PS-11161 [8.4]: mem_root_deque performance optimizations (part 1).#5950
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Reduce target size of mem_root_deque block to 256 bytes from 1Kb. In the majority of cases mem_root_deque is used to store pointers (e.g. to Item objects), hence the 1kB block can store up to 128 elements for 64-bit arch. OTOH in many cases the number of elements which is really used in mem_root_deque is much lower. For example, for many queries number of fields in SELECT list is far smaller than 64. This means that in many cases the bigger part of such a 1kB block is just wasted. Taking into account that there are 5-10 of mem_root_deque instances even for fairly simple queries the effect of such waste becomes more pronounced - allocating these extra unnecessary 5-10Kb on main MEM_ROOT can trigger it to request another block from malloc(). And the latter has small but visible impact on performance in some of sysbench tests.
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Here are Jenkins results for this patch: https://ps80.cd.percona.com/view/8.0%20+%208.4%20parallel%20MTR/job/percona-server-8.x-pipeline-parallel-mtr/1136/#showFailuresLink - they look OK to me as these failures also present in other runs for pristine release-8.4.9-9 tree. |
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Reduce target size of mem_root_deque block to 256 bytes from 1Kb.
In the majority of cases mem_root_deque is used to store pointers (e.g. to Item objects), hence the 1kB block can store up to 128 elements for 64-bit arch.
OTOH in many cases the number of elements which is really used in mem_root_deque is much lower. For example, for many queries number of fields in SELECT list is far smaller than 64.
This means that in many cases the bigger part of such a 1kB block is just wasted. Taking into account that there are 5-10 of mem_root_deque instances even for fairly simple queries the effect of such waste becomes more pronounced - allocating these extra unnecessary 5-10Kb on main MEM_ROOT can trigger it to request another block from malloc(). And the latter has small but visible impact on performance in some of sysbench tests.