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Speed ISeq marking by using a bitmap and rearranging inline caches #6053

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merged 2 commits into from Jun 23, 2022

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@tenderlove tenderlove commented Jun 22, 2022

A large percentage of major GC time is spent marking instruction sequence objects. This PR aims to speed up major GC by speeding up marking instruction sequence objects.

Marking ISeq objects

Today we have to disassemble instruction sequences in order to mark them. The disassembly process looks for GC allocated objects and marks them. To disassemble an iseq, we have to iterate over each instruction, convert the instruction from an address back to the original op code integer, then look up the parameters for the op code. Once we know the parameter types, we can iterate though them and mark "interesting" references. We can see this process in the iseq_extract_values function.

According to profile results, the biggest bottleneck in this function is converting addresses back to instruction ids.

Speeding up ISeq marking

To speed up ISeq marking, this PR introduces two changes. The first change is adding a bitmap, and the second change is rearranging inline caches to be more "convenient".

Bitmaps

At compilation time, we allocate a bitmap along side of the iseq object. The bitmap indicates offsets of VALUE objects inside the instruction sequences. When marking an instruction, we can simply iterate over the bitmap to find VALUE objects that need to be marked.

Inline Cache Rearrangement

Inline cache types IC, IVC, ICVARC, and ISE are allocated from a buffer that is stored on the iseq constant body. These caches are a union type. Unfortunately, these union types don't have a "type" field, so they can only be distinguished by looking at the parameter types of an instruction.

Take the following Ruby code for example:

Foo =~ /#{foo}/o;

The instruction sequences for this code are as follows:

== disasm: #<ISeq:<main>@-e:1 (1,0)-(1,17)> (catch: FALSE)
0000 opt_getinlinecache                     9, <is:0>                 (   1)[Li]
0003 putobject                              true
0005 getconstant                            :Foo
0007 opt_setinlinecache                     <is:0>
0009 once                                   block in <main>, <is:1>
0012 opt_regexpmatch2                       <calldata!mid:=~, argc:1, ARGS_SIMPLE>[CcCr]
0014 leave

The ISeq object contains two entries in the is_entries buffer, one for the ISE cache associated with the once instruction, and one for the IC cache associated with the opt_getinlinecache and opt_setinlinecache instructions.

Unfortunately we cannot iterate through the caches in the is_entries list because the union types don't have the same layout. Marking an ISE is very different than marking an IC, and we can only differentiate them by disassembling and checking the instruction sequences themselves.

To solve this problem, this PR introduces 3 counters for the different types of inline caches. Then, we group inline cache types within the is_entries buffer.
Since the inline cache types are grouped, we can use the counters to iterate over the buffer and we know what type is being used.

Combining bitmap marking and inline cache arrangement means that we can mark instruction sequences without disassembling the instructions.

Speed impact

I benchmarked this change with a basic Rails application using the following script:

puts RUBY_DESCRIPTION

require "benchmark/ips"

Benchmark.ips do |x|
  x.report("major GC") { GC.start }
end

Here are the results with the master version of Ruby:

$ RAILS_ENV=production gel exec bin/rails r test.rb
ruby 3.2.0dev (2022-06-22T12:30:39Z master 744d17ff6c) [arm64-darwin21]
Warming up --------------------------------------
            major GC     4.000  i/100ms
Calculating -------------------------------------
            major GC     47.748  (± 2.1%) i/s -    240.000  in   5.028520s

Here it is with these patches applied:

$ RAILS_ENV=production gel exec bin/rails r test.rb
ruby 3.2.0dev (2022-06-22T20:52:13Z iseq-bitmap 2ba736a7f9) [arm64-darwin21]
Warming up --------------------------------------
            major GC     7.000  i/100ms
Calculating -------------------------------------
            major GC     77.208  (± 1.3%) i/s -    392.000  in   5.079023s

With these patches applied, major GC is about 60% faster.

Memory impact

The memory increase is proportional to the number of instructions stored on an iseq. This works about to be about 1% increase in the size of an iseq (ceil(iseq_length / 64) on 64 bit platforms).

Future work

This PR always mallocs a bitmap table. We can eliminate the malloc when:

  1. There is nothing to mark
  2. iseq_length is <= 64

We may also want to consider using a succ_index_table for storing the bitmap

Redmine issue is here

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@tomascco tomascco left a comment

Thanks for the great work! I love your contributions 😄. Unfortunately the only part of the PR that I can understand are the comments. 😳 (I'm not experienced in C at all)

vm_core.h Outdated Show resolved Hide resolved
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while(bits) {
if (bits & 0x1) {
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@casperisfine casperisfine Jun 23, 2022

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Wouldn't it be faster to use ffs here? https://man7.org/linux/man-pages/man3/ffs.3.html Assuming the bitmap is relatively sparse you'd save quite a few iterations. Or am I missing something?

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@tenderlove tenderlove Jun 23, 2022

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Ya it should be pretty sparse. I'll try it with ffs and see what the numbers look like.

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@tenderlove tenderlove Jun 23, 2022

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I tried using ffs but it's turning out to be a pain because you can't right shift by 64. So we'd need to add a special case if just the top bit is set. I think we should investigate using ffs, but I want to merge this as-is for now.

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@casperisfine casperisfine Jun 23, 2022

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Hum, not sure I follow what the problem is. If only the first bit is set, ffs (or ffl more likely) return 64, but the shift should always be bits >>= ffs(bits) - 1 (unless ffs returns 0).

Anyway no big deal. I'd like to try backporting this patch on top of 3.1 though.

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@Maumagnaguagno Maumagnaguagno Jun 23, 2022

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This post about iterating over set bits quickly could be useful here, it uses __builtin_ctzl.

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@tenderlove tenderlove Jun 24, 2022

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Hum, not sure I follow what the problem is. If only the first bit is set, ffs (or ffl more likely) return 64, but the shift should always be bits >>= ffs(bits) - 1 (unless ffs returns 0).

ffs returns the nth bit and starts at a 1 index. So ffs(0x1) => 1. If we did bits >>= ffs(bits) - 1, then it would never shift anything in the case of 0x1, so we have to do bits >>= ffs(bits). ffs of 0x8000000000000000 returns 64, so the right shift doesn't work.

We can probably use __builtin_ctzl, but I think we should see if this iteration is really a bottleneck at the moment.

tenderlove and others added 2 commits Jun 23, 2022
This commit adds a bitfield to the iseq body that stores offsets inside
the iseq buffer that contain values we need to mark.  We can use this
bitfield to mark objects instead of disassembling the instructions.

This commit also groups inline storage entries and adds a counter for
each entry.  This allows us to iterate and mark each entry without
disassembling instructions

Since we have a bitfield and grouped inline caches, we can mark all
VALUE objects associated with instructions without actually
disassembling the instructions at mark time.

[Feature #18875] [ruby-core:109042]
Co-authored-by: Tomás Coêlho <36938811+tomascco@users.noreply.github.com>
@tenderlove tenderlove merged commit 1ccdb1a into ruby:master Jun 23, 2022
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@tenderlove tenderlove deleted the iseq-bitmap branch Jun 23, 2022
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