Merging upstream version 1.15.0.

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

1 files changed, 792 insertions, 0 deletions

diff --git a/database/engine/pagecache.c b/database/engine/pagecache.c
new file mode 100644
index 000000000..c90947a67
--- /dev/null
+++ b/database/engine/pagecache.c
@@ -0,0 +1,792 @@
+// SPDX-License-Identifier: GPL-3.0-or-later
+#define NETDATA_RRD_INTERNALS
+
+#include "rrdengine.h"
+
+/* Forward declerations */
+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_cache_descr *descr)
+{
+    struct page_cache *pg_cache = &ctx->pg_cache;
+
+    if (likely(NULL != pg_cache->replaceQ.tail)) {
+        descr->prev = pg_cache->replaceQ.tail;
+        pg_cache->replaceQ.tail->next = descr;
+    }
+    if (unlikely(NULL == pg_cache->replaceQ.head)) {
+        pg_cache->replaceQ.head = descr;
+    }
+    pg_cache->replaceQ.tail = descr;
+}
+
+static inline void pg_cache_replaceQ_delete_unsafe(struct rrdengine_instance *ctx,
+                                                   struct rrdeng_page_cache_descr *descr)
+{
+    struct page_cache *pg_cache = &ctx->pg_cache;
+    struct rrdeng_page_cache_descr *prev, *next;
+
+    prev = descr->prev;
+    next = descr->next;
+
+    if (likely(NULL != prev)) {
+        prev->next = next;
+    }
+    if (likely(NULL != next)) {
+        next->prev = prev;
+    }
+    if (unlikely(descr == pg_cache->replaceQ.head)) {
+        pg_cache->replaceQ.head = next;
+    }
+    if (unlikely(descr == pg_cache->replaceQ.tail)) {
+        pg_cache->replaceQ.tail = prev;
+    }
+    descr->prev = descr->next = NULL;
+}
+
+void pg_cache_replaceQ_insert(struct rrdengine_instance *ctx,
+                              struct rrdeng_page_cache_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_cache_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_cache_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_cache_descr *pg_cache_create_descr(void)
+{
+    struct rrdeng_page_cache_descr *descr;
+
+    descr = mallocz(sizeof(*descr));
+    descr->page = NULL;
+    descr->page_length = 0;
+    descr->start_time = INVALID_TIME;
+    descr->end_time = INVALID_TIME;
+    descr->id = NULL;
+    descr->extent = NULL;
+    descr->flags = 0;
+    descr->prev = descr->next = descr->private = NULL;
+    descr->refcnt = 0;
+    descr->waiters = 0;
+    descr->handle = NULL;
+    assert(0 == uv_cond_init(&descr->cond));
+    assert(0 == uv_mutex_init(&descr->mutex));
+
+    return descr;
+}
+
+void pg_cache_destroy_descr(struct rrdeng_page_cache_descr *descr)
+{
+    uv_cond_destroy(&descr->cond);
+    uv_mutex_destroy(&descr->mutex);
+    free(descr);
+}
+
+/* The caller must hold page descriptor lock. */
+void pg_cache_wake_up_waiters_unsafe(struct rrdeng_page_cache_descr *descr)
+{
+    if (descr->waiters)
+        uv_cond_broadcast(&descr->cond);
+}
+
+/*
+ * 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_cache_descr *descr)
+{
+    ++descr->waiters;
+    uv_cond_wait(&descr->cond, &descr->mutex);
+    --descr->waiters;
+}
+
+/*
+ * 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 rrdeng_page_cache_descr *descr)
+{
+    unsigned long flags;
+
+    uv_mutex_lock(&descr->mutex);
+    pg_cache_wait_event_unsafe(descr);
+    flags = descr->flags;
+    uv_mutex_unlock(&descr->mutex);
+
+    return flags;
+}
+
+/*
+ * 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_cache_descr *descr, int exclusive_access)
+{
+    if ((descr->flags & (RRD_PAGE_LOCKED | RRD_PAGE_READ_PENDING)) ||
+        (exclusive_access && descr->refcnt)) {
+        return 0;
+    }
+    if (exclusive_access)
+        descr->flags |= RRD_PAGE_LOCKED;
+    ++descr->refcnt;
+
+    return 1;
+}
+
+/*
+ * The caller must hold page descriptor lock.
