and triggers disk I/O if necessary and possible.
+ * Does not get a reference.
+ * @param ctx DB context
+ * @param id UUID
+ * @param start_time_ut inclusive starting time in usec
+ * @param end_time_ut inclusive ending time in usec
+ * @param page_info_arrayp It allocates (*page_arrayp) and populates it with information of pages that overlap
+ *        with the time range [start_time,end_time]. The caller must free (*page_info_arrayp) with freez().
+ *        If page_info_arrayp is set to NULL nothing was allocated.
+ * @param ret_page_indexp Sets the page index pointer (*ret_page_indexp) for the given UUID.
+ * @return the number of pages that overlap with the time range [start_time,end_time].
+ */
+unsigned pg_cache_preload(struct rrdengine_instance *ctx, uuid_t *id, usec_t start_time_ut, usec_t end_time_ut,
+                          struct rrdeng_page_info **page_info_arrayp, struct pg_cache_page_index **ret_page_indexp)
+{
+    struct page_cache *pg_cache = &ctx->pg_cache;
+    struct rrdeng_page_descr *descr = NULL, *preload_array[PAGE_CACHE_MAX_PRELOAD_PAGES];
+    struct page_cache_descr *pg_cache_descr = NULL;
+    unsigned i, j, k, preload_count, count, page_info_array_max_size;
+    unsigned long flags;
+    Pvoid_t *PValue;
+    struct pg_cache_page_index *page_index = NULL;
+    Word_t Index;
+    uint8_t failed_to_reserve;
+
+    fatal_assert(NULL != ret_page_indexp);
+
+    uv_rwlock_rdlock(&pg_cache->metrics_index.lock);
+    PValue = JudyHSGet(pg_cache->metrics_index.JudyHS_array, id, sizeof(uuid_t));
+    if (likely(NULL != PValue)) {
+        *ret_page_indexp = page_index = *PValue;
+    }
+    uv_rwlock_rdunlock(&pg_cache->metrics_index.lock);
+    if (NULL == PValue) {
+        debug(D_RRDENGINE, "%s: No page was found to attempt preload.", __func__);
+        *ret_page_indexp = NULL;
+        return 0;
+    }
+
+    uv_rwlock_rdlock(&page_index->lock);
+    descr = find_first_page_in_time_range(page_index, start_time_ut, end_time_ut);
+    if (NULL == descr) {
+        uv_rwlock_rdunlock(&page_index->lock);
+        debug(D_RRDENGINE, "%s: No page was found to attempt preload.", __func__);
+        *ret_page_indexp = NULL;
+        return 0;
+    } else {
+        Index = (Word_t)(descr->start_time_ut / USEC_PER_SEC);
+    }
+    if (page_info_arrayp) {
+        page_info_array_max_size = PAGE_CACHE_MAX_PRELOAD_PAGES * sizeof(struct rrdeng_page_info);
+        *page_info_arrayp = mallocz(page_info_array_max_size);
+    }
+
+    for (count = 0, preload_count = 0 ;
+         descr != NULL && is_page_in_time_range(descr, start_time_ut, end_time_ut) ;
+         PValue = JudyLNext(page_index->JudyL_array, &Index, PJE0),
+         descr = unlikely(NULL == PValue) ? NULL : *PValue) {
+        /* Iterate all pages in range */
+
+        if (unlikely(0 == descr->page_length))
+            continue;
+        if (page_info_arrayp) {
+            if (unlikely(count >= page_info_array_max_size / sizeof(struct rrdeng_page_info))) {
+                page_info_array_max_size += PAGE_CACHE_MAX_PRELOAD_PAGES * sizeof(struct rrdeng_page_info);
+                *page_info_arrayp = reallocz(*page_info_arrayp, page_info_array_max_size);
+            }
+            (*page_info_arrayp)[count].start_time_ut = descr->start_time_ut;
+            (*page_info_arrayp)[count].end_time_ut = descr->end_time_ut;
+            (*page_info_arrayp)[count].page_length = descr->page_length;
+        }
+        ++count;
+
+        rrdeng_page_descr_mutex_lock(ctx, descr);
+        pg_cache_descr = descr->pg_cache_descr;
+        flags = pg_cache_descr->flags;
+        if (pg_cache_can_get_unsafe(descr, 0)) {
+            if (flags & RRD_PAGE_POPULATED) {
+                /* success */
+                rrdeng_page_descr_mutex_unlock(ctx, descr);
+                debug(D_RRDENGINE, "%s: Page was found in memory.", __func__);
