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.