+ * Same return values as pg_cache_try_get_unsafe() without doing anything.
+ */
+int pg_cache_can_get_unsafe(struct rrdeng_page_cache_descr *descr, int exclusive_access)
+{
+    if ((descr->flags & (RRD_PAGE_LOCKED | RRD_PAGE_READ_PENDING)) ||
+        (exclusive_access && descr->refcnt)) {
+        return 0;
+    }
+
+    return 1;
+}
+
+/*
+ * The caller must hold the page descriptor lock.
+ * This function may block doing cleanup.
+ */
+void pg_cache_put_unsafe(struct rrdeng_page_cache_descr *descr)
+{
+    descr->flags &= ~RRD_PAGE_LOCKED;
+    if (0 == --descr->refcnt) {
+        pg_cache_wake_up_waiters_unsafe(descr);
+    }
+    /* TODO: perform cleanup */
+}
+
+/*
+ * This function may block doing cleanup.
+ */
+void pg_cache_put(struct rrdeng_page_cache_descr *descr)
+{
+    uv_mutex_lock(&descr->mutex);
+    pg_cache_put_unsafe(descr);
+    uv_mutex_unlock(&descr->mutex);
+}
+
+/* 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 will block until it reserves #number populated pages.
+ * It will trigger evictions or dirty page flushing if the ctx->max_cache_pages limit is hit.
+ */
+static void pg_cache_reserve_pages(struct rrdengine_instance *ctx, unsigned number)
+{
+    struct page_cache *pg_cache = &ctx->pg_cache;
+
+    assert(number < ctx->max_cache_pages);
+
+    uv_rwlock_wrlock(&pg_cache->pg_cache_rwlock);
+    if (pg_cache->populated_pages + number >= ctx->max_cache_pages + 1)
+        debug(D_RRDENGINE, "=================================\nPage cache full. Reserving %u pages.\n=================================",
+                number);
+    while (pg_cache->populated_pages + number >= ctx->max_cache_pages + 1) {
+        if (!pg_cache_try_evict_one_page_unsafe(ctx)) {
+            /* failed to evict */
+            struct completion compl;
+            struct rrdeng_cmd cmd;
+
+            uv_rwlock_wrunlock(&pg_cache->pg_cache_rwlock);
+
+            init_completion(&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__);
+            wait_for_completion(&compl);
+            destroy_completion(&compl);
+
+            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 ctx->cache_pages_low_watermark 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 >= ctx->cache_pages_low_watermark + 1) {
+        debug(D_RRDENGINE,
+              "=================================\nPage cache full. Trying to reserve %u pages.\n=================================",
+              number);
+        do {
+            if (!pg_cache_try_evict_one_page_unsafe(ctx))
+                break;
+            ++count;
+        } while (pg_cache->populated_pages + number >= ctx->cache_pages_low_watermark + 1);
+        debug(D_RRDENGINE, "Evicted %u pages.", count);
+    }
+
+    if (pg_cache->populated_pages + number < ctx->max_cache_pages + 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_cache_descr *descr)
+{
+    free(descr->page);
+    descr->page = NULL;
+    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 -> 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_cache_descr *descr;
+
+    uv_rwlock_wrlock(&pg_cache->replaceQ.lock);
+    for (descr = pg_cache->replaceQ.head ; NULL != descr ; descr = descr->next) {
+        uv_mutex_lock(&descr->mutex);
+        old_flags = 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);
+            uv_mutex_unlock(&descr->mutex);
+            pg_cache_replaceQ_delete_unsafe(ctx, descr);
+            uv_rwlock_wrunlock(&pg_cache->replaceQ.lock);
+
+            return 1;
+        }
+        uv_mutex_unlock(&descr->mutex);
+    };
+    uv_rwlock_wrunlock(&pg_cache->replaceQ.lock);
+
+    /* failed to evict */
+    return 0;
+}
+
+/*
+ * TODO: last waiter frees descriptor ?
