Merging upstream version 1.18.0.

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

7 files changed, 221 insertions, 156 deletions

diff --git a/database/engine/README.md b/database/engine/README.md
index 7791a549..78f3b15e 100644
--- a/database/engine/README.md
+++ b/database/engine/README.md
@@ -1,18 +1,17 @@
 # Database engine
 
-The Database Engine works like a traditional
-database. There is some amount of RAM dedicated to data caching and indexing and the rest of
-the data reside compressed on disk. The number of history entries is not fixed in this case,
-but depends on the configured disk space and the effective compression ratio of the data stored.
-This is the **only mode** that supports changing the data collection update frequency
-(`update_every`) **without losing** the previously stored metrics.
+The Database Engine works like a traditional database. There is some amount of RAM dedicated to data caching and
+indexing and the rest of the data reside compressed on disk. The number of history entries is not fixed in this case,
+but depends on the configured disk space and the effective compression ratio of the data stored. This is the **only
+mode** that supports changing the data collection update frequency (`update_every`) **without losing** the previously
+stored metrics.
 
 ## Files
 
-With the DB engine memory mode the metric data are stored in database files. These files are
-organized in pairs, the datafiles and their corresponding journalfiles, e.g.:
+With the DB engine memory mode the metric data are stored in database files. These files are organized in pairs, the
+datafiles and their corresponding journalfiles, e.g.:
 
-```
+```sh
 datafile-1-0000000001.ndf
 journalfile-1-0000000001.njf
 datafile-1-0000000002.ndf
@@ -22,21 +21,19 @@ journalfile-1-0000000003.njf
 ...
 ```
 
-They are located under their host's cache directory in the directory `./dbengine`
-(e.g. for localhost the default location is `/var/cache/netdata/dbengine/*`). The higher
-numbered filenames contain more recent metric data. The user can safely delete some pairs
-of files when Netdata is stopped to manually free up some space.
+They are located under their host's cache directory in the directory `./dbengine` (e.g. for localhost the default
+location is `/var/cache/netdata/dbengine/*`). The higher numbered filenames contain more recent metric data. The user
+can safely delete some pairs of files when Netdata is stopped to manually free up some space.
 
 _Users should_ **back up** _their `./dbengine` folders if they consider this data to be important._
 
 ## Configuration
 
-There is one DB engine instance per Netdata host/node. That is, there is one `./dbengine` folder
-per node, and all charts of `dbengine` memory mode in such a host share the same storage space
-and DB engine instance memory state. You can select the memory mode for localhost by editing
-netdata.conf and setting:
+There is one DB engine instance per Netdata host/node. That is, there is one `./dbengine` folder per node, and all
+charts of `dbengine` memory mode in such a host share the same storage space and DB engine instance memory state. You
+can select the memory mode for localhost by editing netdata.conf and setting:
 
-```
+```conf
 [global]
     memory mode = dbengine
 ```
@@ -44,110 +41,157 @@ netdata.conf and setting:
 For setting the memory mode for the rest of the nodes you should look at
 [streaming](../../streaming/).
 
-The `history` configuration option is meaningless for `memory mode = dbengine` and is ignored
-for any metrics being stored in the DB engine.
+The `history` configuration option is meaningless for `memory mode = dbengine` and is ignored for any metrics being
+stored in the DB engine.
 
-All DB engine instances, for localhost and all other streaming recipient nodes inherit their
-configuration from `netdata.conf`:
+All DB engine instances, for localhost and all other streaming recipient nodes inherit their configuration from
+`netdata.conf`:
 
-```
+```conf
 [global]
     page cache size = 32
     dbengine disk space = 256
 ```
 
-The above values are the default and minimum values for Page Cache size and DB engine disk space
-quota. Both numbers are in **MiB**. All DB engine instances will allocate the configured resources
-separately.
+The above values are the default and minimum values for Page Cache size and DB engine disk space quota. Both numbers are
+in **MiB**. All DB engine instances will allocate the configured resources separately.
 
-The `page cache size` option determines the amount of RAM in **MiB** that is dedicated to caching
-Netdata metric values themselves.
+The `page cache size` option determines the amount of RAM in **MiB** that is dedicated to caching Netdata metric values
+themselves as far as queries are concerned. The total page cache size will be greater since data collection itself will
+consume additional memory as is described in the [Memory requirements](#memory-requirements) section.
