Re: performance "regression" in cfq compared to anticipatory, deadline and noop

[Jens Axboe <jens.axboe@oracle.com>]

On Tue, May 13 2008, Matthew wrote:
> On Tue, May 13, 2008 at 8:40 PM, Jens Axboe <jens.axboe@oracle.com> wrote:
> >
> > On Tue, May 13 2008, Jens Axboe wrote:
> >  > On Tue, May 13 2008, Matthew wrote:
> >  > > On Tue, May 13, 2008 at 3:05 PM, Jens Axboe <jens.axboe@oracle.com> wrote:
> >  > > >
> >  > > > On Tue, May 13 2008, Matthew wrote:
> >  > > >  > On Tue, May 13, 2008 at 2:20 PM, Jens Axboe <jens.axboe@oracle.com> wrote:
> >  > > >  > >
> >  > > >  > > On Sun, May 11 2008, Kasper Sandberg wrote:
> >  > > >  > >  > On Sun, 2008-05-11 at 14:14 +0100, Daniel J Blueman wrote:
> >  > > >  > >  > > I've been experiencing this for a while also; an almost 50% regression
> >  > > >  > >  > > is seen for single-process reads (ie sync) if slice_idle is 1ms or
> >  > > >  > >  > > more (eg default of 8) [1], which seems phenomenal.
> >  > > >  > >  > >
> >  > > >  > >  > > Jens, is this the expected price to pay for optimal busy-spindle
> >  > > >  > >  > > scheduling, a design issue, bug or am I missing something totally?
> >  > > >  > >  > >
> >  > > >  > >  > > Thanks,
> >  > > >  > >  > >   Daniel
> >  > > >  > [snip]
> >  > > >  > ...
> >  > > >  > [snip]
> >  > > >  > >  >
> >  > > [snip]
> >  > >
> >  > > ...
> >  > >
> >  > > [snip]
> >  > > >  > well - back to topic:
> >  > > >  >
> >  > > >  > for a blktrace one need to enable  CONFIG_BLK_DEV_IO_TRACE , right ?
> >  > > >  > blktrace can be obtained from your git-repo ?
> >  > > >
> >  > > >  Yes on both accounts, or just grab a blktrace snapshot from:
> >  > > >
> >  > > >  http://brick.kernel.dk/snaps/blktrace-git-latest.tar.gz
> >  > > >
> >  > > >  if you don't use git.
> >  > > >
> >  > > >  --
> >  > > >  Jens Axboe
> >  > > >
> >  > > >
> >  > >
> [snip]
> ...
> [snip]
> >  >
> >  > They seem to start out the same, but then CFQ gets interrupted by a
> >  > timer unplug (which is also odd) and after that the request size drops.
> >  > On most devices you don't notice, but some are fairly picky about
> >  > request sizes. The end result is that CFQ has an average dispatch
> >  > request size of 142kb, where AS is more than double that at 306kb. I'll
> >  > need to analyze the data and look at the code a bit more to see WHY this
> >  > happens.
> >
> >  Here's a test patch, I think we get into this situation due to CFQ being
> >  a bit too eager to start queuing again. Not tested, I'll need to spend
> >  some testing time on this. But I'd appreciate some feedback on whether
> >  this changes the situation! The final patch will be a little more
> >  involved.
> [snip]
> ...
> [snip]
> >
> >  --
> >  Jens Axboe
> >
> >
> 
> unfortunately that patch didn't help:
> 
> hdparm -t /dev/sde
> 
> /dev/sde:
>  Timing buffered disk reads:  178 MB in  3.03 seconds =  58.67 MB/sec
> 
> 
> hdparm -t /dev/sdd
> 
> /dev/sdd:
>  Timing buffered disk reads:  164 MB in  3.00 seconds =  54.61 MB/sec
> 
> -> the first should be around 74 MB/sec, the second around 102 MB/sec

Can you capture blktrace for that run as well, please? Just to have
something to compare with.

-- 
Jens Axboe