linux-kernel - Re: read()/readv() only from page cache

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Message-Id: <E1XQjOX-00CcnG-UC@intern.SerNet.DE>
Date:	Sun, 7 Sep 2014 22:48:51 +0200
From:	Volker Lendecke <Volker.Lendecke@...Net.DE>
To:	Milosz Tanski <milosz@...in.com>
Cc:	Christoph Hellwig <hch@...radead.org>,
	Mel Gorman <mgorman@...e.de>,
	LKML <linux-kernel@...r.kernel.org>, Tejun Heo <tj@...nel.org>,
	linux-aio@...ck.org
Subject: Re: read()/readv() only from page cache

On Fri, Sep 05, 2014 at 12:27:21PM -0400, Milosz Tanski wrote:
> In a VLDB like workload this would enable me to lower the latency of
> common fast requests and. By fast requests I mean ones that do not
> require much data, the data is cached, or there's a predictable read
> pattern (read-ahead). Obviously it would be at the expense of the
> latency of large/slow requests (they have to make 2 read calls, the
> first one always EWOULDBLOCK) ... but in that case it doesn't matter
> since the time to do actual IO would trump any kind of extra latency.

That was my thinking as well when I discussed it with Christoph. Sorry
for not getting around to actually test his patch. Samba right
now uses a thread pool for aio, we get parallel read requests from
clients over a single TCP connection and we need to keep all disks busy
simultaneously. We'd like to avoid the thread overhead when it's not
necessary because the data is already around. A per-request flag would
fit Samba better than a flag settable by fcntl in this pattern. Samba
can't easily open a file twice (once with NONBLOCK and once without).

Volker

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