Re: [RFC Patch 1/3] mm/slub: increase the maximum slab order to 4 for big systems

Feng Tang
2023-09-12T16:05:00Z

Hi Hyeonggon,

Many thanks for the review!

On Tue, Sep 12, 2023 at 01:52:19PM +0900, Hyeonggon Yoo wrote:
> On Tue, Sep 5, 2023 at 11:07 PM Feng Tang <feng.tang@intel.com> wrote:
> >
> > There are reports about severe lock contention for slub's per-node
> > 'list_lock' in 'hackbench' test, [1][2], on server systems. And
> > similar contention is also seen when running 'mmap1' case of
> > will-it-scale on big systems. As the trend is one processor (socket)
> > will have more and more CPUs (100+, 200+), the contention could be
> > much more severe and becomes a scalability issue.
> >
> > One way to help reducing the contention is to increase the maximum
> > slab order from 3 to 4, for big systems.
>
> Hello Feng,
>
> Increasing order with a higher number of CPUs (and so with more
> memory) makes sense to me.
> IIUC the contention here becomes worse when the number of slabs
> increases, so it makes sense to
> decrease the number of slabs by increasing order.
>
> By the way, my silly question here is:
> In the first place, is it worth taking 1/2 of s->cpu_partial_slabs in
> the slowpath when slab is frequently used?
> wouldn't the cpu partial slab list be re-filled again by free if free
> operations are frequently performed?

My understanding is the contention is related to the number of
objects for each cpu (the current slab and on the per-cpu partial
list), if it's easier to be used up, then the per-node lock will be
contended.

This patch increase the order (I should have also considered the
CPU number), while keeping the per-cpu partial numbers unchanged,
as it doubles the 'nr_objects' in set_cpu_partial().

But the 2/3 patch only increases the per-cpu partial number, and
keeps the order unchanged. From the performance data in cover
letter, 1/3 and 2/3 can individually reduce the contention for
will-it-scale/mmap1, as they both increase the available per-cpu
object numbers.

>
> > Unconditionally increasing the order could bring trouble to client
> > devices with very limited size of memory, which may care more about
> > memory footprint, also allocating order 4 page could be harder under
> > memory pressure. So the increase will only be done for big systems
> > like servers, which usually are equipped with plenty of memory and
> > easier to hit lock contention issues.
>
> Also, does it make sense not to increase the order when PAGE_SIZE > 4096?

Good point! Some other discussion on mm list earlier this week
also reminded me that there are ARCHs supporting bigger pages like
64KB, and these patches needs to consider more about it.

> > Following is some performance data:
> >
> > will-it-scale/mmap1
> > -------------------
> > Run will-it-scale benchmark's 'mmap1' test case on a 2 socket Sapphire
> > Rapids server (112 cores / 224 threads) with 256 GB DRAM, run 3
> > configurations with parallel test threads of 25%, 50% and 100% of
> > number of CPUs, and the data is (base is vanilla v6.5 kernel):
> >
> > base base+patch
> > wis-mmap1-25% 223670 +33.3% 298205 per_process_ops
> > wis-mmap1-50% 186020 +51.8% 282383 per_process_ops
> > wis-mmap1-100% 89200 +65.0% 147139 per_process_ops
> >
> > Take the perf-profile comparasion of 50% test case, the lock contention
> > is greatly reduced:
> >
> > 43.80 -30.8 13.04 pp.self.native_queued_spin_lock_slowpath
> > 0.85 -0.2 0.65 pp.self.___slab_alloc
> > 0.41 -0.1 0.27 pp.self.__unfreeze_partials
> > 0.20 ± 2% -0.1 0.12 ± 4% pp.self.get_any_partial
> >
> > hackbench
> > ---------
> >
> > Run same hackbench testcase mentioned in [1], use same HW/SW as will-it-scale:
> >
> > base base+patch
> > hackbench 759951 +10.5% 839601 hackbench.throughput
> >
> > perf-profile diff:
> > 22.20 ± 3% -15.2 7.05 pp.self.native_queued_spin_lock_slowpath
> > 0.82 -0.2 0.59 pp.self.___slab_alloc
> > 0.33 -0.2 0.13 pp.self.__unfreeze_partials
> >
> > [1]. https://lore.kernel.org/all/202307172140.3b34825a-oliver.sang@intel.com/
> > [2]. ttps://lore.kernel.org/lkml/ZORaUsd+So+tnyMV@chenyu5-mobl2/
> > Signed-off-by: Feng Tang <feng.tang@intel.com>
>
> > ---
> > mm/slub.c | 51 ++++++++++++++++++++++++++++++++++++++-------------
> > 1 file changed, 38 insertions(+), 13 deletions(-)
> >
> > diff --git a/mm/slub.c b/mm/slub.c
> > index f7940048138c..09ae1ed642b7 100644
> > --- a/mm/slub.c
> > +++ b/mm/slub.c
> > @@ -4081,7 +4081,7 @@ EXPORT_SYMBOL(kmem_cache_alloc_bulk);
> > */
> > static unsigned int slub_min_order;
> > static unsigned int slub_max_order =
> > - IS_ENABLED(CONFIG_SLUB_TINY) ? 1 : PAGE_ALLOC_COSTLY_ORDER;
> > + IS_ENABLED(CONFIG_SLUB_TINY) ? 1 : 4;
> > static unsigned int slub_min_objects;
> >
> > /*
> > @@ -4134,6 +4134,26 @@ static inline unsigned int calc_slab_order(unsigned int size,
> > return order;
> > }
>