linux-kernel - Re: [PATCH] fix readahead pipeline break caused by block plug

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Date:	Tue, 31 Jan 2012 22:36:53 -0500
From:	Vivek Goyal <vgoyal@...hat.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Shaohua Li <shaohua.li@...el.com>,
	lkml <linux-kernel@...r.kernel.org>,
	linux-mm <linux-mm@...ck.org>, Jens Axboe <axboe@...nel.dk>,
	Herbert Poetzl <herbert@...hfloor.at>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Wu Fengguang <wfg@...ux.intel.com>
Subject: Re: [PATCH] fix readahead pipeline break caused by block plug

On Tue, Jan 31, 2012 at 05:22:17PM -0500, Vivek Goyal wrote:
[..]

> > 
> > We've never really bothered making the /dev/sda[X] I/O very efficient
> > for large I/O's under the (probably wrong) assumption that it isn't a
> > very interesting case.  Regular files will (or should) use the mpage
> > functions, via address_space_operations.readpages().  fs/blockdev.c
> > doesn't even implement it.
> > 
> > > and by the time all the pages
> > > are submitted and one big merged request is formed it wates lot of time.
> > 
> > But that was the case in eariler kernels too.  Why did it change?
> 
> Actually, I assumed that the case of reading /dev/sda[X] worked well in
> earlier kernels. Sorry about that. Will build a 2.6.38 kernel tonight
> and run the test case again to make sure we had same overhead and
> relatively poor performance while reading /dev/sda[X].

Ok, I tried it with 2.6.38 kernel and results look more or less same.
Throughput varied between 105MB to 145MB. Many a times it was close to
110MB and other times it was 145MB. Don't know what causes that spike
sometimes.

I still see that IO is being submitted one page at a time. The only
real difference seems to be that queue unplug happening at random times
and many a times we are submitting much smaller requests (40 sectors, 48
sectors etc).

Thanks
Vivek
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