+ */
+void pg_cache_punch_hole(struct rrdengine_instance *ctx, struct rrdeng_page_cache_descr *descr)
+{
+    struct page_cache *pg_cache = &ctx->pg_cache;
+    Pvoid_t *PValue;
+    struct pg_cache_page_index *page_index;
+    int ret;
+
+    uv_rwlock_rdlock(&pg_cache->metrics_index.lock);
+    PValue = JudyHSGet(pg_cache->metrics_index.JudyHS_array, descr->id, sizeof(uuid_t));
+    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 / USEC_PER_SEC), PJE0);
+    uv_rwlock_wrunlock(&page_index->lock);
+    if (unlikely(0 == ret)) {
+        error("Page under deletion was not in index.");
+        if (unlikely(debug_flags & D_RRDENGINE))
+            print_page_cache_descr(descr);
+        goto destroy;
+    }
+    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);
+
+    uv_mutex_lock(&descr->mutex);
+    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);
+        pg_cache_wait_event_unsafe(descr);
+    }
+    /* even a locked page could be dirty */
+    while (unlikely(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);
+        pg_cache_wait_event_unsafe(descr);
+    }
+    uv_mutex_unlock(&descr->mutex);
+
+    if (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(descr);
+destroy:
+    pg_cache_destroy_descr(descr);
+    pg_cache_update_metric_times(page_index);
+}
+
+static inline int is_page_in_time_range(struct rrdeng_page_cache_descr *descr, usec_t start_time, usec_t end_time)
+{
+    usec_t pg_start, pg_end;
+
+    pg_start = descr->start_time;
+    pg_end = descr->end_time;
+
+    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_cache_descr *descr, usec_t point_in_time)
+{
+    return (point_in_time >= descr->start_time && point_in_time <= descr->end_time);
+}
+
+/* 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_cache_descr *descr)
+{
+    usec_t oldest_time = page_index->oldest_time;
+    usec_t latest_time = page_index->latest_time;
+
+    if (unlikely(oldest_time == INVALID_TIME || descr->start_time < oldest_time)) {
+        page_index->oldest_time = descr->start_time;
+    }
+    if (likely(descr->end_time > latest_time || latest_time == INVALID_TIME)) {
+        page_index->latest_time = descr->end_time;
+    }
+}
+
+/* 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_cache_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;
+    }
+    lastIndex = (Word_t)-1;
+    lastPValue = JudyLLast(page_index->JudyL_array, &lastIndex, PJE0);
+    if (likely(NULL != lastPValue)) {
+        descr = *lastPValue;
+        latest_time = descr->end_time;
+    }
+    uv_rwlock_rdunlock(&page_index->lock);
+
+    if (unlikely(NULL == firstPValue)) {
+        assert(NULL == lastPValue);
+        page_index->oldest_time = page_index->latest_time = INVALID_TIME;
+        return;
+    }
+    page_index->oldest_time = oldest_time;
+    page_index->latest_time = 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_cache_descr *descr)
+{
+    struct page_cache *pg_cache = &ctx->pg_cache;
+    Pvoid_t *PValue;
+    struct pg_cache_page_index *page_index;
+
+    if (descr->flags & RRD_PAGE_POPULATED) {
+        pg_cache_reserve_pages(ctx, 1);
+        if (!(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));
+        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 / USEC_PER_SEC), PJE0);
+    *PValue = descr;
+    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);
+}
+
+/*
+ * Searches for a page and triggers disk I/O if necessary and possible.
+ * Does not get a reference.
+ * Returns page index pointer for given metric UUID.
