ruby.git/gc, branch v4.0.4 The Ruby Programming Language merge revision(s) 8a586af33b59cae93a1bee13c39e87dd087a4a6b: [Backport #21838] 2026-02-09T23:39:41+00:00 Takashi Kokubun takashikkbn@gmail.com 2026-02-09T23:39:41+00:00 d5d3fb8b80e81426517bf150ebcab83be4ed996d [PATCH] Don't force major GC when there are allocatable slots [Bug #21838] When we have allocatable slots, we can grow the heap instead of forcing a major GC. This prevents major GC to be ran very often in certain situations. See the ticket for more details. On ruby-bench, we can see that this patch doesn't cause any significant regressions: -------------- ----------- ---------- --------- ----------- ---------- --------- -------------- ------------- bench master (ms) stddev (%) RSS (MiB) branch (ms) stddev (%) RSS (MiB) branch 1st itr master/branch activerecord 148.2 0.3 59.2 150.0 0.8 69.7 1.015 0.988 chunky-png 435.2 0.3 72.9 438.8 0.1 66.7 0.993 0.992 erubi-rails 733.8 1.2 118.7 704.8 0.2 98.3 1.077 1.041 hexapdf 1400.4 1.1 247.0 1405.0 0.9 223.7 0.986 0.997 liquid-c 32.5 3.3 32.8 32.5 2.1 30.7 1.042 0.999 liquid-compile 31.0 1.7 35.1 33.4 3.9 32.8 0.938 0.928 liquid-render 84.7 0.4 30.8 86.3 0.4 30.8 0.981 0.982 lobsters 594.7 0.6 310.5 596.6 0.4 306.0 1.057 0.997 mail 75.6 2.8 53.3 76.9 0.7 53.2 0.968 0.982 psych-load 1122.8 1.2 29.2 1145.1 0.4 31.7 0.964 0.981 railsbench 1244.7 0.3 115.5 1254.8 1.1 115.2 0.939 0.992 rubocop 103.7 0.5 94.1 104.3 0.5 92.4 0.985 0.994 ruby-lsp 88.3 0.6 78.5 88.5 1.2 77.9 0.992 0.997 sequel 26.9 0.9 33.6 28.3 1.4 32.1 0.954 0.952 shipit 1119.3 1.5 171.4 1075.7 2.1 162.5 1.873 1.040 -------------- ----------- ---------- --------- ----------- ---------- --------- -------------- ------------- 
	[PATCH] Don't force major GC when there are allocatable slots

	[Bug #21838]

	When we have allocatable slots, we can grow the heap instead of forcing
	a major GC. This prevents major GC to be ran very often in certain situations.
	See the ticket for more details.

	On ruby-bench, we can see that this patch doesn't cause any significant
	regressions:

	    --------------  -----------  ----------  ---------  -----------  ----------  ---------  --------------  -------------
	    bench           master (ms)  stddev (%)  RSS (MiB)  branch (ms)  stddev (%)  RSS (MiB)  branch 1st itr  master/branch
	    activerecord    148.2        0.3         59.2       150.0        0.8         69.7       1.015           0.988
	    chunky-png      435.2        0.3         72.9       438.8        0.1         66.7       0.993           0.992
	    erubi-rails     733.8        1.2         118.7      704.8        0.2         98.3       1.077           1.041
	    hexapdf         1400.4       1.1         247.0      1405.0       0.9         223.7      0.986           0.997
	    liquid-c        32.5         3.3         32.8       32.5         2.1         30.7       1.042           0.999
	    liquid-compile  31.0         1.7         35.1       33.4         3.9         32.8       0.938           0.928
	    liquid-render   84.7         0.4         30.8       86.3         0.4         30.8       0.981           0.982
	    lobsters        594.7        0.6         310.5      596.6        0.4         306.0      1.057           0.997
	    mail            75.6         2.8         53.3       76.9         0.7         53.2       0.968           0.982
	    psych-load      1122.8       1.2         29.2       1145.1       0.4         31.7       0.964           0.981
	    railsbench      1244.7       0.3         115.5      1254.8       1.1         115.2      0.939           0.992
	    rubocop         103.7        0.5         94.1       104.3        0.5         92.4       0.985           0.994
	    ruby-lsp        88.3         0.6         78.5       88.5         1.2         77.9       0.992           0.997
	    sequel          26.9         0.9         33.6       28.3         1.4         32.1       0.954           0.952
	    shipit          1119.3       1.5         171.4      1075.7       2.1         162.5      1.873           1.040
	    --------------  -----------  ----------  ---------  -----------  ----------  ---------  --------------  -------------
Move special const check to gc.c for rb_gc_impl_object_moved_p 2025-12-23T18:54:08+00:00 Peter Zhu peter@peterzhu.ca 2025-12-23T17:59:41+00:00 e2cf92eddc5403316b0d449b02ba403a27610d7b 






