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

Vlastimil Babka
2020-11-18T11:25:51Z

On 11/18/20 9:27 AM, Bharata B Rao 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>

Acked-by: Vlastimil Babka <vbabka@suse.cz>

Ideally, we would react to hotplug events and update existing caches
accordingly. But for that, recalculation of order for existing caches
would have to be made safe, while not affecting hot paths. We have
removed the sysfs interface with 32a6f409b693 ("mm, slub: remove runtime
allocation order changes") as it didn't seem easy and worth the trouble.

In case somebody wants to start with a large order right from the boot
because they know they will hotplug lots of cpus later, they can use
slub_min_objects= boot param to override this heuristic. So in case this
change regresses somebody's performance, there's a way around it and
thus the risk is low IMHO.

> ---
> 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(-)
>
> 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);
>
>