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

[Hyeonggon Yoo]

On Tue, Sep 5, 2023 at 11:07 PM Feng Tang <feng.tang@xxxxxxxxx> 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?

> 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?

> 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@xxxxxxxxx/
> [2]. ttps://lore.kernel.org/lkml/ZORaUsd+So+tnyMV@chenyu5-mobl2/
> Signed-off-by: Feng Tang <feng.tang@xxxxxxxxx>

> ---
>  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;
>  }