linux-kernel - Re: [PATCH v6 0/7] vfs: Non-blockling buffered fs read (page cache only)

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Date:	Wed, 12 Nov 2014 08:40:12 +1100
From:	Dave Chinner <david@...morbit.com>
To:	Milosz Tanski <milosz@...in.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Christoph Hellwig <hch@...radead.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
Subject: Re: [PATCH v6 0/7] vfs: Non-blockling buffered fs read (page cache
 only)

On Tue, Nov 11, 2014 at 11:02:00AM -0500, Milosz Tanski wrote:
> On Tue, Nov 11, 2014 at 1:44 AM, Dave Chinner <david@...morbit.com> wrote:
> > On Mon, Nov 10, 2014 at 11:40:23AM -0500, Milosz Tanski wrote:
> >> This patcheset introduces an ability to perform a non-blocking read from
> >> regular files in buffered IO mode. This works by only for those filesystems
> >> that have data in the page cache.
> >>
> >> It does this by introducing new syscalls new syscalls preadv2/pwritev2. These
> >> new syscalls behave like the network sendmsg, recvmsg syscalls that accept an
> >> extra flag argument (RWF_NONBLOCK).
> >>
> >> It's a very common patern today (samba, libuv, etc..) use a large threadpool to
> >> perform buffered IO operations. They submit the work form another thread
> >> that performs network IO and epoll or other threads that perform CPU work. This
> >> leads to increased latency for processing, esp. in the case of data that's
> >> already cached in the page cache.
> >>
> >> With the new interface the applications will now be able to fetch the data in
> >> their network / cpu bound thread(s) and only defer to a threadpool if it's not
> >> there. In our own application (VLDB) we've observed a decrease in latency for
> >> "fast" request by avoiding unnecessary queuing and having to swap out current
> >> tasks in IO bound work threads.
> >
> > Can you write a test (or set of) for fstests that exercises this new
> > functionality? I'm not worried about performance, just
> > correctness....
> 
> Sure thing. Can you point me at the fstests repo? A quick google
> search reveals lots of projects named fstests, most of them abandoned.

git://git.kernel.org/pub/scm/fs/xfs/xfstests-dev.git

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com
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