 
-The `dbengine disk space` option determines the amount of disk space in **MiB** that is dedicated
-to storing Netdata metric values and all related metadata describing them.
+The `dbengine disk space` option determines the amount of disk space in **MiB** that is dedicated to storing Netdata
+metric values and all related metadata describing them.
 
 ## Operation
 
-The DB engine stores chart metric values in 4096-byte pages in memory. Each chart dimension gets
-its own page to store consecutive values generated from the data collectors. Those pages comprise
-the **Page Cache**.
+The DB engine stores chart metric values in 4096-byte pages in memory. Each chart dimension gets its own page to store
+consecutive values generated from the data collectors. Those pages comprise the **Page Cache**.
 
-When those pages fill up they are slowly compressed and flushed to disk.
-It can take `4096 / 4 = 1024 seconds = 17 minutes`, for a chart dimension that is being collected
-every 1 second, to fill a page. Pages can be cut short when we stop Netdata or the DB engine
-instance so as to not lose the data. When we query the DB engine for data we trigger disk read
-I/O requests that fill the Page Cache with the requested pages and potentially evict cold
-(not recently used) pages. 
+When those pages fill up they are slowly compressed and flushed to disk. It can take `4096 / 4 = 1024 seconds = 17
+minutes`, for a chart dimension that is being collected every 1 second, to fill a page. Pages can be cut short when we
+stop Netdata or the DB engine instance so as to not lose the data. When we query the DB engine for data we trigger disk
+read I/O requests that fill the Page Cache with the requested pages and potentially evict cold (not recently used)
+pages. 
 
-When the disk quota is exceeded the oldest values are removed from the DB engine at real time, by
-automatically deleting the oldest datafile and journalfile pair. Any corresponding pages residing
-in the Page Cache will also be invalidated and removed. The DB engine logic will try to maintain
-between 10 and 20 file pairs at any point in time. 
+When the disk quota is exceeded the oldest values are removed from the DB engine at real time, by automatically deleting
+the oldest datafile and journalfile pair. Any corresponding pages residing in the Page Cache will also be invalidated
+and removed. The DB engine logic will try to maintain between 10 and 20 file pairs at any point in time. 
 
-The Database Engine uses direct I/O to avoid polluting the OS filesystem caches and does not 
-generate excessive I/O traffic so as to create the minimum possible interference with other 
-applications.
+The Database Engine uses direct I/O to avoid polluting the OS filesystem caches and does not generate excessive I/O
+traffic so as to create the minimum possible interference with other applications.
 
 ## Memory requirements
 
-Using memory mode `dbengine` we can overcome most memory restrictions and store a dataset that
-is much larger than the available memory.
+Using memory mode `dbengine` we can overcome most memory restrictions and store a dataset that is much larger than the
+available memory.
 
-There are explicit memory requirements **per** DB engine **instance**, meaning **per** Netdata 
-**node** (e.g. localhost and streaming recipient nodes):
+There are explicit memory requirements **per** DB engine **instance**, meaning **per** Netdata **node** (e.g. localhost
+and streaming recipient nodes):
 
--   `page cache size` must be at least `#dimensions-being-collected x 4096 x 2` bytes.
+-   The total page cache memory footprint will be an additional `#dimensions-being-collected x 4096 x 2` bytes over what
+    the user configured with `page cache size`.
 
 -   an additional `#pages-on-disk x 4096 x 0.03` bytes of RAM are allocated for metadata.
 
     -   roughly speaking this is 3% of the uncompressed disk space taken by the DB files.
 
-    -   for very highly compressible data (compression ratio > 90%) this RAM overhead
-        is comparable to the disk space footprint.
+    -   for very highly compressible data (compression ratio > 90%) this RAM overhead is comparable to the disk space
+        footprint.
 
-An important observation is that RAM usage depends on both the `page cache size` and the 
-`dbengine disk space` options. 
+An important observation is that RAM usage depends on both the `page cache size` and the `dbengine disk space` options.
 
 ## File descriptor requirements
 
-The Database Engine may keep a **significant** amount of files open per instance (e.g. per streaming
-slave or master server). When configuring your system you should make sure there are at least 50
-file descriptors available per `dbengine` instance.
