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From: "Huang\, Ying" <ying.huang@intel.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Waiman Long <longman@redhat.com>,
	Matthew Wilcox <willy@infradead.org>, "Chen\,
	Rong A" <rong.a.chen@intel.com>, "lkp\@01.org" <lkp@01.org>,
	LKML <linux-kernel@vger.kernel.org>,
	Andi Kleen <ak@linux.intel.com>,
	Dave Hansen <dave.hansen@intel.com>,
	Tim C Chen <tim.c.chen@intel.com>
Subject: Re: [LKP] [page cache] eb797a8ee0: vm-scalability.throughput -16.5% regression
Date: Sat, 02 Mar 2019 16:26:33 +0800	[thread overview]
Message-ID: <87a7idn0li.fsf@yhuang-dev.intel.com> (raw)
In-Reply-To: <CAHk-=wiXgigVGwumT0y_N64LjCiB5tR0snjCAm3u4YyhDB+dwQ@mail.gmail.com> (Linus Torvalds's message of "Wed, 27 Feb 2019 17:32:57 -0800")

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

> On Wed, Feb 27, 2019 at 5:19 PM Huang, Ying <ying.huang@intel.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

  reply	other threads:[~2019-03-02  8:27 UTC|newest]

Thread overview: 10+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-11-14  9:22 kernel test robot
2018-11-14 14:17 ` Matthew Wilcox
2019-02-26  8:17   ` Huang, Ying
2019-02-26 17:30     ` Linus Torvalds
2019-02-26 20:29       ` Waiman Long
2019-02-28  1:18         ` Huang, Ying
2019-02-28  1:32           ` Linus Torvalds
2019-03-02  8:26             ` Huang, Ying [this message]
2019-02-28  2:37           ` Waiman Long
2019-02-28  3:26             ` Huang, Ying

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