From: Jeff Moyer <jmoyer@redhat.com>
Subject: Re: [PATCH 2/2][v2] blk-plug: don't flush nested plug lists
Date: Fri, 10 Apr 2015 17:50:06 -0400
Message-ID: <x498udzlkkx.fsf@segfault.boston.devel.redhat.com>
References: <1428347694-17704-1-git-send-email-jmoyer@redhat.com>
	<1428347694-17704-2-git-send-email-jmoyer@redhat.com>
	<x49wq1nrcoe.fsf_-_@segfault.boston.devel.redhat.com>
	<20150408230203.GG15810@dastard>
Mime-Version: 1.0
Content-Type: text/plain
Cc: Jens Axboe <axboe@kernel.dk>, Ming Lei <tom.leiming@gmail.com>,
        Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
        Roger Pau Monn?? <roger.pau@citrix.com>,
        Alasdair Kergon <agk@redhat.com>, Mike Snitzer <snitzer@redhat.com>,
        Neil Brown <neilb@suse.de>,
        "Nicholas A. Bellinger" <nab@linux-iscsi.org>,
        Alexander Viro <viro@zeniv.linux.org.uk>, Chris Mason <clm@fb.com>,
        Josef Bacik <jbacik@fb.com>, David Sterba <dsterba@suse.cz>,
        "Theodore Ts'o" <tytso@mit.edu>,
        Andreas Dilger <adilger.kernel@dilger.ca>,
        Jaegeuk Kim <jaegeuk@kernel.org>, Changman Lee <cm224.lee@samsung.com>,
        Steven Whitehouse <swhiteho@redhat.com>,
        Mikulas Patocka <mikulas@artax.karlin.mff.cuni.cz>,
        Andrew Morton <akpm@linux-foundation.org>,
        Rik van Riel <riel@redhat.com>, Johannes Weiner <hannes@cmpxchg.org>,
To: Dave Chinner <david@fromorbit.com>
Return-path: <owner-linux-aio@kvack.org>
In-Reply-To: <20150408230203.GG15810@dastard> (Dave Chinner's message of "Thu,
	9 Apr 2015 09:02:03 +1000")
Sender: owner-linux-aio@kvack.org
List-Id: linux-ext4.vger.kernel.org

Dave Chinner <david@fromorbit.com> writes:

> On Tue, Apr 07, 2015 at 02:55:13PM -0400, Jeff Moyer wrote:
>> The way the on-stack plugging currently works, each nesting level
>> flushes its own list of I/Os.  This can be less than optimal (read
>> awful) for certain workloads.  For example, consider an application
>> that issues asynchronous O_DIRECT I/Os.  It can send down a bunch of
>> I/Os together in a single io_submit call, only to have each of them
>> dispatched individually down in the bowels of the dirct I/O code.
>> The reason is that there are blk_plug-s instantiated both at the upper
>> call site in do_io_submit and down in do_direct_IO.  The latter will
>> submit as little as 1 I/O at a time (if you have a small enough I/O
>> size) instead of performing the batching that the plugging
>> infrastructure is supposed to provide.
>
> I'm wondering what impact this will have on filesystem metadata IO
> that needs to be issued immediately. e.g. we are doing writeback, so
> there is a high level plug in place and we need to page in btree
> blocks to do extent allocation. We do readahead at this point,
> but it looks like this change will prevent the readahead from being
> issued by the unplug in xfs_buf_iosubmit().

I'm not ignoring you, Dave, I'm just doing some more investigation and
testing.  It's taking longer than I had hoped.

-Jeff

--
To unsubscribe, send a message with 'unsubscribe linux-aio' in
the body to majordomo@kvack.org.  For more info on Linux AIO,
see: http://www.kvack.org/aio/
Don't email: <a href=mailto:"aart@kvack.org">aart@kvack.org</a>