+The Database Engine may keep a **significant** amount of files open per instance (e.g. per streaming slave or master
+server). When configuring your system you should make sure there are at least 50 file descriptors available per
+`dbengine` instance.
 
-Netdata allocates 25% of the available file descriptors to its Database Engine instances. This means that only 25%
-of the file descriptors that are available to the Netdata service are accessible by dbengine instances.
-You should take that into account when configuring your service
-or system-wide file descriptor limits. You can roughly estimate that the Netdata service needs 2048 file
-descriptors for every 10 streaming slave hosts when streaming is configured to use `memory mode = dbengine`.
+Netdata allocates 25% of the available file descriptors to its Database Engine instances. This means that only 25% of
+the file descriptors that are available to the Netdata service are accessible by dbengine instances. You should take
+that into account when configuring your service or system-wide file descriptor limits. You can roughly estimate that the
+Netdata service needs 2048 file descriptors for every 10 streaming slave hosts when streaming is configured to use
+`memory mode = dbengine`.
 
-If for example one wants to allocate 65536 file descriptors to the Netdata service on a systemd system
-one needs to override the Netdata service by running `sudo systemctl edit netdata` and creating a
-file with contents:
+If for example one wants to allocate 65536 file descriptors to the Netdata service on a systemd system one needs to
+override the Netdata service by running `sudo systemctl edit netdata` and creating a file with contents:
 
-```
+```sh
 [Service]
 LimitNOFILE=65536
 ```
 
 For other types of services one can add the line:
 
-```
+```sh
 ulimit -n 65536
 ```
 
-at the beginning of the service file. Alternatively you can change the system-wide limits of the kernel by changing `/etc/sysctl.conf`. For linux that would be:
+at the beginning of the service file. Alternatively you can change the system-wide limits of the kernel by changing
+ `/etc/sysctl.conf`. For linux that would be:
 
-```
+```conf
 fs.file-max = 65536
 ```
 
 In FreeBSD and OS X you change the lines like this:
 
-```
+```conf
 kern.maxfilesperproc=65536
 kern.maxfiles=65536
 ```
 
 You can apply the settings by running `sysctl -p` or by rebooting.
 
+## Evaluation
+
+We have evaluated the performance of the `dbengine` API that the netdata daemon uses internally. This is **not** the
+web API of netdata. Our benchmarks ran on a **single** `dbengine` instance, multiple of which can be running in a
+netdata master server. We used a server with an AMD Ryzen Threadripper 2950X 16-Core Processor and 2 disk drives, a
+Seagate Constellation ES.3 2TB magnetic HDD and a SAMSUNG MZQLB960HAJR-00007 960GB NAND Flash SSD.
+
+For our workload, we defined 32 charts with 128 metrics each, giving us a total of 4096 metrics. We defined 1 worker
+thread per chart (32 threads) that generates new data points with a data generation interval of 1 second. The time axis
+of the time-series is emulated and accelerated so that the worker threads can generate as many data points as possible
+without delays. 
+
+We also defined 32 worker threads that perform queries on random metrics with semi-random time ranges. The
+starting time of the query is randomly selected between the beginning of the time-series and the time of the latest data
+point. The ending time is randomly selected between 1 second and 1 hour after the starting time. The pseudo-random
+numbers are generated with a uniform distribution.
+
+The data are written to the database at the same time as they are read from it. This is a concurrent read/write mixed
+workload with a duration of 60 seconds. The faster `dbengine` runs, the bigger the dataset size becomes since more
+data points will be generated. We set a page cache size of 64MiB for the two disk-bound scenarios. This way, the dataset
+size of the metric data is much bigger than the RAM that is being used for caching so as to trigger I/O requests most
+of the time. In our final scenario, we set the page cache size to 16 GiB. That way, the dataset fits in the page cache
+so as to avoid all disk bottlenecks.
+
+The reported numbers are the following:
+
+| device | page cache | dataset | reads/sec | writes/sec |
+| :---: | :---: | ---: | ---: | ---: |
+| HDD | 64 MiB | 4.1 GiB | 813K | 18.0M |
+| SSD | 64 MiB | 9.8 GiB | 1.7M | 43.0M |
+| N/A | 16 GiB | 6.8 GiB |118.2M | 30.2M |
+
+where "reads/sec" is the number of metric data points being read from the database via its API per second and
+"writes/sec" is the number of metric data points being written to the database per second. 
