Hi, On Wed, 18 Nov 2020 at 09:28, Bharata B Rao <bharata@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxx> > --- > 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 >
