Re: [PATCH][v2] testsuite: start as many runtest instances as there are job slots

On Tue, 18 Aug 2026, [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.  On a 352 core machine the testsuite runs 128
> wide whatever -j says, and the rest of the machine is idle.
> 
> Take the number from MFLAGS, which GNU make 4.0 and later set, but never go
> below the old constant.  The instances claim work in batches of ten tests
> rather than taking a fixed share each, so an instance that draws a long batch
> is the tail of the whole target, and instances beyond the token count are
> there to pick up the slack.  On x86_64 at -j12  check-gcc
> takes a median of 772.8 s with 12 slots, 718.5 s with 24, 717.7 s with 48 and
> 732.0 s with 128.  Following -j alone is the slowest of the four.
> So the floor is retained, and at 128 no -j below 128 changes behaviour at all.
> 
> A bare -j and older make do not give a number, and there the floor is
> retained as well.  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 far 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.
> 
> Ok for trunk?

OK.

Please give others a chance to comment.

Thanks,
Richard.

> Thanks,
> Kyrill
> 
> gcc/ChangeLog:
> 
> 	* Makefile.in (check_p_min_slots, 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 | 19 ++++++++++++++++++-
>  1 file changed, 18 insertions(+), 1 deletion(-)
> 
> diff --git a/gcc/Makefile.in b/gcc/Makefile.in
> index 5c7cb98d22b..c718585e607 100644
> --- a/gcc/Makefile.in
> +++ b/gcc/Makefile.in
> @@ -4760,8 +4760,25 @@ check_p_tool=$(firstword $(subst _, ,$*))
>  check_p_count=$(check_$(check_p_tool)_parallelize)
>  check_p_subno=$(word 2,$(subst _, ,$*))
>  check_p_subdir=$(subst _,,$*)
> +# Number of runtest instances to start.  An explicit GCC_TEST_PARALLEL_SLOTS
> +# wins.  Otherwise start one per job token, but never fewer than
> +# check_p_min_slots: the instances claim work in batches of ten tests, so a
> +# batch that runs long is the tail of the whole target, and having more
> +# instances than tokens lets the ones that are left pick up the slack.  Above
> +# that floor the token count is the useful number, since 128 instances on a
> +# machine given -j256 leave half of it idle.  GNU make 4.0 and later put the
> +# level in MFLAGS.  A bare -j, and older make, do not, and there the floor
> +# stands on its own.
> +check_p_min_slots=128
> +check_p_jobs=$(patsubst -j%,%,$(filter -j%,$(MFLAGS)))
> +# max(check_p_jobs, check_p_min_slots).  A number is <= the floor exactly when
> +# it appears among the first check_p_min_slots counting numbers.
> +check_p_slots=$(strip $(if $(GCC_TEST_PARALLEL_SLOTS),$(GCC_TEST_PARALLEL_SLOTS),\
> +	$(if $(check_p_jobs),$(if $(filter $(check_p_jobs),\
> +		$(wordlist 1,$(check_p_min_slots),$(one_to_9999))),\
> +		$(check_p_min_slots),$(check_p_jobs)),$(check_p_min_slots))))
>  check_p_subdirs=$(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
> 

-- 
Richard Biener <[email protected]>
SUSE Software Solutions Germany GmbH,
Frankenstrasse 146, 90461 Nuernberg, Germany;
GF: Jochen Jaser, Andrew McDonald, Abhinav Puri; (HRB 36809, AG Nuernberg)