[ruby/mmtk] Implement Ruby heap
2025-12-22T21:54:27+00:00

Peter Zhu
peter@peterzhu.ca

2025-12-22T16:24:08+00:00

af30e4714ccb0dabff85298ee2305d84103b98d1

This heap emulates the growth characteristics of the Ruby default GC's
heap. By default, the heap grows by 40%, requires at least 20% empty
after a GC, and allows at most 65% empty before it shrinks the heap. This
is all configurable via the same environment variables the default GC
uses (`RUBY_GC_HEAP_FREE_SLOTS_GOAL_RATIO`, `RUBY_GC_HEAP_FREE_SLOTS_MIN_RATIO`,
`RUBY_GC_HEAP_FREE_SLOTS_MAX_RATIO`, respectively).

The Ruby heap can be enabled via the `MMTK_HEAP_MODE=ruby` environment
variable.

Compared to the dynamic heap in MMTk (which uses the MemBalancer algorithm),
the Ruby heap allows the heap to grow more generously, which uses a bit
more memory but offers significant performance gains because it runs GC
much less frequently.

We can see in the benchmarks below that this Ruby heap heap gives faster
performance than the dynamic heap in every benchmark, with over 2x faster
in many of them. We see that memory is often around 10-20% higher with
certain outliers that use significantly more memory like hexapdf and
erubi-rails. We can also see that this brings MMTk's Ruby heap much
closer in performance to the default GC.

Ruby heap benchmark results:

    --------------  --------------  ----------  ---------
    bench           ruby heap (ms)  stddev (%)  RSS (MiB)
    activerecord    233.6           10.7        85.9
    chunky-png      457.1           1.1         79.3
    erubi-rails     1148.0          3.8         133.3
    hexapdf         1570.5          2.4         403.0
    liquid-c        42.8            5.3         43.4
    liquid-compile  41.3            7.6         52.6
    liquid-render   102.8           3.8         55.3
    lobsters        651.9           8.0         426.3
    mail            106.4           1.8         67.2
    psych-load      1552.1          0.8         43.4
    railsbench      1707.2          6.0         145.6
    rubocop         127.2           15.3        148.8
    ruby-lsp        136.6           11.7        113.7
    sequel          47.2            5.9         44.4
    shipit          1197.5          3.6         301.0
    --------------  --------------  ----------  ---------

Dynamic heap benchmark results:

    --------------  -----------------  ----------  ---------
    bench           dynamic heap (ms)  stddev (%)  RSS (MiB)
    activerecord    845.3              3.1         76.7
    chunky-png      525.9              0.4         38.9
    erubi-rails     2694.9             3.4         115.8
    hexapdf         2344.8             5.6         164.9
    liquid-c        73.7               5.0         40.5
    liquid-compile  107.1              6.8         40.3
    liquid-render   147.2              1.7         39.5
    lobsters        697.6              4.5         342.0
    mail            224.6              2.1         64.0
    psych-load      4326.7             0.6         37.4
    railsbench      3218.0             5.5         124.7
    rubocop         203.6              6.1         110.9
    ruby-lsp        350.7              3.2         79.0
    sequel          121.8              2.5         39.6
    shipit          1510.1             3.1         220.8
    --------------  -----------------  ----------  ---------

Default GC benchmark results:

