|
Adds MMTK_HEAP_MODE=cpu, a dynamic heap-sizing policy that grows
or shrinks the heap after each GC cycle to keep measured GC CPU
overhead near a configurable target. The control law follows
Tavakolisomeh et al., 'Heap Size Adjustment with CPU Control', MPLR
'23: a sigmoid of the (averaged) GC CPU overhead error in (-inf, +inf)
maps to a heap-size adjustment factor in (0.5, 1.5).
Implementation lives alongside the existing 'ruby' delegated trigger
in gc/mmtk/src/heap/. T_GC is wall-clock GC duration; T_APP is process
CPU time delta read via clock_gettime(CLOCK_PROCESS_CPUTIME_ID), which
correctly credits multi-threaded mutator parallelism. Nursery-only
generational GCs are skipped so the trigger only re-sizes at full
collections.
Configuration:
MMTK_GC_CPU_TARGET target GC CPU overhead, percent. Default 5.
MMTK_GC_CPU_WINDOW number of recent cycles averaged. Default 3.
The default differs from the paper's recommended 15. The paper
targets ZGC, a concurrent generational collector; MMTk-Ruby currently
ships stop-the-world Immix, where every percent of GC CPU also blocks
the mutator. An empirical sweep of MMTK_GC_CPU_TARGET across
ruby-bench (railsbench, lobsters, psych-load, liquid-render, lee)
found 5-6 to be Pareto-optimal vs the existing 'ruby' heap mode:
about 6 percent geomean throughput improvement at essentially equal
peak RSS. Targets >=10 trade large amounts of throughput for modest
RSS savings on this collector.
bin/smoke-test, bin/ruby-mmtk-mode, bin/compare-heap-modes, and
doc/testing-cpu-heap-mode.md are included so reviewers and future
contributors can reproduce the sweep against ruby/ruby-bench.
https://github.com/ruby/mmtk/commit/1f223f5ad5
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
|
|
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
|
