linux-ext4 - Re: Latency writing to an mlocked ext4 mapping

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Date:	Tue, 1 Nov 2011 18:51:04 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Jan Kara <jack@...e.cz>
Cc:	Andreas Dilger <adilger@...ger.ca>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>
Subject: Re: Latency writing to an mlocked ext4 mapping

On Tue, Nov 1, 2011 at 4:10 PM, Andy Lutomirski <luto@...capital.net> wrote:
> On Tue, Nov 1, 2011 at 4:03 PM, Jan Kara <jack@...e.cz> wrote:

>>
>> Avoiding IO during a minor fault would be a decent thing which might be
>> worth pursuing. As you properly noted "stable pages during writeback"
>> requirement is one obstacle which won't be that trivial to avoid though...
>
> There's an easy solution that would be good enough for me: add a mount
> option to turn off stable pages.
>
> Is the other problem just a race, perhaps?  __block_page_mkwrite calls
> __block_write_begin (which calls get_block, which I think is where the
> latency comes from) *before* wait_on_page_writeback, which means that
> there might not be any space allocated yet.

I think I'm right (other than calling it a race).  If I change my code to do:

- map the file (with MCL_FUTURE set)
- fallocate
- dirty all pages
- fsync
- dirty all pages again

in the non-real-time thread, then a short test that was a mediocre
reproducer seems to work.

This is annoying, though -- I'm not generating twice as much write I/O
as I used to.  Is there any way to force the delalloc code to do its
thing without triggering writeback?  I don't think fallocate has this
effect.

--Andy
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