    --------------  ---------------  ----------  ---------
    bench           default GC (ms)  stddev (%)  RSS (MiB)
    activerecord    148.4            0.6         67.9
    chunky-png      440.2            0.7         57.0
    erubi-rails     722.7            0.3         97.8
    hexapdf         1466.2           1.7         254.3
    liquid-c        32.5             3.6         42.3
    liquid-compile  31.2             1.9         35.4
    liquid-render   88.3             0.7         30.8
    lobsters        633.6            7.0         305.4
    mail            76.6             1.6         53.2
    psych-load      1166.2           1.3         29.1
    railsbench      1262.9           2.3         114.7
    rubocop         105.6            0.8         95.4
    ruby-lsp        101.6            1.4         75.4
    sequel          27.4             1.2         33.1
    shipit          1083.1           1.5         163.4
    --------------  ---------------  ----------  ---------

https://github.com/ruby/mmtk/commit/c0ca29922d





This heap emulates the growth characteristics of the Ruby default GC's
heap. By default, the heap grows by 40%, requires at least 20% empty
after a GC, and allows at most 65% empty before it shrinks the heap. This
is all configurable via the same environment variables the default GC
uses (`RUBY_GC_HEAP_FREE_SLOTS_GOAL_RATIO`, `RUBY_GC_HEAP_FREE_SLOTS_MIN_RATIO`,
`RUBY_GC_HEAP_FREE_SLOTS_MAX_RATIO`, respectively).

The Ruby heap can be enabled via the `MMTK_HEAP_MODE=ruby` environment
variable.

Compared to the dynamic heap in MMTk (which uses the MemBalancer algorithm),
the Ruby heap allows the heap to grow more generously, which uses a bit
more memory but offers significant performance gains because it runs GC
much less frequently.

We can see in the benchmarks below that this Ruby heap heap gives faster
performance than the dynamic heap in every benchmark, with over 2x faster
in many of them. We see that memory is often around 10-20% higher with
certain outliers that use significantly more memory like hexapdf and
erubi-rails. We can also see that this brings MMTk's Ruby heap much
closer in performance to the default GC.

Ruby heap benchmark results:

    --------------  --------------  ----------  ---------
    bench           ruby heap (ms)  stddev (%)  RSS (MiB)
    activerecord    233.6           10.7        85.9
    chunky-png      457.1           1.1         79.3
    erubi-rails     1148.0          3.8         133.3
    hexapdf         1570.5          2.4         403.0
    liquid-c        42.8            5.3         43.4
    liquid-compile  41.3            7.6         52.6
    liquid-render   102.8           3.8         55.3
    lobsters        651.9           8.0         426.3
    mail            106.4           1.8         67.2
    psych-load      1552.1          0.8         43.4
    railsbench      1707.2          6.0         145.6
    rubocop         127.2           15.3        148.8
    ruby-lsp        136.6           11.7        113.7
    sequel          47.2            5.9         44.4
    shipit          1197.5          3.6         301.0
    --------------  --------------  ----------  ---------

Dynamic heap benchmark results:

    --------------  -----------------  ----------  ---------
    bench           dynamic heap (ms)  stddev (%)  RSS (MiB)
    activerecord    845.3              3.1         76.7
    chunky-png      525.9              0.4         38.9
    erubi-rails     2694.9             3.4         115.8
    hexapdf         2344.8             5.6         164.9
    liquid-c        73.7               5.0         40.5
    liquid-compile  107.1              6.8         40.3
    liquid-render   147.2              1.7         39.5
    lobsters        697.6              4.5         342.0
    mail            224.6              2.1         64.0
    psych-load      4326.7             0.6         37.4
    railsbench      3218.0             5.5         124.7
    rubocop         203.6              6.1         110.9
    ruby-lsp        350.7              3.2         79.0
    sequel          121.8              2.5         39.6
    shipit          1510.1             3.1         220.8
    --------------  -----------------  ----------  ---------

Default GC benchmark results:

    --------------  ---------------  ----------  ---------
    bench           default GC (ms)  stddev (%)  RSS (MiB)
    activerecord    148.4            0.6         67.9
    chunky-png      440.2            0.7         57.0
    erubi-rails     722.7            0.3         97.8
    hexapdf         1466.2           1.7         254.3
    liquid-c        32.5             3.6         42.3
    liquid-compile  31.2             1.9         35.4
    liquid-render   88.3             0.7         30.8
    lobsters        633.6            7.0         305.4
    mail            76.6             1.6         53.2
    psych-load      1166.2           1.3         29.1
    railsbench      1262.9           2.3         114.7
    rubocop         105.6            0.8         95.4
    ruby-lsp        101.6            1.4         75.4
    sequel          27.4             1.2         33.1
    shipit          1083.1           1.5         163.4
    --------------  ---------------  ----------  ---------

https://github.com/ruby/mmtk/commit/c0ca29922d







[ruby/mmtk] Add a 32 byte heap for allocating smaller objects
2025-12-21T13:59:28+00:00

