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

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Message-ID: <x49sijrxt1f.fsf@segfault.boston.devel.redhat.com>
Date:	Tue, 16 Sep 2014 15:30:04 -0400
From:	Jeff Moyer <jmoyer@...hat.com>
To:	Milosz Tanski <milosz@...in.com>
Cc:	linux-kernel@...r.kernel.org,
	Christoph Hellwig <hch@...radead.org>,
	linux-fsdevel@...r.kernel.org, linux-aio@...ck.org,
	Mel Gorman <mgorman@...e.de>,
	Volker Lendecke <Volker.Lendecke@...net.de>,
	Tejun Heo <tj@...nel.org>
Subject: Re: [RFC PATCH 0/7] Non-blockling buffered fs read (page cache only)

Milosz Tanski <milosz@...in.com> writes:

> 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 readv2/writev2 and
> preadv2/pwritev2. These new syscalls behave like the network sendmsg, recvmsg
> syscalls that accept an extra flag argument (O_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.
>
> I have co-developed these changes with Christoph Hellwig, a whole lot of his
> fixes went into the first patch in the series (were squashed with his
> approval).
>
> I am going to post the perf report in a reply-to to this RFC.

You can send the performance data along with the patch series, no need to
separate it off in a reply.

One additional patch I'd like to see is a man page update.  That would
help clarify exactly what you're trying to accomplish.

I look forward to v2!

Cheers,
Jeff
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