Background:
Following the upgrade to jemalloc 5.2, there was a test that used to be flaky and
started failing consistently (on 32bit), so we disabled it (see #9645).
This is a test that i introduced in #7289 when i attempted to solve a rare stagnation
problem, and it later turned out i failed to solve it, ans what's more i added a test that
caused it to be not so rare, and as i mentioned, now in jemalloc 5.2 it became consistent on 32bit.
Stagnation can happen when all the slabs of the bin are equally utilized, so the decision
to move an allocation from a relatively empty slab to a relatively full one, will never
happen, and in that test all the slabs are at 50% utilization, so the defragger could just
keep scanning the keyspace and not move anything.
What this PR changes:
* First, finally in jemalloc 5.2 we have the count of non-full slabs, so when we compare
the utilization of the current slab, we can compare it to the average utilization of the non-full
slabs in our bin, instead of the total average of our bin. this takes the full slabs out of the game,
since they're not candidates for migration (neither source nor target).
* Secondly, We add some 12% (100/8) to the decision to defrag an allocation, this is the part
that aims to avoid stagnation, and it's especially important since the above mentioned change
can get us closer to stagnation.
* Thirdly, since jemalloc 5.2 adds sharded bins, we take into account all shards (something
that's missing from the original PR that merged it), this isn't expected to make any difference
since anyway there should be just one shard.
How this was benchmarked.
What i did was run the memefficiency test unit with `--verbose` and compare the defragger hits
and misses the tests reported.
At first, when i took into consideration only the non-full slabs, it got a lot worse (i got into
stagnation, or just got a lot of misses and a lot of hits), but when i added the 10% i got back
to results that were slightly better than the ones of the jemalloc 5.1 branch. i.e. full defragmentation
was achieved with fewer hits (relocations), and fewer misses (keyspace scans).
There's a rare case which leads to stagnation in the defragger, causing
it to keep scanning the keyspace and do nothing (not moving any
allocation), this happens when all the allocator slabs of a certain bin
have the same % utilization, but the slab from which new allocations are
made have a lower utilization.
this commit fixes it by removing the current slab from the overall
average utilization of the bin, and also eliminate any precision loss in
the utilization calculation and move the decision about the defrag to
reside inside jemalloc.
and also add a test that consistently reproduce this issue.
There's a rare case which leads to stagnation in the defragger, causing
it to keep scanning the keyspace and do nothing (not moving any
allocation), this happens when all the allocator slabs of a certain bin
have the same % utilization, but the slab from which new allocations are
made have a lower utilization.
this commit fixes it by removing the current slab from the overall
average utilization of the bin, and also eliminate any precision loss in
the utilization calculation and move the decision about the defrag to
reside inside jemalloc.
and also add a test that consistently reproduce this issue.
The original jemalloc source tree was modified to:
1. Remove the configure error that prevents nested builds.
2. Insert the Redis private Jemalloc API in order to allow the
Redis fragmentation function to work.
This gives us a 24 bytes size class which is dict.c dictEntry size, thus
improving the memory efficiency of Redis significantly.
Moreover other non 16 bytes aligned tiny classes are added that further
reduce the fragmentation of the allocator.
Technically speaking LG_QUANTUM should be 4 on i386 / AMD64 because of
SSE types and other 16 bytes types, however we don't use those, and our
jemalloc only targets Redis.
New versions of Jemalloc will have an explicit configure switch in order
to specify the quantum value for a platform without requiring any change
to the Jemalloc source code: we'll switch to this system when available.
This change was originally proposed by Oran Agra (@oranagra) as a change
to the Jemalloc script to generate the size classes define. We ended
doing it differently by changing LG_QUANTUM since it is apparently the
supported Jemalloc method to obtain a 24 bytes size class, moreover it
also provides us other potentially useful size classes.
Related to issue #2510.