+
+Notice that the HDD numbers are pretty high and not much slower than the SSD numbers. This is thanks to the database
+engine design being optimized for rotating media. In the database engine disk I/O requests are:
+
+-   asynchronous to mask the high I/O latency of HDDs.
+-   mostly large to reduce the amount of HDD seeking time.
+-   mostly sequential to reduce the amount of HDD seeking time.
+-   compressed to reduce the amount of required throughput.
+
+As a result, the HDD is not thousands of times slower than the SSD, which is typical for other workloads.
+
+An interesting observation to make is that the CPU-bound run (16 GiB page cache) generates fewer data than the SSD run
+(6.8 GiB vs 9.8 GiB). The reason is that the 32 reader threads in the SSD scenario are more frequently blocked by I/O,
+and generate a read load of 1.7M/sec, whereas in the CPU-bound scenario the read load is 70 times higher at 118M/sec.
+Consequently, there is a significant degree of interference by the reader threads, that slow down the writer threads.
+This is also possible because the interference effects are greater than the SSD impact on data generation throughput.
+
 [![analytics](https://www.google-analytics.com/collect?v=1&aip=1&t=pageview&_s=1&ds=github&dr=https%3A%2F%2Fgithub.com%2Fnetdata%2Fnetdata&dl=https%3A%2F%2Fmy-netdata.io%2Fgithub%2Fdatabase%2Fengine%2FREADME&_u=MAC~&cid=5792dfd7-8dc4-476b-af31-da2fdb9f93d2&tid=UA-64295674-3)](<>)
diff --git a/database/engine/pagecache.c b/database/engine/pagecache.c
index 457bcb21..a419ba98 100644
--- a/database/engine/pagecache.c
+++ b/database/engine/pagecache.c
@@ -209,9 +209,31 @@ static void pg_cache_release_pages(struct rrdengine_instance *ctx, unsigned numb
     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.
+ * The caller must hold the page cache lock.
+ */
+static inline unsigned long pg_cache_hard_limit(struct rrdengine_instance *ctx)
+{
+    /* it's twice the number of producers since we pin 2 pages per producer */
+    return ctx->max_cache_pages + 2 * (unsigned long)ctx->stats.metric_API_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.
+ * The caller must hold the page cache lock.
+ */
+static inline unsigned long pg_cache_soft_limit(struct rrdengine_instance *ctx)
+{
+    /* it's twice the number of producers since we pin 2 pages per producer */
+    return ctx->cache_pages_low_watermark + 2 * (unsigned long)ctx->stats.metric_API_producers;
+}
+
 /*
  * 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.
+ * 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)
 {
@@ -223,10 +245,10 @@ static void pg_cache_reserve_pages(struct rrdengine_instance *ctx, unsigned numb
     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)
+    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 >= ctx->max_cache_pages + 1) {
+    while (pg_cache->populated_pages + number >= pg_cache_hard_limit(ctx) + 1) {
 
         if (!pg_cache_try_evict_one_page_unsafe(ctx)) {
             /* failed to evict */
@@ -260,7 +282,7 @@ static void pg_cache_reserve_pages(struct rrdengine_instance *ctx, unsigned numb
 
 /*
  * This function will attempt to reserve #number populated pages.
- * It may trigger evictions if the ctx->cache_pages_low_watermark limit is hit.
