Messages in this thread | | | Date | Wed, 1 Feb 2012 12:22:41 -0800 | From | Andrew Morton <> | Subject | Re: [PATCH] fix readahead pipeline break caused by block plug |
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On Wed, 1 Feb 2012 15:10:17 -0500 Vivek Goyal <vgoyal@redhat.com> 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 http://us.generation-nt.com/answer/patch-add-readpages-support-block-devices-help-201462802.html
> 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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