+                continue;
+            }
+        }
+        if (!(flags & RRD_PAGE_POPULATED) && pg_cache_try_get_unsafe(descr, 1)) {
+            preload_array[preload_count++] = descr;
+            if (PAGE_CACHE_MAX_PRELOAD_PAGES == preload_count) {
+                rrdeng_page_descr_mutex_unlock(ctx, descr);
+                break;
+            }
+        }
+        rrdeng_page_descr_mutex_unlock(ctx, descr);
+
+    }
+    uv_rwlock_rdunlock(&page_index->lock);
+
+    failed_to_reserve = 0;
+    for (i = 0 ; i < preload_count && !failed_to_reserve ; ++i) {
+        struct rrdeng_cmd cmd;
+        struct rrdeng_page_descr *next;
+
+        descr = preload_array[i];
+        if (NULL == descr) {
+            continue;
+        }
+        if (!pg_cache_try_reserve_pages(ctx, 1)) {
+            failed_to_reserve = 1;
+            break;
+        }
+        cmd.opcode = RRDENG_READ_EXTENT;
+        cmd.read_extent.page_cache_descr[0] = descr;
+        /* don't use this page again */
+        preload_array[i] = NULL;
+        for (j = 0, k = 1 ; j < preload_count ; ++j) {
+            next = preload_array[j];
+            if (NULL == next) {
+                continue;
+            }
+            if (descr->extent == next->extent) {
+                /* same extent, consolidate */
+                if (!pg_cache_try_reserve_pages(ctx, 1)) {
+                    failed_to_reserve = 1;
+                    break;
+                }
+                cmd.read_extent.page_cache_descr[k++] = next;
+                /* don't use this page again */
+                preload_array[j] = NULL;
+            }
+        }
+        cmd.read_extent.page_count = k;
+        rrdeng_enq_cmd(&ctx->worker_config, &cmd);
+    }
+    if (failed_to_reserve) {
+        debug(D_RRDENGINE, "%s: Failed to reserve enough memory, canceling I/O.", __func__);
+        for (i = 0 ; i < preload_count ; ++i) {
+            descr = preload_array[i];
+            if (NULL == descr) {
+                continue;
+            }
+            pg_cache_put(ctx, descr);
+        }
+    }
+    if (!preload_count) {
+        /* no such page */
+        debug(D_RRDENGINE, "%s: No page was eligible to attempt preload.", __func__);
+    }
+    if (unlikely(0 == count && page_info_arrayp)) {
+        freez(*page_info_arrayp);
+        *page_info_arrayp = NULL;
+    }
+    return count;
+}
+
+/*
+ * Searches for a page and gets a reference.
+ * When point_in_time is INVALID_TIME get any page.
+ * If index is NULL lookup by UUID (id).
+ */
+struct rrdeng_page_descr *
+        pg_cache_lookup(struct rrdengine_instance *ctx, struct pg_cache_page_index *index, uuid_t *id,
+                        usec_t point_in_time_ut)
+{
+    struct page_cache *pg_cache = &ctx->pg_cache;
+    struct rrdeng_page_descr *descr = NULL;
+    struct page_cache_descr *pg_cache_descr = NULL;
+    unsigned long flags;
+    Pvoid_t *PValue;
+    struct pg_cache_page_index *page_index = NULL;
+    Word_t Index;
+    uint8_t page_not_in_cache;
+
+    if (unlikely(NULL == index)) {
+        uv_rwlock_rdlock(&pg_cache->metrics_index.lock);
+        PValue = JudyHSGet(pg_cache->metrics_index.JudyHS_array, id, sizeof(uuid_t));
+        if (likely(NULL != PValue)) {
+            page_index = *PValue;
+        }
+        uv_rwlock_rdunlock(&pg_cache->metrics_index.lock);
+        if (NULL == PValue) {
+            return NULL;
+        }
+    } else {
+        page_index = index;
+    }
+    pg_cache_reserve_pages(ctx, 1);
+
+    page_not_in_cache = 0;
+    uv_rwlock_rdlock(&page_index->lock);
+    while (1) {
+        Index = (Word_t)(point_in_time_ut / USEC_PER_SEC);
+        PValue = JudyLLast(page_index->JudyL_array, &Index, PJE0);
+        if (likely(NULL != PValue)) {
+            descr = *PValue;
+        }
+        if (NULL == PValue ||