+ * 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)
@@ -272,7 +294,7 @@ static int pg_cache_try_reserve_pages(struct rrdengine_instance *ctx, unsigned n
     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) {
+    if (pg_cache->populated_pages + number >= pg_cache_soft_limit(ctx) + 1) {
         debug(D_RRDENGINE,
               "==Page cache full. Trying to reserve %u pages.==",
               number);
@@ -280,11 +302,11 @@ static int pg_cache_try_reserve_pages(struct rrdengine_instance *ctx, unsigned n
             if (!pg_cache_try_evict_one_page_unsafe(ctx))
                 break;
             ++count;
-        } while (pg_cache->populated_pages + number >= ctx->cache_pages_low_watermark + 1);
+        } 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 < ctx->max_cache_pages + 1) {
+    if (pg_cache->populated_pages + number < pg_cache_hard_limit(ctx) + 1) {
         pg_cache->populated_pages += number;
         ret = 1; /* success */
     }
diff --git a/database/engine/pagecache.h b/database/engine/pagecache.h
index d464211e..ab1a5c1a 100644
--- a/database/engine/pagecache.h
+++ b/database/engine/pagecache.h
@@ -183,4 +183,42 @@ extern void free_page_cache(struct rrdengine_instance *ctx);
 extern void pg_cache_add_new_metric_time(struct pg_cache_page_index *page_index, struct rrdeng_page_descr *descr);
 extern void pg_cache_update_metric_times(struct pg_cache_page_index *page_index);
 
+static inline void
+    pg_cache_atomic_get_pg_info(struct rrdeng_page_descr *descr, usec_t *end_timep, uint32_t *page_lengthp)
+{
+    usec_t end_time, old_end_time;
+    uint32_t page_length;
+
+    if (NULL == descr->extent) {
+        /* this page is currently being modified, get consistent info locklessly */
+        do {
+            end_time = descr->end_time;
+            __sync_synchronize();
+            old_end_time = end_time;
+            page_length = descr->page_length;
+            __sync_synchronize();
+            end_time = descr->end_time;
+            __sync_synchronize();
+        } while ((end_time != old_end_time || (end_time & 1) != 0));
+
+        *end_timep = end_time;
+        *page_lengthp = page_length;
+    } else {
+        *end_timep = descr->end_time;
+        *page_lengthp = descr->page_length;
+    }
+}
+
+/* The caller must hold a reference to the page and must have already set the new data */
+static inline void pg_cache_atomic_set_pg_info(struct rrdeng_page_descr *descr, usec_t end_time, uint32_t page_length)
+{
+    assert(!(end_time & 1));
+    __sync_synchronize();
+    descr->end_time |= 1; /* mark start of uncertainty period by adding 1 microsecond */
+    __sync_synchronize();
+    descr->page_length = page_length;
+    __sync_synchronize();
+    descr->end_time = end_time; /* mark end of uncertainty period */
+}
+
 #endif /* NETDATA_PAGECACHE_H */
diff --git a/database/engine/rrdengine.c b/database/engine/rrdengine.c
index 36d91754..896d71f1 100644
--- a/database/engine/rrdengine.c
+++ b/database/engine/rrdengine.c
@@ -5,6 +5,8 @@
 
 rrdeng_stats_t global_io_errors = 0;
 rrdeng_stats_t global_fs_errors = 0;
+rrdeng_stats_t global_pg_cache_warnings = 0;
+rrdeng_stats_t global_pg_cache_errors = 0;
 rrdeng_stats_t rrdeng_reserved_file_descriptors = 0;
 
 void sanity_check(void)
@@ -251,13 +253,10 @@ void flush_pages_cb(uv_fs_t* req)
 {
     struct rrdengine_worker_config* wc = req->loop->data;
     struct rrdengine_instance *ctx = wc->ctx;
-    struct page_cache *pg_cache = &ctx->pg_cache;
     struct extent_io_descriptor *xt_io_descr;
     struct rrdeng_page_descr *descr;
     struct page_cache_descr *pg_cache_descr;
-    int ret;
     unsigned i, count;
-    Word_t commit_id;
 
     xt_io_descr = req->data;
     if (req->result < 0) {
@@ -277,13 +276,6 @@ void flush_pages_cb(uv_fs_t* req)
         /* care, we don't hold the descriptor mutex */
