"This patchset allows Scylla to determine the size of a memtable instead
of relying in the user-provided memtable_cleanup_threshold. It does that
by allowing the region_group to specify a soft limit which will trigger
the allocation as early as it is reached.
Given that, we'll keep the memtables in memory for as long as it takes
to reach that limit, regardless of the individual size of any single one
of them. That limit is set to 1/4 of dirty memory. That's the same as
last submission, except this time I have run some experiments to gauge
behavior of that versus 1/2 of dirty memory, which was a preferred
theoretical value.
After that is done, the flush logic is reworked to guarantee that
flushes are not initiated if we already have one memtable under flush.
That allow us to better take advantage of coalescing opportunities with
new requests and prevents the pending memtable explosion that is
ultimately responsible for Issue 1817.
I have run mainly two workloads with this. The first one a local RF=1
workload with large partitions, sized 128kB and 100 threads. The results
are:
Before:
op rate : 632 [WRITE:632]
partition rate : 632 [WRITE:632]
row rate : 632 [WRITE:632]
latency mean : 157.8 [WRITE:157.8]
latency median : 115.5 [WRITE:115.5]
latency 95th percentile : 486.7 [WRITE:486.7]
latency 99th percentile : 534.8 [WRITE:534.8]
latency 99.9th percentile : 599.0 [WRITE:599.0]
latency max : 722.6 [WRITE:722.6]
Total partitions : 189667 [WRITE:189667]
Total errors : 0 [WRITE:0]
total gc count : 0
total gc mb : 0
total gc time (s) : 0
avg gc time(ms) : NaN
stdev gc time(ms) : 0
Total operation time : 00:05:00
END
After:
op rate : 951 [WRITE:951]
partition rate : 951 [WRITE:951]
row rate : 951 [WRITE:951]
latency mean : 104.8 [WRITE:104.8]
latency median : 102.5 [WRITE:102.5]
latency 95th percentile : 155.8 [WRITE:155.8]
latency 99th percentile : 177.8 [WRITE:177.8]
latency 99.9th percentile : 686.4 [WRITE:686.4]
latency max : 1081.4 [WRITE:1081.4]
Total partitions : 285324 [WRITE:285324]
Total errors : 0 [WRITE:0]
total gc count : 0
total gc mb : 0
total gc time (s) : 0
avg gc time(ms) : NaN
stdev gc time(ms) : 0
Total operation time : 00:05:00
END
The other workload was the workload described in #1817. And the result
is that we now have a load that is very stable around 100k ops/s and
hardly any timeouts, instead of the 1.4 baseline of wild variations
around 100k ops/s and lots of timeouts, or the deep reduction of
1.5-rc1."
* 'issue-1817-v4' of github.com:glommer/scylla:
database: rework memtable flush logic
get rid of max_memtable_size
pass a region to dirty_memory_manager accounting API
memtable: add a method to expose the region_group
logalloc: allow region group reclaimer to specify a soft limit
database: remove outdated comment
database: uphold virtual dirty for system tables.
(cherry picked from commit 5d067eebf2)
affc0d9138