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From: Linus Torvalds <torvalds@linux-foundation.org>
To: "Huang, Ying" <ying.huang@intel.com>
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: Wed, 27 Feb 2019 17:32:57 -0800	[thread overview]
Message-ID: <CAHk-=wiXgigVGwumT0y_N64LjCiB5tR0snjCAm3u4YyhDB+dwQ@mail.gmail.com> (raw)
In-Reply-To: <87imx4pv5q.fsf@yhuang-dev.intel.com>

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.

               Linus

  reply	other threads:[~2019-02-28  1:33 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 [this message]
2019-03-02  8:26             ` Huang, Ying
2019-02-28  2:37           ` Waiman Long
2019-02-28  3:26             ` Huang, Ying

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