         descr = xt_io_descr->descr_array[i];
 
-        uv_rwlock_wrlock(&pg_cache->commited_page_index.lock);
-        commit_id = xt_io_descr->descr_commit_idx_array[i];
-        ret = JudyLDel(&pg_cache->commited_page_index.JudyL_array, commit_id, PJE0);
-        assert(1 == ret);
-        --pg_cache->commited_page_index.nr_commited_pages;
-        uv_rwlock_wrunlock(&pg_cache->commited_page_index.lock);
-
         pg_cache_replaceQ_insert(ctx, descr);
 
         rrdeng_page_descr_mutex_lock(ctx, descr);
@@ -331,7 +323,7 @@ static int do_flush_pages(struct rrdengine_worker_config* wc, int force, struct
     if (force) {
         debug(D_RRDENGINE, "Asynchronous flushing of extent has been forced by page pressure.");
     }
-    uv_rwlock_rdlock(&pg_cache->commited_page_index.lock);
+    uv_rwlock_wrlock(&pg_cache->commited_page_index.lock);
     for (Index = 0, count = 0, uncompressed_payload_length = 0,
          PValue = JudyLFirst(pg_cache->commited_page_index.JudyL_array, &Index, PJE0),
          descr = unlikely(NULL == PValue) ? NULL : *PValue ;
@@ -340,11 +332,15 @@ static int do_flush_pages(struct rrdengine_worker_config* wc, int force, struct
 
          PValue = JudyLNext(pg_cache->commited_page_index.JudyL_array, &Index, PJE0),
          descr = unlikely(NULL == PValue) ? NULL : *PValue) {
+        uint8_t page_write_pending;
+
         assert(0 != descr->page_length);
+        page_write_pending = 0;
 
         rrdeng_page_descr_mutex_lock(ctx, descr);
         pg_cache_descr = descr->pg_cache_descr;
         if (!(pg_cache_descr->flags & RRD_PAGE_WRITE_PENDING)) {
+            page_write_pending = 1;
             /* care, no reference being held */
             pg_cache_descr->flags |= RRD_PAGE_WRITE_PENDING;
             uncompressed_payload_length += descr->page_length;
@@ -352,8 +348,14 @@ static int do_flush_pages(struct rrdengine_worker_config* wc, int force, struct
             eligible_pages[count++] = descr;
         }
         rrdeng_page_descr_mutex_unlock(ctx, descr);
+
+        if (page_write_pending) {
+            ret = JudyLDel(&pg_cache->commited_page_index.JudyL_array, Index, PJE0);
+            assert(1 == ret);
+            --pg_cache->commited_page_index.nr_commited_pages;
+        }
     }
-    uv_rwlock_rdunlock(&pg_cache->commited_page_index.lock);
+    uv_rwlock_wrunlock(&pg_cache->commited_page_index.lock);
 
     if (!count) {
         debug(D_RRDENGINE, "%s: no pages eligible for flushing.", __func__);
@@ -813,47 +815,6 @@ error_after_loop_init:
     complete(&ctx->rrdengine_completion);
 }
 
-
-#define NR_PAGES (256)
-static void basic_functional_test(struct rrdengine_instance *ctx)
-{
-    int i, j, failed_validations;
-    uuid_t uuid[NR_PAGES];
-    void *buf;
-    struct rrdeng_page_descr *handle[NR_PAGES];
-    char uuid_str[UUID_STR_LEN];
-    char backup[NR_PAGES][UUID_STR_LEN * 100]; /* backup storage for page data verification */
-
-    for (i = 0 ; i < NR_PAGES ; ++i) {
-        uuid_generate(uuid[i]);
-        uuid_unparse_lower(uuid[i], uuid_str);
-//      fprintf(stderr, "Generated uuid[%d]=%s\n", i, uuid_str);
-        buf = rrdeng_create_page(ctx, &uuid[i], &handle[i]);
-        /* Each page contains 10 times its own UUID stringified */
-        for (j = 0 ; j < 100 ; ++j) {
-            strcpy(buf + UUID_STR_LEN * j, uuid_str);
-            strcpy(backup[i] + UUID_STR_LEN * j, uuid_str);
-        }
-        rrdeng_commit_page(ctx, handle[i], (Word_t)i);
-    }
-    fprintf(stderr, "\n********** CREATED %d METRIC PAGES ***********\n\n", NR_PAGES);
-    failed_validations = 0;
-    for (i = 0 ; i < NR_PAGES ; ++i) {
-        buf = rrdeng_get_latest_page(ctx, &uuid[i], (void **)&handle[i]);
-        if (NULL == buf) {
-            ++failed_validations;
-            fprintf(stderr, "Page %d was LOST.\n", i);
-        }
-        if (memcmp(backup[i], buf, UUID_STR_LEN * 100)) {
