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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
--------------  ---------------  ----------  ---------

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
    --------------  ---------------  ----------  ---------
@peterzhu2118 peterzhu2118 merged commit 67c9c6a into main Dec 22, 2025
20 of 22 checks passed
@peterzhu2118 peterzhu2118 deleted the ruby-heap branch December 22, 2025 21:53
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2 participants