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

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Message-ID: <20210128144245.GH3592@techsingularity.net>
Date:   Thu, 28 Jan 2021 14:42:45 +0000
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     Michal Hocko <mhocko@...e.com>
Cc:     Vincent Guittot <vincent.guittot@...aro.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Christoph Lameter <cl@...ux.com>,
        Bharata B Rao <bharata@...ux.ibm.com>,
        linux-kernel <linux-kernel@...r.kernel.org>, linux-mm@...ck.org,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Andrew Morton <akpm@...ux-foundation.org>, guro@...com,
        Shakeel Butt <shakeelb@...gle.com>,
        Johannes Weiner <hannes@...xchg.org>,
        aneesh.kumar@...ux.ibm.com, Jann Horn <jannh@...gle.com>
Subject: Re: [RFC PATCH v0] mm/slub: Let number of online CPUs determine the
 slub page order

On Thu, Jan 28, 2021 at 02:57:10PM +0100, Michal Hocko wrote:
> On Thu 28-01-21 13:45:12, Mel Gorman wrote:
> [...]
> > So mostly this is down to the number of times SLUB calls into the page
> > allocator which only caches order-0 pages on a per-cpu basis. I do have
> > a prototype for a high-order per-cpu allocator but it is very rough --
> > high watermarks stop making sense, code is rough, memory needed for the
> > pcpu structures quadruples etc.
> 
> Thanks this is really useful. But it really begs a question whether this
> is a general case or more an exception. And as such maybe we want to
> define high throughput caches which would gain a higher order pages to
> keep pace with allocation and reduce the churn or deploy some other
> techniques to reduce the direct page allocator involvement.

I don't think we want to define "high throughput caches" because it'll
be workload dependant and a game of whack-a-mole. If the "high throughput
cache" is a kmalloc cache for some set of workloads and one of the inode
caches or dcaches for another one, there will be no setting that is
universally good.

-- 
Mel Gorman
SUSE Labs