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

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Date:	Mon, 22 Sep 2014 12:32:02 -0400
From:	Milosz Tanski <milosz@...in.com>
To:	Jeff Moyer <jmoyer@...hat.com>
Cc:	Christoph Hellwig <hch@...radead.org>,
	"Elliott, Robert (Server Storage)" <Elliott@...com>,
	Andreas Dilger <adilger@...ger.ca>,
	"linux-kernel@...r.kernel.org" <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>
Subject: Re: [RFC PATCH 0/7] Non-blockling buffered fs read (page cache only)

On Mon, Sep 22, 2014 at 11:48 AM, Jeff Moyer <jmoyer@...hat.com> wrote:
> Christoph Hellwig <hch@...radead.org> writes:
>
>> Without the atomic WRITE SCATTERED use case adding the syscalls seems
>> rather pointless, and I'd really avoid blocking nice software only
>> features like the per-I/O nonblock flag (and the similarly trivial
>> per-I/O sync option I have a prototype for) on it.
>
> Andreas and Zach pointed out that the scatter/gather system calls also
> help network file systems.  I'm not yet sure how much work it would be,
> but it certainly seems worth considering readx/writex (or whatever we
> want to call them) to avoid needlessly adding a ton of system calls.
>
> Cheers,
> Jeff

I spent some time thinking about multi-position scatter/gather in
context of this over the weekend. The non-blocking case seams easy,
the implementation I purposed needs an extra loop. Where this gets
hairy is making the non-trivial blocking case work well (as in have
concurrent requests for each of the ranges) in the filesystem code. If
that's the road we're going to go down I have a gut feeling we're
going to get stuck in the same spot(s) as the other non-blocking
buffered r/w attempts from the past.

Best,
- Milosz

-- 
Milosz Tanski
CTO
16 East 34th Street, 15th floor
New York, NY 10016

p: 646-253-9055
e: milosz@...in.com
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