linux-kernel - Re: [PATCH v7 0/5] vfs: Non-blockling buffered fs read (page cache only)

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Message-Id: <20150330155700.92f4c8a0bf13418aaf01ae04@linux-foundation.org>
Date:	Mon, 30 Mar 2015 15:57:00 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Milosz Tanski <milosz@...in.com>
Cc:	Jeremy Allison <jra@...ba.org>,
	Christoph Hellwig <hch@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	"linux-aio@...ck.org" <linux-aio@...ck.org>,
	Mel Gorman <mgorman@...e.de>,
	Volker Lendecke <Volker.Lendecke@...net.de>,
	Tejun Heo <tj@...nel.org>, Jeff Moyer <jmoyer@...hat.com>,
	"Theodore Ts'o" <tytso@....edu>, Al Viro <viro@...iv.linux.org.uk>,
	Linux API <linux-api@...r.kernel.org>,
	Michael Kerrisk <mtk.manpages@...il.com>,
	linux-arch@...r.kernel.org, Dave Chinner <david@...morbit.com>
Subject: Re: [PATCH v7 0/5] vfs: Non-blockling buffered fs read (page cache
 only)

On Mon, 30 Mar 2015 18:49:06 -0400 Milosz Tanski <milosz@...in.com> wrote:

> > A fincore+pread solution that blocks is simply unsafe
> > to use for us. We'll have to stay with the threadpool :-(.
> 
> We're getting data from a network filesystem Ceph in our case, but it
> could be pNFS. In many cases those filesystems have some kind
> hierarchy and it's not uncommon for us to se requests that take 20 to
> 25 milliseconds to complete. In this case the miss becomes very
> expensive. And it's not just that one requests experiences the slow
> down all the request being serviced by that (single) epoll thread
> experience head-of-line blocking because of one stalled request.
> 
> 10K request a second is a common load for many web services / video
> servers servings chunks of data. If we experience one miss a second,
> that 25 million stall will impact 250 other requests (all of them will
> have a 25ms latency tacked on).

I'd expect a fincore() which doesn't do SetPageReferenced() to be
orders of magnitude better than this.  A fincore() which does use
SetPageReferenced() will be in the "basically never happens" region -
it would take massive and artificial memory stress to trigger.
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