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Feature #19435

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Expose counts for each GC reason in GC.stat

Added by byroot (Jean Boussier) almost 2 years ago. Updated over 1 year ago.

Status:
Open
Assignee:
-
Target version:
-
[ruby-core:112398]

Description

Context

We recently tuned the GC settings on our monolith application because we were seeing some very long GC pauses (multiple seconds) during some requests.

Very early we realized that we could know how often the GC was triggered, and how long it was taking, but we had no information as to why, hence no good way
to know which specific configuration to tune. As of today, the only way to get this information is to compile Ruby with debug counters, but that's not really
accessible for most users, and not very suitable to be deployed in production.

So we patched our Ruby to expose counters for each specific reason in GC.stat and this data was extremely valuable.

For instance we discovered that the number 1 cause of major GC was shady objects, which allowed us to both better tune or GC and to drive some
targeted patches to Ruby.

Proposal

We'd like to merge the patch we used on our Ruby build. It expose 8 new keys in GC.stat:

  • :major_gc_nofree_count
  • :major_gc_oldgen_count
  • :major_gc_shady_count
  • :major_gc_newobj_count
  • :major_gc_malloc_count
  • :major_gc_oldmalloc_count
  • :minor_gc_newobj_count
  • :minor_gc_malloc_count

Some very uncommon reasons like force etc are ignored as they're not valuable.

Also note that sometimes multiple conditions can be met to trigger GC, in such case we my increment several counters, so the sum of major_gc_*_count can be higher than major_gc_count.

Proposed patch: https://github.com/ruby/ruby/pull/7250

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