From: Michal Hocko <mhocko@suse.com>
To: Vlastimil Babka <vbabka@suse.cz>
Cc: Christoph Lameter <cl@linux.com>,
Bharata B Rao <bharata@linux.ibm.com>,
Vincent Guittot <vincent.guittot@linaro.org>,
linux-kernel <linux-kernel@vger.kernel.org>,
linux-mm@kvack.org, David Rientjes <rientjes@google.com>,
Joonsoo Kim <iamjoonsoo.kim@lge.com>,
Andrew Morton <akpm@linux-foundation.org>,
guro@fb.com, shakeelb@google.com,
Johannes Weiner <hannes@cmpxchg.org>,
aneesh.kumar@linux.ibm.com, Jann Horn <jannh@google.com>
Subject: Re: [RFC PATCH v0] mm/slub: Let number of online CPUs determine the slub page order
Date: Tue, 26 Jan 2021 09:52:43 +0100 [thread overview]
Message-ID: <20210126085243.GE827@dhcp22.suse.cz> (raw)
In-Reply-To: <d7fb9425-9a62-c7b8-604d-5828d7e6b1da@suse.cz>
On Thu 21-01-21 19:19:21, Vlastimil Babka wrote:
[...]
> We could also start questioning the very assumption that number of cpus should
> affect slab page size in the first place. Should it? After all, each CPU will
> have one or more slab pages privately cached, as we discuss in the other
> thread... So why make the slab pages also larger?
I do agree. What is the acutal justification for this scaling?
/*
* Attempt to find best configuration for a slab. This
* works by first attempting to generate a layout with
* the best configuration and backing off gradually.
*
* First we increase the acceptable waste in a slab. Then
* we reduce the minimum objects required in a slab.
*/
doesn't speak about CPUs. 9b2cd506e5f2 ("slub: Calculate min_objects
based on number of processors.") does talk about hackbench "This has
been shown to address the performance issues in hackbench on 16p etc."
but it doesn't give any more details to tell actually _why_ that works.
This thread shows that this is still somehow related to performance but
the real reason is not clear. I believe we should be focusing on the
actual reasons for the performance impact than playing with some fancy
math and tuning for a benchmark on a particular machine which doesn't
work for others due to subtle initialization timing issues.
Fundamentally why should higher number of CPUs imply the size of slab in
the first place?
--
Michal Hocko
SUSE Labs
next prev parent reply other threads:[~2021-01-26 11:23 UTC|newest]
Thread overview: 37+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-11-18 8:27 [RFC PATCH v0] mm/slub: Let number of online CPUs determine the slub page order Bharata B Rao
2020-11-18 11:25 ` Vlastimil Babka
2020-11-18 19:34 ` Roman Gushchin
2020-11-18 19:53 ` David Rientjes
2021-01-20 17:36 ` Vincent Guittot
2021-01-21 5:30 ` Bharata B Rao
2021-01-21 9:09 ` Vincent Guittot
2021-01-21 10:01 ` Christoph Lameter
2021-01-21 10:48 ` Vincent Guittot
2021-01-21 18:19 ` Vlastimil Babka
2021-01-22 8:03 ` Vincent Guittot
2021-01-22 12:03 ` Vlastimil Babka
2021-01-22 13:16 ` Vincent Guittot
2021-01-23 5:16 ` Bharata B Rao
2021-01-23 12:32 ` Vincent Guittot
2021-01-25 11:20 ` Vlastimil Babka
2021-01-26 23:03 ` Will Deacon
2021-01-27 9:10 ` Christoph Lameter
2021-01-27 11:04 ` Vlastimil Babka
2021-02-03 11:10 ` Bharata B Rao
2021-02-04 7:32 ` Vincent Guittot
2021-02-04 9:07 ` Christoph Lameter
2021-02-04 9:33 ` Vlastimil Babka
2021-02-08 13:41 ` [PATCH] mm, slub: better heuristic for number of cpus when calculating slab order Vlastimil Babka
2021-02-08 14:54 ` Vincent Guittot
2021-02-10 14:07 ` Mel Gorman
2021-01-22 13:05 ` [RFC PATCH v0] mm/slub: Let number of online CPUs determine the slub page order Jann Horn
2021-01-22 13:09 ` Jann Horn
2021-01-22 15:27 ` Vlastimil Babka
2021-01-25 4:28 ` Bharata B Rao
2021-01-26 8:52 ` Michal Hocko [this message]
2021-01-26 13:38 ` Vincent Guittot
2021-01-26 13:59 ` Michal Hocko
2021-01-27 13:38 ` Vlastimil Babka
2021-01-28 13:45 ` Mel Gorman
2021-01-28 13:57 ` Michal Hocko
2021-01-28 14:42 ` Mel Gorman
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