Add constant page type

[vkalintiris]

Summary

Adds a constant page type that stores efficiently pages that contain the same value.

Experimental results show that we achieve the same memory reduction as the gorilla page type, ie. around 50-75% of the pages are of constant type. However: (a) the implementation is far simpler, (b) it's easy to extend the concept to non-zero tiers (whereas the gorilla implementation is not applicable to non-zero tiers).

Test Plan

• CI jobs
• Running agent on parent 3.

[thiagoftsm]

Hello @vkalintiris ,

When I run this PR using db.mode = ram everything is working as expected, but when I shift to db.mode = dbegnine, I observed this SIGABRT:

0x00007ffff6c96aab in pthread_kill@@GLIBC_2.34 () from /lib64/libc.so.6
(gdb) bt
#0  0x00007ffff6c96aab in pthread_kill@@GLIBC_2.34 () from /lib64/libc.so.6
#1  0x00007ffff6c42e12 in raise () from /lib64/libc.so.6
#2  0x00007ffff6c2849f in abort () from /lib64/libc.so.6
#3  0x0000000000414f68 in netdata_logger_fatal (
    file=file@entry=0xb67ac0 "/home/thiago/Netdata/tests_netdata/src/database/engine/page.c", 
    function=function@entry=0xbb0f10 <__FUNCTION__.1> "pgdc_get_next_point", line=line@entry=714, 
    fmt=fmt@entry=0xb68090 "Wrong expected cursor position")
    at /home/thiago/Netdata/tests_netdata/src/libnetdata/log/log.c:2351
#4  0x00000000006cf95a in pgdc_get_next_point (pgdc=<optimized out>, expected_position=<optimized out>, 
    sp=sp@entry=0x7fffe1bdc400) at /home/thiago/Netdata/tests_netdata/src/database/engine/page.c:714
#5  0x00000000006c9190 in rrdeng_load_metric_next (seqh=0x51963e8)
    at /home/thiago/Netdata/tests_netdata/src/database/engine/rrdengineapi.c:886
#6  0x00000000004a207b in storage_engine_query_next_metric (seqh=<optimized out>)
    at /home/thiago/Netdata/tests_netdata/src/database/rrd.h:592
#7  rrd2rrdr_query_execute (ops=0x51962c0, dim_id_in_rrdr=0, r=0x5196090)
    at /home/thiago/Netdata/tests_netdata/src/web/api/queries/query.c:1662
#8  rrd2rrdr (owa=owa@entry=0x5196050, qt=qt@entry=0x5197060)
    at /home/thiago/Netdata/tests_netdata/src/web/api/queries/query.c:3532
#7  rrd2rrdr_query_execute (ops=0x51962c0, dim_id_in_rrdr=0, r=0x5196090)
    at /home/thiago/Netdata/tests_netdata/src/web/api/queries/query.c:1662
#8  rrd2rrdr (owa=owa@entry=0x5196050, qt=qt@entry=0x5197060)
    at /home/thiago/Netdata/tests_netdata/src/web/api/queries/query.c:3532
#9  0x00000000004a83ba in rrd2rrdr_legacy (owa=owa@entry=0x5196050, st=st@entry=0x2eb7220, 
    points=points@entry=1, after=after@entry=-60, before=before@entry=0, 
    group_method=group_method@entry=RRDR_GROUPING_SUM, resampling_time=0, options=16809984, 
    dimensions=0x1927f08 "unmatched", group_options=0x0, timeout_ms=0, tier=0, 
    query_source=QUERY_SOURCE_HEALTH, priority=STORAGE_PRIORITY_SYNCHRONOUS)
    at /home/thiago/Netdata/tests_netdata/src/web/api/queries/query.c:3429
#10 0x00000000004bc459 in rrdset2value_api_v1 (st=0x2eb7220, wb=wb@entry=0x0, n=n@entry=0x25a0d00, 
    dimensions=0x1927f08 "unmatched", points=points@entry=1, after=after@entry=-60, before=0, 
    group_method=RRDR_GROUPING_SUM, group_options=0x0, resampling_time=0, options=16809984, 
--Type <RET> for more, q to quit, c to continue without paging--
    db_after=0x25a0d48, db_before=0x25a0d50, db_points_read=0x0, db_points_per_tier=0x0, 
    result_points_generated=0x0, value_is_null=0x7fffe1bdcb6c, anomaly_rate=0x0, timeout=0, tier=0, 
    query_source=QUERY_SOURCE_HEALTH, priority=STORAGE_PRIORITY_SYNCHRONOUS)
    at /home/thiago/Netdata/tests_netdata/src/web/api/formatters/rrd2json.c:80
#11 0x00000000004fafc7 in health_event_loop ()
    at /home/thiago/Netdata/tests_netdata/src/health/health_event_loop.c:366
#12 0x00000000004fc733 in health_main (ptr=0xdf6130)
    at /home/thiago/Netdata/tests_netdata/src/health/health_event_loop.c:768
#13 0x000000000085793d in netdata_thread_init (ptr=0x1827f70)
    at /home/thiago/Netdata/tests_netdata/src/libnetdata/threads/threads.c:285
#14 0x00007ffff6c94d57 in start_thread () from /lib64/libc.so.6
#15 0x00007ffff6d196a8 in clone3 () from /lib64/libc.so.6

I do not have these issues with our current master branch.

Best regards!

[vkalintiris]

Thanks for reporting the issue @thiagoftsm! The latest commit should fix the error you encountered.

[thiagoftsm]

Thanks for reporting the issue @thiagoftsm! The latest commit should fix the error you encountered.

After last commit, everything is working as expected, I am going to test streaming during a few hours to be sure I did not miss anything. 🤝

[thiagoftsm]

After last commit, everything is working as expected; LGTM!