Peter Zhu
peter@peterzhu.ca

2025-12-20T16:42:36+00:00

37b98f0df71e85a831677562dc08f6ce6cdae842

https://github.com/ruby/mmtk/commit/c4cca6c1c3





https://github.com/ruby/mmtk/commit/c4cca6c1c3







[ruby/mmtk] Implement fast path for bump pointer allocator
2025-12-21T02:55:12+00:00

Peter Zhu
peter@peterzhu.ca

2025-12-20T21:08:01+00:00

038b158fa30d1b1cda153aeb10dac5f59f966036

Adding a fast path for bump pointer allocator can improve allocation
performance.

For the following microbenchmark with MMTK_HEAP_MIN=100MiB:

    10_000_000.times { String.new }

Before:

    810.7 ms ±   8.3 ms    [User: 790.9 ms, System: 40.3 ms]

After:

    777.9 ms ±  10.4 ms    [User: 759.0 ms, System: 37.9 ms]

https://github.com/ruby/mmtk/commit/0ff5c9f579





Adding a fast path for bump pointer allocator can improve allocation
performance.

For the following microbenchmark with MMTK_HEAP_MIN=100MiB:

    10_000_000.times { String.new }

Before:

    810.7 ms ±   8.3 ms    [User: 790.9 ms, System: 40.3 ms]

After:

    777.9 ms ±  10.4 ms    [User: 759.0 ms, System: 37.9 ms]

https://github.com/ruby/mmtk/commit/0ff5c9f579







[ruby/mmtk] Make rb_gc_impl_heap_id_for_size use MMTK_HEAP_COUNT
2025-12-20T16:41:11+00:00

Peter Zhu
peter@peterzhu.ca

2025-12-20T16:39:58+00:00

6d5605b28a800be19357ec57ff6c8e1118aabca0

https://github.com/ruby/mmtk/commit/2185189df4





https://github.com/ruby/mmtk/commit/2185189df4







[ruby/mmtk] Call rb_bug when Ruby mutator thread panics
2025-12-20T13:40:00+00:00

Peter Zhu
peter@peterzhu.ca

2025-12-19T22:16:01+00:00

6bf921051ceba1742318e3c92dddd50ba4f05d17

This will allow the Ruby backtrace, memory mapping, etc. to be outputted
when a Ruby mutator thread panics.

https://github.com/ruby/mmtk/commit/d10fd325dd





This will allow the Ruby backtrace, memory mapping, etc. to be outputted
when a Ruby mutator thread panics.

https://github.com/ruby/mmtk/commit/d10fd325dd







[ruby/mmtk] Extract max object size to MMTK_MAX_OBJ_SIZE
2025-12-19T22:14:02+00:00

Peter Zhu
peter@peterzhu.ca

2025-12-19T01:01:13+00:00

8274c5e1428b5b88e885857d466822cbadc19761

https://github.com/ruby/mmtk/commit/ed9036c295





https://github.com/ruby/mmtk/commit/ed9036c295







[ruby/mmtk] Extract heap count to MMTK_HEAP_COUNT macro
2025-12-19T22:13:49+00:00

Peter Zhu
peter@peterzhu.ca

2025-12-19T01:00:13+00:00

ed0fae5b501dea14617103ab5ec2d042f445ed7f

https://github.com/ruby/mmtk/commit/4e789e118b





https://github.com/ruby/mmtk/commit/4e789e118b







Use RBIMPL_ASSERT_OR_ASSUME instead of ASSUME for better errors when it does not hold
2025-12-16T20:00:27+00:00

Benoit Daloze
eregontp@gmail.com

2025-12-16T10:58:49+00:00

4d4f414a6062173743af7fe2b88835f16287a6cc