Re: [RFC PATCH v0] mm/slub: Let number of online CPUs determine the slub page order

[Vincent Guittot <vincent.guittot@linaro.org>]

Hi,

On Wed, 18 Nov 2020 at 09:28, Bharata B Rao <bharata@linux.ibm.com> wrote:
>
> The page order of the slab that gets chosen for a given slab
> cache depends on the number of objects that can be fit in the
> slab while meeting other requirements. We start with a value
> of minimum objects based on nr_cpu_ids that is driven by
> possible number of CPUs and hence could be higher than the
> actual number of CPUs present in the system. This leads to
> calculate_order() chosing a page order that is on the higher
> side leading to increased slab memory consumption on systems
> that have bigger page sizes.
>
> Hence rely on the number of online CPUs when determining the
> mininum objects, thereby increasing the chances of chosing
> a lower conservative page order for the slab.
>
> Signed-off-by: Bharata B Rao <bharata@linux.ibm.com>
> ---
> This is a generic change and I am unsure how it would affect
> other archs, but as a start, here are some numbers from
> PowerPC pseries KVM guest with and without this patch:
>
> This table shows how this change has affected some of the slab
> caches.
> ===================================================================
>                 Current                         Patched
> Cache   <objperslab> <pagesperslab>     <objperslab> <pagesperslab>
> ===================================================================
> TCPv6           53    2                 26    1
> net_namespace   53    4                 26    2
> dtl             32    2                 16    1
> names_cache     32    2                 16    1
> task_struct     53    8                 13    2
> thread_stack    32    8                 8     2
> pgtable-2^11    16    8                 8     4
> pgtable-2^8     32    2                 16    1
> kmalloc-32k     16    8                 8     4
> kmalloc-16k     32    8                 8     2
> kmalloc-8k      32    4                 8     1
> kmalloc-4k      32    2                 16    1
> ===================================================================
>
> Slab memory (kB) consumption comparision
> ==================================================================
>                         Current         Patched
> ==================================================================
> After-boot              205760          156096
> During-hackbench        629145          506752 (Avg of 5 runs)
> After-hackbench         474176          331840 (after drop_caches)
> ==================================================================
>
> Hackbench Time (Avg of 5 runs)
> (hackbench -s 1024 -l 200 -g 200 -f 25 -P)
> ==========================================
> Current         Patched
> ==========================================
> 10.990          11.010
> ==========================================
>
> Measuring the effect due to CPU hotplug
> ----------------------------------------
> Since the patch doesn't consider all the possible CPUs for page
> order calcluation, let's see how affects the case when CPUs are
> hotplugged. Here I compare a system that is booted with 64CPUs
> with a system that is booted with 16CPUs but hotplugged with
> 48CPUs after boot. These numbers are with the patch applied.
>
> Slab memory (kB) consumption comparision
> ===================================================================
>                         64bootCPUs      16bootCPUs+48HotPluggedCPUs
> ===================================================================
> After-boot              390272          159744
> After-hotplug           -               251328
> During-hackbench        1001267         941926 (Avg of 5 runs)
> After-hackbench         913600          827200 (after drop_caches)
> ===================================================================
>
> Hackbench Time (Avg of 5 runs)
> (hackbench -s 1024 -l 200 -g 200 -f 25 -P)
> ===========================================
> 64bootCPUs      16bootCPUs+48HotPluggedCPUs
> ===========================================
> 12.554          12.589
> ===========================================
>  mm/slub.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>

I'm facing significant performances regression on a large arm64 server
system (224 CPUs). Regressions is also present on small arm64 system
(8 CPUs) but in a far smaller order of magnitude

On 224 CPUs system : 9 iterations of hackbench -l 16000 -g 16
v5.11-rc4 : 9.135sec (+/- 0.45%)
v5.11-rc4 + revert this patch: 3.173sec (+/- 0.48%)
v5.10: 3.136sec (+/- 0.40%)

This is a 191% regression compared to v5.10.

The problem is that calculate_order() is called a number of times
before secondaries CPUs are booted and it returns 1 instead of 224.
This makes the use of num_online_cpus() irrelevant for those cases

After adding in my command line "slub_min_objects=36" which equals to
4 * (fls(num_online_cpus()) + 1) with a correct num_online_cpus == 224
, the regression diseapears:

9 iterations of hackbench -l 16000 -g 16: 3.201sec (+/- 0.90%)



> diff --git a/mm/slub.c b/mm/slub.c
> index 34dcc09e2ec9..8342c0a167b2 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -3433,7 +3433,7 @@ static inline int calculate_order(unsigned int size)
>          */
>         min_objects = slub_min_objects;
>         if (!min_objects)
> -               min_objects = 4 * (fls(nr_cpu_ids) + 1);
> +               min_objects = 4 * (fls(num_online_cpus()) + 1);
>         max_objects = order_objects(slub_max_order, size);
>         min_objects = min(min_objects, max_objects);
>
> --
> 2.26.2
>