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

From: Andrew Morton
Date: Wed Feb 01 2012 - 15:22:40 EST

On Wed, 1 Feb 2012 15:10:17 -0500
Vivek Goyal <vgoyal@xxxxxxxxxx> wrote:

> On Wed, Feb 01, 2012 at 04:18:07AM -0500, Christoph Hellwig wrote:
> > On Tue, Jan 31, 2012 at 10:36:53PM -0500, Vivek Goyal wrote:
> > > 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).
> >
> > This is expected given that the block device node uses
> > block_read_full_page, and not mpage_readpage(s).
> What is the difference between block_read_full_page() and
> mpage_readpage().

block_read_full_page() will attach buffer_heads to the page and will
perform IO via those buffer_heads. mpage_readpage() feeds the page
directly to the BIO layer and leaves it without attached buffer_heads.

> IOW, why block device does not use mpage_readpage(s)
> interface?

We've tried it in the past and problems ensued. A quick google search
for blkdev_readpages turns up stuff like

> Applying following patch improved the speed from 110MB/s to more than
> 230MB/s.

Yeah. It should be doable - it would be a matter of hunting down and
squishing the oddball corner cases.

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