+            0 == descr->page_length ||
+            (INVALID_TIME != point_in_time_ut &&
+             !is_point_in_time_in_page(descr, point_in_time_ut))) {
+            /* non-empty page not found */
+            uv_rwlock_rdunlock(&page_index->lock);
+
+            pg_cache_release_pages(ctx, 1);
+            return NULL;
+        }
+        rrdeng_page_descr_mutex_lock(ctx, descr);
+        pg_cache_descr = descr->pg_cache_descr;
+        flags = pg_cache_descr->flags;
+        if ((flags & RRD_PAGE_POPULATED) && pg_cache_try_get_unsafe(descr, 0)) {
+            /* success */
+            rrdeng_page_descr_mutex_unlock(ctx, descr);
+            debug(D_RRDENGINE, "%s: Page was found in memory.", __func__);
+            break;
+        }
+        if (!(flags & RRD_PAGE_POPULATED) && pg_cache_try_get_unsafe(descr, 1)) {
+            struct rrdeng_cmd cmd;
+
+            uv_rwlock_rdunlock(&page_index->lock);
+
+            cmd.opcode = RRDENG_READ_PAGE;
+            cmd.read_page.page_cache_descr = descr;
+            rrdeng_enq_cmd(&ctx->worker_config, &cmd);
+
+            debug(D_RRDENGINE, "%s: Waiting for page to be asynchronously read from disk:", __func__);
+            if(unlikely(debug_flags & D_RRDENGINE))
+                print_page_cache_descr(descr, "", true);
+            while (!(pg_cache_descr->flags & RRD_PAGE_POPULATED)) {
+                pg_cache_wait_event_unsafe(descr);
+            }
+            /* success */
+            /* Downgrade exclusive reference to allow other readers */
+            pg_cache_descr->flags &= ~RRD_PAGE_LOCKED;
+            pg_cache_wake_up_waiters_unsafe(descr);
+            rrdeng_page_descr_mutex_unlock(ctx, descr);
+            rrd_stat_atomic_add(&ctx->stats.pg_cache_misses, 1);
+            return descr;
+        }
+        uv_rwlock_rdunlock(&page_index->lock);
+        debug(D_RRDENGINE, "%s: Waiting for page to be unlocked:", __func__);
+        if(unlikely(debug_flags & D_RRDENGINE))
+            print_page_cache_descr(descr, "", true);
+        if (!(flags & RRD_PAGE_POPULATED))
+            page_not_in_cache = 1;
+        pg_cache_wait_event_unsafe(descr);
+        rrdeng_page_descr_mutex_unlock(ctx, descr);
+
+        /* reset scan to find again */
+        uv_rwlock_rdlock(&page_index->lock);
+    }
+    uv_rwlock_rdunlock(&page_index->lock);
+
+    if (!(flags & RRD_PAGE_DIRTY))
+        pg_cache_replaceQ_set_hot(ctx, descr);
+    pg_cache_release_pages(ctx, 1);
+    if (page_not_in_cache)
+        rrd_stat_atomic_add(&ctx->stats.pg_cache_misses, 1);
+    else
+        rrd_stat_atomic_add(&ctx->stats.pg_cache_hits, 1);
+    return descr;
+}
+
+/*
+ * Searches for the first page between start_time and end_time and gets a reference.
+ * start_time and end_time are inclusive.
+ * If index is NULL lookup by UUID (id).
+ */
+struct rrdeng_page_descr *
+pg_cache_lookup_next(struct rrdengine_instance *ctx, struct pg_cache_page_index *index, uuid_t *id,
+                     usec_t start_time_ut, usec_t end_time_ut)
+{
+    struct page_cache *pg_cache = &ctx->pg_cache;
+    struct rrdeng_page_descr *descr = NULL;
+    struct page_cache_descr *pg_cache_descr = NULL;
+    unsigned long flags;
+    Pvoid_t *PValue;
+    struct pg_cache_page_index *page_index = NULL;
+    uint8_t page_not_in_cache;
+
+    if (unlikely(NULL == index)) {
+        uv_rwlock_rdlock(&pg_cache->metrics_index.lock);
+        PValue = JudyHSGet(pg_cache->metrics_index.JudyHS_array, id, sizeof(uuid_t));
+        if (likely(NULL != PValue)) {
+            page_index = *PValue;
+        }
+        uv_rwlock_rdunlock(&pg_cache->metrics_index.lock);
+        if (NULL == PValue) {
+            return NULL;
+        }
+    } else {
+        page_index = index;
+    }
+    pg_cache_reserve_pages(ctx, 1);
+
+    page_not_in_cache = 0;
+    uv_rwlock_rdlock(&page_index->lock);