-            ++failed_validations;
-            fprintf(stderr, "Page %d data comparison with backup FAILED validation.\n", i);
-        }
-        rrdeng_put_page(ctx, handle[i]);
-    }
-    fprintf(stderr, "\n********** CORRECTLY VALIDATED %d/%d METRIC PAGES ***********\n\n",
-            NR_PAGES - failed_validations, NR_PAGES);
-
-}
 /* C entry point for development purposes
  * make "LDFLAGS=-errdengine_main"
  */
@@ -866,8 +827,6 @@ void rrdengine_main(void)
     if (ret) {
         exit(ret);
     }
-    basic_functional_test(ctx);
-
     rrdeng_exit(ctx);
     fprintf(stderr, "Hello world!");
     exit(0);
diff --git a/database/engine/rrdengineapi.c b/database/engine/rrdengineapi.c
index bf373f31..5fa23d8f 100644
--- a/database/engine/rrdengineapi.c
+++ b/database/engine/rrdengineapi.c
@@ -4,7 +4,7 @@
 /* Default global database instance */
 static struct rrdengine_instance default_global_ctx;
 
-int default_rrdeng_page_cache_mb = RRDENG_MIN_PAGE_CACHE_SIZE_MB;
+int default_rrdeng_page_cache_mb = 32;
 int default_rrdeng_disk_quota_mb = RRDENG_MIN_DISK_SPACE_MB;
 
 /*
@@ -95,9 +95,8 @@ void rrdeng_store_metric_flush_current_page(RRDDIM *rd)
     if (likely(descr->page_length)) {
         int ret, page_is_empty;
 
-#ifdef NETDATA_INTERNAL_CHECKS
         rrd_stat_atomic_add(&ctx->stats.metric_API_producers, -1);
-#endif
+
         if (handle->prev_descr) {
             /* unpin old second page */
             pg_cache_put(ctx, handle->prev_descr);
@@ -185,16 +184,14 @@ void rrdeng_store_metric_next(RRDDIM *rd, usec_t point_in_time, storage_number n
     }
     page = descr->pg_cache_descr->page;
     page[descr->page_length / sizeof(number)] = number;
-    descr->end_time = point_in_time;
-    descr->page_length += sizeof(number);
+    pg_cache_atomic_set_pg_info(descr, point_in_time, descr->page_length + sizeof(number));
+
     if (perfect_page_alignment)
         rd->rrdset->rrddim_page_alignment = descr->page_length;
     if (unlikely(INVALID_TIME == descr->start_time)) {
         descr->start_time = point_in_time;
 
-#ifdef NETDATA_INTERNAL_CHECKS
         rrd_stat_atomic_add(&ctx->stats.metric_API_producers, 1);
-#endif
         pg_cache_insert(ctx, handle->page_index, descr);
     } else {
         pg_cache_add_new_metric_time(handle->page_index, descr);
@@ -312,8 +309,9 @@ unsigned rrdeng_variable_step_boundaries(RRDSET *st, time_t start_time, time_t e
         curr = &page_info_array[i];
         *pginfo_to_points(curr) = 0; /* initialize to invalid page */
         *pginfo_to_dt(curr) = 0; /* no known data collection interval yet */
-        if (unlikely(INVALID_TIME == curr->start_time || INVALID_TIME == curr->end_time)) {
-            info("Ignoring page with invalid timestamp.");
+        if (unlikely(INVALID_TIME == curr->start_time || INVALID_TIME == curr->end_time ||
+                     curr->end_time < curr->start_time)) {
+            info("Ignoring page with invalid timestamps.");
             prev = old_prev;
             continue;
         }
@@ -366,7 +364,7 @@ unsigned rrdeng_variable_step_boundaries(RRDSET *st, time_t start_time, time_t e
             continue;
         }
 
-        if (unlikely(0 == dt)) { /* unknown data collection interval */
+        if (unlikely(0 == *pginfo_to_dt(curr))) { /* unknown data collection interval */
             assert(1 == page_points);
 
             if (likely(NULL != prev)) { /* get interval from previous page */
@@ -454,7 +452,8 @@ storage_number rrdeng_load_metric_next(struct rrddim_query_handle *rrdimm_handle
     struct rrdeng_page_descr *descr;
     storage_number *page, ret;
     unsigned position, entries;
-    usec_t next_page_time, current_position_time;
+    usec_t next_page_time, current_position_time, page_end_time;
+    uint32_t page_length;
 
     handle = &rrdimm_handle->rrdeng;
     if (unlikely(INVALID_TIME == handle->next_page_time)) {