+ */
+struct pg_cache_page_index *
+        pg_cache_preload(struct rrdengine_instance *ctx, uuid_t *id, usec_t start_time, usec_t end_time)
+{
+    struct page_cache *pg_cache = &ctx->pg_cache;
+    struct rrdeng_page_cache_descr *descr = NULL, *preload_array[PAGE_CACHE_MAX_PRELOAD_PAGES];
+    int i, j, k, count, found;
+    unsigned long flags;
+    Pvoid_t *PValue;
+    struct pg_cache_page_index *page_index;
+    Word_t Index;
+    uint8_t failed_to_reserve;
+
+    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) {
+        debug(D_RRDENGINE, "%s: No page was found to attempt preload.", __func__);
+        return NULL;
+    }
+
+    uv_rwlock_rdlock(&page_index->lock);
+    /* Find first page in range */
+    found = 0;
+    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)) {
+            found = 1;
+        }
+    }
+    if (!found) {
+        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)) {
+                found = 1;
+            }
+        }
+    }
+    if (!found) {
+        uv_rwlock_rdunlock(&page_index->lock);
+        debug(D_RRDENGINE, "%s: No page was found to attempt preload.", __func__);
+        return page_index;
+    }
+
+    for (count = 0 ;
+         descr != NULL && is_page_in_time_range(descr, start_time, end_time);
+         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;
+        uv_mutex_lock(&descr->mutex);
+        flags = descr->flags;
+        if (pg_cache_can_get_unsafe(descr, 0)) {
+            if (flags & RRD_PAGE_POPULATED) {
+                /* success */
+                uv_mutex_unlock(&descr->mutex);
+                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[count++] = descr;
+            if (PAGE_CACHE_MAX_PRELOAD_PAGES == count) {
+                uv_mutex_unlock(&descr->mutex);
+                break;
+            }
+        }
+        uv_mutex_unlock(&descr->mutex);
+
+    };
+    uv_rwlock_rdunlock(&page_index->lock);
+
+    failed_to_reserve = 0;
+    for (i = 0 ; i < count && !failed_to_reserve ; ++i) {
+        struct rrdeng_cmd cmd;
+        struct rrdeng_page_cache_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 < 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 < count ; ++i) {
+            descr = preload_array[i];
+            if (NULL == descr) {
+                continue;
+            }
+            pg_cache_put(descr);
+        }
+    }
+    if (!count) {
+        /* no such page */
+        debug(D_RRDENGINE, "%s: No page was eligible to attempt preload.", __func__);
+    }
+    return page_index;
+}
+
+/*
+ * 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_cache_descr *
+        pg_cache_lookup(struct rrdengine_instance *ctx, struct pg_cache_page_index *index, uuid_t *id,
+                        usec_t point_in_time)
+{
+    struct page_cache *pg_cache = &ctx->pg_cache;
+    struct rrdeng_page_cache_descr *descr = NULL;
+    unsigned long flags;
+    Pvoid_t *PValue;
+    struct pg_cache_page_index *page_index;
+    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 / 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 &&
+             !is_point_in_time_in_page(descr, point_in_time))) {
+            /* non-empty page not found */
+            uv_rwlock_rdunlock(&page_index->lock);
+
+            pg_cache_release_pages(ctx, 1);
+            return NULL;
+        }
+        uv_mutex_lock(&descr->mutex);
+        flags = descr->flags;
+        if ((flags & RRD_PAGE_POPULATED) && pg_cache_try_get_unsafe(descr, 0)) {
+            /* success */
+            uv_mutex_unlock(&descr->mutex);
+            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);
+            while (!(descr->flags & RRD_PAGE_POPULATED)) {
+                pg_cache_wait_event_unsafe(descr);
+            }
+            /* success */
+            /* Downgrade exclusive reference to allow other readers */
+            descr->flags &= ~RRD_PAGE_LOCKED;
+            pg_cache_wake_up_waiters_unsafe(descr);
+            uv_mutex_unlock(&descr->mutex);
+            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);
+        if (!(flags & RRD_PAGE_POPULATED))
+            page_not_in_cache = 1;
+        pg_cache_wait_event_unsafe(descr);
+        uv_mutex_unlock(&descr->mutex);
+
+        /* 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 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);
+    assert(0 == uv_rwlock_init(&page_index->lock));
+    page_index->oldest_time = INVALID_TIME;
+    page_index->latest_time = INVALID_TIME;
+
+    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;
+    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;
+    assert(0 == uv_rwlock_init(&pg_cache->replaceQ.lock));
+}
+
+static void init_commited_page_index(struct rrdengine_instance *ctx)
+{
+    struct page_cache *pg_cache = &ctx->pg_cache;
+
+    pg_cache->commited_page_index.JudyL_array = (Pvoid_t) NULL;
+    assert(0 == uv_rwlock_init(&pg_cache->commited_page_index.lock));
+    pg_cache->commited_page_index.latest_corr_id = 0;
+    pg_cache->commited_page_index.nr_commited_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;
+    assert(0 == uv_rwlock_init(&pg_cache->pg_cache_rwlock));
+
+    init_metrics_index(ctx);
+    init_replaceQ(ctx);
+    init_commited_page_index(ctx);
+}
\ No newline at end of file