Re: [PATCH] fix readahead pipeline break caused by block plug
From: Wu Fengguang
Date: Wed Feb 01 2012 - 02:20:08 EST
On Tue, Jan 31, 2012 at 10:36:53PM -0500, Vivek Goyal wrote:
> 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
The block device really has some aged performance bug. Which
interestingly only show up in some test environments...
> 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).
Would you share the blktrace data?
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