Introduce malloc_trim(0) in full gc cycles
Per Hongli's excellent article it looks like malloc_trim can help tremendously with memory bloat issues.
I would like to get this patch tested side-by-side at Discourse, GitHub and Shopify. If it looks good I think this is both a great candidate for 2.7 and and 2.4,2.5,2.6 backports.
Will coordinate with Shopify and GitHub to see if we can get numbers posted here, I will run tests on a live Discourse instance over the next week and report numbers here.
Koichi, what are your thoughts, to me this looks like an incredibly safe patch, the amount of work added to major GCs is tiny compared to the potential benefit, walking all pages is a very cheap operation.
Updated by sam.saffron (Sam Saffron) about 1 month ago
Updated by sam.saffron (Sam Saffron) 24 days ago
- File Screenshot_2019-03-28 Grafana - Compare Discourse Perf.png Screenshot_2019-03-28 Grafana - Compare Discourse Perf.png added
containers run multiple ruby processes (a few unicorn workers and a sidekiq worker)
standard14 = Ruby 2.6.2 + jemalloc
standard14_a = Ruby 2.6.2 + glibc malloc
standard14_b = Ruby 2.6.2 + glibc malloc + patch
My conclusions from these graphs:
- Memory is clearly down with the patch
- 99th percentile performance is slightly impacted
- cpu is very slightly higher
- jemalloc still fairs better than glibc even after the patch
I think I would support a slightly amended patch that only does the trim once every say 10 minutes (maybe even in a background thread), happy to test that out as well.
That said... selfishly for Discourse this does not matter that much we will still stick with jemalloc cause memory is better and performance is better under jemalloc.
For the wider ruby community though a safer default is very appealing.
Updated by bluz71 (Dennis B) 23 days ago
Thanks Sam, a very nice set of results.
Notice that 99th percentile Topic list was faster with the patch, whilst slower with Topic view. So I'm not sure we can say that the patch will always be slower on the worst runs.
Query, what is the version of jemalloc that you are using? One of the interesting observations in #14759 is the variance between jemalloc versions (say 3.6.0 vs 5.1.0).
Updated by tessi (Philipp Tessenow) 21 days ago
FYI: For easier testing this idea, I just pushed a small gem to rubygems
malloc_trim (https://github.com/tessi/malloc_trim). This gives access to
malloc_trim to ruby land to let us play with it without the need to re-compile ruby and/or deploying a custom patched ruby.
MallocTrim.enable_trimming, there is also a built in way to run
malloc_trim(0) after every GC MARK (the most relevant internal event I found to hook into). It probably calls malloc_trim somewhat too often -- I'd be happy for suggestions to find a better hook.
In any way, doing a manual
MallocTrim.trim after a
GC.start (e.g. between two rails requests) is still possible with this gem.
Updated by sam.saffron (Sam Saffron) 20 days ago
My tests were with 3.6.0, I will do a side by side now that I have all the infrastructure of 5 vs 3.6 and even tcmalloc.
Nice to see that gem! Looking at my graphs I think best bang would just be to spin a thread that does trimming every 10-30 minutes or something, especially if you can release the GVL prior to calling it (provided this thing is thread safe, which carlos (Carlos Sánchez) should know)