+    int retry_count = 0;
+    while (1) {
+        descr = find_first_page_in_time_range(page_index, start_time_ut, end_time_ut);
+        if (NULL == descr || 0 == descr->page_length || retry_count == default_rrdeng_page_fetch_retries) {
+            /* non-empty page not found */
+            if (retry_count == default_rrdeng_page_fetch_retries)
+                error_report("Page cache timeout while waiting for page %p : returning FAIL", descr);
+            uv_rwlock_rdunlock(&page_index->lock);
+
+            pg_cache_release_pages(ctx, 1);
+            return NULL;
+        }
+        rrdeng_page_descr_mutex_lock(ctx, descr);
+        pg_cache_descr = descr->pg_cache_descr;
+        flags = pg_cache_descr->flags;
+        if ((flags & RRD_PAGE_POPULATED) && pg_cache_try_get_unsafe(descr, 0)) {
+            /* success */
+            rrdeng_page_descr_mutex_unlock(ctx, descr);
+            debug(D_RRDENGINE, "%s: Page was found in memory.", __func__);
+            break;
+        }
+        if (!(flags & RRD_PAGE_POPULATED) && pg_cache_try_get_unsafe(descr, 1)) {
+            struct rrdeng_cmd cmd;
+
+            uv_rwlock_rdunlock(&page_index->lock);
+
+            cmd.opcode = RRDENG_READ_PAGE;
+            cmd.read_page.page_cache_descr = descr;
+            rrdeng_enq_cmd(&ctx->worker_config, &cmd);
+
+            debug(D_RRDENGINE, "%s: Waiting for page to be asynchronously read from disk:", __func__);
+            if(unlikely(debug_flags & D_RRDENGINE))
+                print_page_cache_descr(descr, "", true);
+            while (!(pg_cache_descr->flags & RRD_PAGE_POPULATED)) {
+                pg_cache_wait_event_unsafe(descr);
+            }
+            /* success */
+            /* Downgrade exclusive reference to allow other readers */
+            pg_cache_descr->flags &= ~RRD_PAGE_LOCKED;
+            pg_cache_wake_up_waiters_unsafe(descr);
+            rrdeng_page_descr_mutex_unlock(ctx, descr);
+            rrd_stat_atomic_add(&ctx->stats.pg_cache_misses, 1);
+            return descr;
+        }
+        uv_rwlock_rdunlock(&page_index->lock);
+        debug(D_RRDENGINE, "%s: Waiting for page to be unlocked:", __func__);
+        if(unlikely(debug_flags & D_RRDENGINE))
+            print_page_cache_descr(descr, "", true);
+        if (!(flags & RRD_PAGE_POPULATED))
+            page_not_in_cache = 1;
+
+        if (pg_cache_timedwait_event_unsafe(descr, default_rrdeng_page_fetch_timeout) == UV_ETIMEDOUT) {
+            error_report("Page cache timeout while waiting for page %p : retry count = %d", descr, retry_count);
+            ++retry_count;
+        }
+        rrdeng_page_descr_mutex_unlock(ctx, descr);
+
+        /* reset scan to find again */
+        uv_rwlock_rdlock(&page_index->lock);
+    }
+    uv_rwlock_rdunlock(&page_index->lock);
+
+    if (!(flags & RRD_PAGE_DIRTY))
+        pg_cache_replaceQ_set_hot(ctx, descr);
+    pg_cache_release_pages(ctx, 1);
+    if (page_not_in_cache)
+        rrd_stat_atomic_add(&ctx->stats.pg_cache_misses, 1);
+    else
+        rrd_stat_atomic_add(&ctx->stats.pg_cache_hits, 1);
+    return descr;
+}
+
+struct pg_cache_page_index *create_page_index(uuid_t *id, struct rrdengine_instance *ctx)
+{
+    struct pg_cache_page_index *page_index;
+
+    page_index = mallocz(sizeof(*page_index));
+    page_index->JudyL_array = (Pvoid_t) NULL;
+    uuid_copy(page_index->id, *id);
+    fatal_assert(0 == uv_rwlock_init(&page_index->lock));
+    page_index->oldest_time_ut = INVALID_TIME;
+    page_index->latest_time_ut = INVALID_TIME;
+    page_index->prev = NULL;
+    page_index->page_count = 0;
+    page_index->refcount = 0;
+    page_index->writers = 0;
+    page_index->ctx = ctx;
+    page_index->latest_update_every_s = default_rrd_update_every;
+
+    return page_index;
+}
+
+static void init_metrics_index(struct rrdengine_instance *ctx)
