Re: [PATCH 2/2] testsuite: start as many runtest instances as there are job slots

On Mon, Aug 17, 2026 at 6:19 PM <[email protected]> wrote:
>
> From: Kyrylo Tkachov <[email protected]>
>
> The parallel check targets start 128 runtest instances, or as many as
> GCC_TEST_PARALLEL_SLOTS says.  The -j level make was given is looked at only
> to decide whether to go parallel at all, never for how parallel to be.
>
> That constant predates machines with more cores than it, and it is now what
> limits the testsuite on them.  The instances claim work from one shared queue,
> so the number worth starting is the number of job tokens.  On a 352 core
> machine the testsuite runs 128 wide whatever -j says, and the rest of the
> machine is idle.  In the other direction, make -j4 check still starts 128
> instances, and each one reads the whole testsuite and repeats every
> effective-target probe before it can claim any work.
>
> Take the number from MFLAGS, which GNU make 4.0 and later set.  A bare -j and
> older make do not give a number, and there the old default stands.  An
> explicit GCC_TEST_PARALLEL_SLOTS still wins, and the per language
> check_$lang_parallelize caps still apply.
>
> On aarch64 at -j352, check-gcc falls by 24%, from a median of 118.1 s to
> 89.3 s over four interleaved rounds, with identical results in every round.
> It also runs more repeatably: the spread across rounds falls from 10.6%
> to 4.5%, and the slowest patched round is still 22% faster than the median
> unpatched one.
>
> Bootstrapped on aarch64-none-linux-gnu.  Ok for trunk?

But with low N, like -j12 my experience is that a larger GCC_TEST_PARALLEL_SLOTS
helps because the actual time needed for the slots is not even.  So
I'd rather _not_
have this taken literally but kept at a minimum number of slots (for
example 128, but
I guess 64 might work as well).  Some actual numbers might help?

Richard.

>
> gcc/ChangeLog:
>
>         * Makefile.in (check_p_jobs, check_p_slots): New variables.
>         (check_p_subdirs): Use check_p_slots.
>
> Signed-off-by: Kyrylo Tkachov <[email protected]>
> ---
>  gcc/Makefile.in | 12 +++++++++++-
>  1 file changed, 11 insertions(+), 1 deletion(-)
>
> diff --git a/gcc/Makefile.in b/gcc/Makefile.in
> index 5c7cb98d22b..ac42f54e53d 100644
> --- a/gcc/Makefile.in
> +++ b/gcc/Makefile.in
> @@ -4760,8 +4760,18 @@ check_p_tool=$(firstword $(subst _, ,$*))
>  check_p_count=$(check_$(check_p_tool)_parallelize)
>  check_p_subno=$(word 2,$(subst _, ,$*))
>  check_p_subdir=$(subst _,,$*)
> +# Number of runtest instances to start.  An explicit GCC_TEST_PARALLEL_SLOTS
> +# wins.  Otherwise follow the -j level make was given.  The instances share one
> +# work queue, so the useful number of them is the number of job tokens, not a
> +# constant: fewer leaves the machine idle, more only adds start-up, since every
> +# instance reads the whole testsuite and repeats the effective-target probes
> +# before it can claim any work.  GNU make 4.0 and later put the number in
> +# MFLAGS.  A bare -j, and older make, do not, and there the historical default
> +# of 128 is kept.
> +check_p_jobs=$(patsubst -j%,%,$(filter -j%,$(MFLAGS)))
> +check_p_slots=$(if $(GCC_TEST_PARALLEL_SLOTS),$(GCC_TEST_PARALLEL_SLOTS),$(if $(check_p_jobs),$(check_p_jobs),128))
>  check_p_subdirs=$(wordlist 1,$(check_p_count),$(wordlist 1, \
> -               $(if $(GCC_TEST_PARALLEL_SLOTS),$(GCC_TEST_PARALLEL_SLOTS),128), \
> +               $(check_p_slots), \
>                 $(one_to_9999)))
>
>  # For parallelized check-% targets, this decides whether parallelization
> --
> 2.50.1 (Apple Git-155)
>