Bug #18599: `vm->loaded_features_snapshot` became very inefficient in Ruby 3.1

• Status: Closed
• Assignee: -
• Target version: -
• ruby -v: ruby 3.1.1p18 (2022-02-18 revision 53f5fc4236) [x86_64-darwin20]
• Backport: 2.6: WONTFIX, 2.7: REQUIRED, 3.0: REQUIRED, 3.1: DONE

[byroot (Jean Boussier)]

A change in how shared arrays behave has caused a fairly major performance regression in Kernel.require. In short on every single call, a full copy of $LOADED_FEATURES is made.

In our app this causes over 6GiB worth of useless allocations.

Reproduction

# frozen_string_literal: true
require "objspace"
files = 5.times.map { |i| "/tmp/foo-#{i}.rb" }
files.each { |f| File.write(f, "") }

require files.shift
4.times { GC.start }
generation = GC.count

ObjectSpace.trace_object_allocations do
  files.each { |f| require f }
end

ObjectSpace.dump_all(output: File.open("/tmp/allocated.heap", "w+"), since: generation)
ObjectSpace.dump_all(output: File.open("/tmp/full.heap", "w+"))

If you inspect the allocated.heap, you'll notice 4 array, each increasingly bigger.

$ grep ARRAY /tmp/allocated.heap 
{"address":"0x10fe2b3b8", "type":"ARRAY", "frozen":true, "length":92, "references":["0x10fcd3da8", "0x10fcca2a8", "0x10fcc21c0", ...], "file":"<internal:/opt/rubies/3.1.0/lib/ruby/3.1.0/rubygems/core_ext/kernel_require.rb>", "line":85, "method":"require", "generation":13, "memsize":776, "flags":{"wb_protected":true}}
...
{"address":"0x10fe2b688", "type":"ARRAY", "frozen":true, "length":91, "references":["0x10fcd3da8", "0x10fcca2a8", "0x10fcc21c0", ...], "file":"<internal:/opt/rubies/3.1.0/lib/ruby/3.1.0/rubygems/core_ext/kernel_require.rb>", "line":85, "method":"require", "generation":13, "memsize":768, "flags":{"wb_protected":true}}
...
{"address":"0x10fe2b958", "type":"ARRAY", "frozen":true, "length":89, "references":["0x10fcd3da8", "0x10fcca2a8", "0x10fcc21c0", ...], "file":"<internal:/opt/rubies/3.1.0/lib/ruby/3.1.0/rubygems/core_ext/kernel_require.rb>", "line":85, "method":"require", "generation":13, "memsize":752, "flags":{"wb_protected":true}}
...
{"address":"0x10fe2bc28", "type":"ARRAY", "frozen":true, "length":88, "references":["0x10fcd3da8", "0x10fcca2a8", "0x10fcc21c0", ...], "file":"<internal:/opt/rubies/3.1.0/lib/ruby/3.1.0/rubygems/core_ext/kernel_require.rb>", "line":85, "method":"require", "generation":13, "memsize":744, "flags":{"wb_protected":true}}
...

And if you lookup the addresses in full.heap, it makes no doubt that it's $LOADED_FEATURES.

Note however how the array is "frozen":true, so it's not $LOADED_FEATURES directly, but the shared array that backs vm->loaded_features and vm->loaded_features_snapshot.

More scoped reproduction

If we scope on shared arrays behavior specifically, we can get a much smaller reproduction:

array = 10.times.to_a
copy = array.dup
before = GC.stat(:total_allocated_objects)
copy << 1
p GC.stat(:total_allocated_objects) - before # 1 on Ruby 3.1, 0 on 3.0 and older

In short, back in 3.0 and older, appending to a shared array wouldn't "unshare" the array, but starting in 3.1 it now does.
The problem being that Kernel.require heavily assumed this implementation detail:

static void
reset_loaded_features_snapshot(rb_vm_t *vm)
{
    rb_ary_replace(vm->loaded_features_snapshot, vm->loaded_features);
}

rb_provide_feature(rb_vm_t *vm, VALUE feature)
{
    //...
    get_loaded_features_index(vm);
    rb_ary_push(features, rb_fstring(feature));
    features_index_add(vm, feature, INT2FIX(RARRAY_LEN(features)-1));
    reset_loaded_features_snapshot(vm);
}

rb_provide_feature first append to $LOADED_FEATURES and then resets vm->loaded_features_snapshot to be a shared copy of $LOADED_FEATURES.

[byroot (Jean Boussier)]

• Backport changed from 2.6: DONTNEED, 2.7: DONTNEED, 3.0: DONTNEED, 3.1: UNKNOWN to 2.6: DONTNEED, 2.7: UNKNOWN, 3.0: UNKNOWN, 3.1: UNKNOWN

Ok, so my assessment about shared arrays wasn't quite correct. What caused the difference between 3.0 and 3.1 here is http://github.com/ruby/ruby/commit/aeae6e2842e1702dfb89b8ae69b48c4f5f64c662

Previously the shared array would be force recycled immediately, which was hiding the problem, but it was already there.

With the help of @peterzhu2118 (Peter Zhu), we figured a fairly simple patch for this: https://github.com/ruby/ruby/pull/5593

[byroot (Jean Boussier)]

I tested the patch against our app, it does solve the problem perfectly, and the time spent in GC during boot is noticeably reduced (~40% -> ~28%).

[byroot (Jean Boussier)]

• Status changed from Open to Closed

Applied in changeset git|b8f0dc59d52266d9fbfc039e2f4b0f727c62baa0.

---

rb_provide_feature: Prevent $LOADED_FEATURES from being copied

[Bug #18599]

vm->loaded_features and vm->loaded_features_snapshot both share the
same root. When a feature is pushed into loaded_features, the sharing
is broken and loaded_features is copied.

So an application requiring 1000 files, will allocate 1000 arrays of
increasing size, which is very wasteful.

To avoid this, we first clear the snapshot, so that loaded_features
can directly be pushed into.

Co-Authored-By: Peter Zhu [e-mail]

[byroot (Jean Boussier)]

• Backport changed from 2.6: DONTNEED, 2.7: UNKNOWN, 3.0: UNKNOWN, 3.1: UNKNOWN to 2.6: DONTNEED, 2.7: REQUIRED, 3.0: REQUIRED, 3.1: REQUIRED

[byroot (Jean Boussier)]

• Backport changed from 2.6: DONTNEED, 2.7: REQUIRED, 3.0: REQUIRED, 3.1: REQUIRED to 2.6: WONTFIX, 2.7: REQUIRED, 3.0: REQUIRED, 3.1: REQUIRED

[thomthom (Thomas Thomassen)]

Is this targeted to be ported to Ruby 3.1? I see it's not included in 3.1.2 nor in the ruby_3_1 branch.

[byroot (Jean Boussier)]

Is this targeted to be ported to Ruby 3.1?

Yes.

[nagachika (Tomoyuki Chikanaga)]

• Backport changed from 2.6: WONTFIX, 2.7: REQUIRED, 3.0: REQUIRED, 3.1: REQUIRED to 2.6: WONTFIX, 2.7: REQUIRED, 3.0: REQUIRED, 3.1: DONE

ruby_3_1 012015d762d000966a33fcea5f3069decd9d4007 merged revision(s) b8f0dc59d52266d9fbfc039e2f4b0f727c62baa0.