statistics: avoid large CMSketch affecting the latency of normal query (#17542) #17543
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cherry-pick #17542 to release-3.0
What problem does this PR solve?
Problem Summary:
We have captured a runtime trace of tidb-server and observed a large latency caused by GC sweep.
The root cause is the large amount 8K span referenced by
CMSketchand there is a limitation of Go's allocator. In this case, 8K allocation issued byIndexLookUpExecwill take a long time to scan memory spans used byCMSketch.What is changed and how it works?
Memory allocation larger than 32K will never use memory span. The default configuration of
CMSketchwill use 40K memory, so use a single slice is a workaround.This problem can be solved in Go 1.15, so this simple workaround is a good choice before Go 1.15 release.
This test is using v3.0.14 TiDB compiled with Go 1.13.10, but I believe v4.0 and Go 1.14 have the same problem.
Release note