@@ -464,15 +463,17 @@ storage_number rrdeng_load_metric_next(struct rrddim_query_handle *rrdimm_handle
     if (unlikely(NULL == (descr = handle->descr))) {
         /* it's the first call */
         next_page_time = handle->next_page_time * USEC_PER_SEC;
+    } else {
+        pg_cache_atomic_get_pg_info(descr, &page_end_time, &page_length);
     }
     position = handle->position + 1;
 
     if (unlikely(NULL == descr ||
-                 position >= (descr->page_length / sizeof(storage_number)))) {
+                 position >= (page_length / sizeof(storage_number)))) {
         /* We need to get a new page */
         if (descr) {
             /* Drop old page's reference */
-            handle->next_page_time = (descr->end_time / USEC_PER_SEC) + 1;
+            handle->next_page_time = (page_end_time / USEC_PER_SEC) + 1;
             if (unlikely(handle->next_page_time > rrdimm_handle->end_time)) {
                 goto no_more_metrics;
             }
@@ -492,26 +493,27 @@ storage_number rrdeng_load_metric_next(struct rrddim_query_handle *rrdimm_handle
         rrd_stat_atomic_add(&ctx->stats.metric_API_consumers, 1);
 #endif
         handle->descr = descr;
+        pg_cache_atomic_get_pg_info(descr, &page_end_time, &page_length);
         if (unlikely(INVALID_TIME == descr->start_time ||
-                     INVALID_TIME == descr->end_time)) {
+                     INVALID_TIME == page_end_time)) {
             goto no_more_metrics;
         }
-        if (unlikely(descr->start_time != descr->end_time && next_page_time > descr->start_time)) {
+        if (unlikely(descr->start_time != page_end_time && next_page_time > descr->start_time)) {
             /* we're in the middle of the page somewhere */
-            entries = descr->page_length / sizeof(storage_number);
-            position = ((uint64_t)(next_page_time - descr->start_time)) * entries /
-                       (descr->end_time - descr->start_time + 1);
+            entries = page_length / sizeof(storage_number);
+            position = ((uint64_t)(next_page_time - descr->start_time)) * (entries - 1) /
+                       (page_end_time - descr->start_time);
         } else {
             position = 0;
         }
     }
     page = descr->pg_cache_descr->page;
     ret = page[position];
-    entries = descr->page_length / sizeof(storage_number);
+    entries = page_length / sizeof(storage_number);
     if (entries > 1) {
         usec_t dt;
 
-        dt = (descr->end_time - descr->start_time) / (entries - 1);
+        dt = (page_end_time - descr->start_time) / (entries - 1);
         current_position_time = descr->start_time + position * dt;
     } else {
         current_position_time = descr->start_time;
diff --git a/database/engine/rrdengineapi.h b/database/engine/rrdengineapi.h
index 9b1ab187..c876705e 100644
--- a/database/engine/rrdengineapi.h
+++ b/database/engine/rrdengineapi.h
@@ -5,7 +5,7 @@
 
 #include "rrdengine.h"
 
-#define RRDENG_MIN_PAGE_CACHE_SIZE_MB (32)
+#define RRDENG_MIN_PAGE_CACHE_SIZE_MB (8)
 #define RRDENG_MIN_DISK_SPACE_MB (256)
 
 #define RRDENG_NR_STATS (33)
diff --git a/database/engine/rrdenginelib.c b/database/engine/rrdenginelib.c
index 96504b27..1a04dc2a 100644
--- a/database/engine/rrdenginelib.c
+++ b/database/engine/rrdenginelib.c
@@ -8,7 +8,7 @@ void print_page_cache_descr(struct rrdeng_page_descr *descr)
 {
     struct page_cache_descr *pg_cache_descr = descr->pg_cache_descr;
     char uuid_str[UUID_STR_LEN];
-    char str[BUFSIZE];
+    char str[BUFSIZE + 1];
     int pos = 0;
 
     uuid_unparse_lower(*descr->id, uuid_str);
@@ -31,7 +31,7 @@ void print_page_cache_descr(struct rrdeng_page_descr *descr)
 void print_page_descr(struct rrdeng_page_descr *descr)
 {
     char uuid_str[UUID_STR_LEN];
-    char str[BUFSIZE];
+    char str[BUFSIZE + 1];
     int pos = 0;
 
     uuid_unparse_lower(*descr->id, uuid_str);