linux-kernel - Re: [LKP] [page cache] eb797a8ee0: vm-scalability.throughput -16.5% regression

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Date:   Sat, 02 Mar 2019 16:26:33 +0800
From:   "Huang\, Ying" <ying.huang@...el.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Waiman Long <longman@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        "Chen\, Rong A" <rong.a.chen@...el.com>,
        "lkp\@01.org" <lkp@...org>, LKML <linux-kernel@...r.kernel.org>,
        Andi Kleen <ak@...ux.intel.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Tim C Chen <tim.c.chen@...el.com>
Subject: Re: [LKP] [page cache] eb797a8ee0: vm-scalability.throughput -16.5% regression

Linus Torvalds <torvalds@...ux-foundation.org> writes:

> On Wed, Feb 27, 2019 at 5:19 PM Huang, Ying <ying.huang@...el.com> wrote:
>>
>> So I think in the heavily contended situation, we should put the fields
>> accessed by rwsem holder in a different cache line of rwsem.  But in
>> un-contended situation, we should put the fields accessed in rwsem
>> holder and rwsem in the same cache line to reduce the cache footprint.
>> The requirement of un-contended and heavily contended situation is
>> contradicted.
>
> Generally, we should strive to optimize for the uncontended state.
>
> The performance profile of a contended state tends to be very
> different, and the actual solution tends to be to try really hard to
> just avoid contention to begin with.
>
> I think we've gotten to the point where we have very few real loads
> that show lock contention on a kernel level. And when people do find
> loads that cause contention, we should try really hard to fix the
> locking rather than try to then treat the symptom of contention.
>
> So on the whole, aim to make the uncontended case go fast, at least to
> a first approximation.

Sounds reasonable!  Thanks for clarification!

Best Regards,
Huang, Ying

>                Linus