+{
+    struct page_cache *pg_cache = &ctx->pg_cache;
+
+    pg_cache->metrics_index.JudyHS_array = (Pvoid_t) NULL;
+    pg_cache->metrics_index.last_page_index = NULL;
+    fatal_assert(0 == uv_rwlock_init(&pg_cache->metrics_index.lock));
+}
+
+static void init_replaceQ(struct rrdengine_instance *ctx)
+{
+    struct page_cache *pg_cache = &ctx->pg_cache;
+
+    pg_cache->replaceQ.head = NULL;
+    pg_cache->replaceQ.tail = NULL;
+    fatal_assert(0 == uv_rwlock_init(&pg_cache->replaceQ.lock));
+}
+
+static void init_committed_page_index(struct rrdengine_instance *ctx)
+{
+    struct page_cache *pg_cache = &ctx->pg_cache;
+
+    pg_cache->committed_page_index.JudyL_array = (Pvoid_t) NULL;
+    fatal_assert(0 == uv_rwlock_init(&pg_cache->committed_page_index.lock));
+    pg_cache->committed_page_index.latest_corr_id = 0;
+    pg_cache->committed_page_index.nr_committed_pages = 0;
+}
+
+void init_page_cache(struct rrdengine_instance *ctx)
+{
+    struct page_cache *pg_cache = &ctx->pg_cache;
+
+    pg_cache->page_descriptors = 0;
+    pg_cache->populated_pages = 0;
+    fatal_assert(0 == uv_rwlock_init(&pg_cache->pg_cache_rwlock));
+
+    init_metrics_index(ctx);
+    init_replaceQ(ctx);
+    init_committed_page_index(ctx);
+}
+
+void free_page_cache(struct rrdengine_instance *ctx)
+{
+    struct page_cache *pg_cache = &ctx->pg_cache;
+    Pvoid_t *PValue;
+    struct pg_cache_page_index *page_index, *prev_page_index;
+    Word_t Index;
+    struct rrdeng_page_descr *descr;
+    struct page_cache_descr *pg_cache_descr;
+
+    // if we are exiting, the OS will recover all memory so do not slow down the shutdown process
+    // Do the cleanup if we are compiling with NETDATA_INTERNAL_CHECKS
+    // This affects the reporting of dbengine statistics which are available in real time
+    // via the /api/v1/dbengine_stats endpoint
+#ifndef NETDATA_DBENGINE_FREE
+    if (netdata_exit)
+        return;
+#endif
+    Word_t metrics_index_bytes = 0, pages_index_bytes = 0, pages_dirty_index_bytes = 0;
+
+    /* Free committed page index */
+    pages_dirty_index_bytes = JudyLFreeArray(&pg_cache->committed_page_index.JudyL_array, PJE0);
+    fatal_assert(NULL == pg_cache->committed_page_index.JudyL_array);
+
+    for (page_index = pg_cache->metrics_index.last_page_index ;
+         page_index != NULL ;
+         page_index = prev_page_index) {
+
+        prev_page_index = page_index->prev;
+
+        /* Find first page in range */
+        Index = (Word_t) 0;
+        PValue = JudyLFirst(page_index->JudyL_array, &Index, PJE0);
+        descr = unlikely(NULL == PValue) ? NULL : *PValue;
+
+        while (descr != NULL) {
+            /* Iterate all page descriptors of this metric */
+
+            if (descr->pg_cache_descr_state & PG_CACHE_DESCR_ALLOCATED) {
+                /* Check rrdenglocking.c */
+                pg_cache_descr = descr->pg_cache_descr;
+                if (pg_cache_descr->flags & RRD_PAGE_POPULATED) {
+                    dbengine_page_free(pg_cache_descr->page);
+                }
+                rrdeng_destroy_pg_cache_descr(ctx, pg_cache_descr);
+            }
+            rrdeng_page_descr_freez(descr);
+
+            PValue = JudyLNext(page_index->JudyL_array, &Index, PJE0);
+            descr = unlikely(NULL == PValue) ? NULL : *PValue;
+        }
+
+        /* Free page index */
+        pages_index_bytes += JudyLFreeArray(&page_index->JudyL_array, PJE0);
+        fatal_assert(NULL == page_index->JudyL_array);
+        freez(page_index);
+    }
+    /* Free metrics index */
+    metrics_index_bytes = JudyHSFreeArray(&pg_cache->metrics_index.JudyHS_array, PJE0);
+    fatal_assert(NULL == pg_cache->metrics_index.JudyHS_array);
+    info("Freed %lu bytes of memory from page cache.", pages_dirty_index_bytes + pages_index_bytes + metrics_index_bytes);
+}