Issue #14718 has been updated by davidtgoldblatt (David Goldblatt).


I don't think this benchmark is a useful way to compare performance between versions 3 and 5 of jemalloc. In between them was the advent of time-based purging, where the allocator waits for a while (by default, around 10 seconds) before returning memory to the OS. That purging occurs (well, in the default but not recommended case where background threads are disabled) on an operation counter tick. By having low activity for a short program lifetime, it effectively disables returning memory to the OS, in a way that wouldn't actually persist if you were running a more realistic program. Even outside of that, we've started using MADV_FREE, which gives the kernel the right to reclaim memory, but in such a way that it will only do so if the machine is actually under memory pressure; this won't show up in test runs unless you're trying to make it happen.

You could verify this by setting dirty decay and muzzy decay to 0 in the MALLOC_CONF environment variable (i.e. MALLOC_CONF="dirty_decay_ms:0,muzzy_decay_ms:0", unless I've typoed something).

In general, malloc benchmarking is notoriously difficult to do usefully (outside of ruling out certain kinds of pathological behavior, such as in an internal allocator test suite). Better would be some representative suite of allocation-heavy test programs that accomplish some real task.

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Feature #14718: Use jemalloc by default?
https://bugs.ruby-lang.org/issues/14718#change-73236

* Author: mperham (Mike Perham)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
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I know Sam opened #9113 4 years ago to suggest this but I'm revisiting the topic to see if there's any movement here for Ruby 2.6 or 2.7.  I supply a major piece of Ruby infrastructure (Sidekiq) and I keep hearing over and over how Ruby is terrible with memory, a huge memory hog with their Rails apps.  My users switch to jemalloc and a miracle occurs: their memory usage drops massively.  Some data points:

https://twitter.com/brandonhilkert/status/987400365627801601
https://twitter.com/d_jones/status/989866391787335680
https://github.com/mperham/sidekiq/issues/3824#issuecomment-383072469

Redis moved to jemalloc many years ago and it solved all of their memory issues too.  Their conclusion: the glibc allocator "sucks really really hard". http://oldblog.antirez.com/post/everything-about-redis-24.html

This is a real pain point for the entire Rails community and would improve Ruby's reputation immensely if we can solve this problem.

---Files--------------------------------
glibc_arena_2.png (7.23 KB)
jemalloc.png (21.1 KB)
glibc-arena-2.log (60.3 KB)
glibc.log (62.3 KB)
jemalloc-3-5.log (58.5 KB)
glibc.png (9.03 KB)


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