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

[Jens Axboe]

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
> > 
> > --- [1]
> > 
> > # cat /sys/block/sda/queue/iosched/slice_idle
> > 8
> > # echo 1 >/proc/sys/vm/drop_caches
> > # dd if=/dev/sda of=/dev/null bs=64k count=5000
> > 5000+0 records in
> > 5000+0 records out
> > 327680000 bytes (328 MB) copied, 4.92922 s, 66.5 MB/s
> > 
> > # echo 0 >/sys/block/sda/queue/iosched/slice_idle
> > # echo 1 >/proc/sys/vm/drop_caches
> > # dd if=/dev/sda of=/dev/null bs=64k count=5000
> > 5000+0 records in
> > 5000+0 records out
> > 327680000 bytes (328 MB) copied, 2.74098 s, 120 MB/s
> > 
> > # hdparm -Tt /dev/sda
> > 
> > /dev/sda:
> >  Timing cached reads:   15464 MB in  2.00 seconds = 7741.05 MB/sec
> >  Timing buffered disk reads:  342 MB in  3.01 seconds = 113.70 MB/sec
> > 
> > [120MB/s is known platter-rate for this disc, so expected]
> 
> This appears to be what i get aswell..
> 
> root@quadstation # dd if=/dev/sda of=/dev/null bs=64k count=5000
> 5000+0 records in
> 5000+0 records out
> 327680000 bytes (328 MB) copied, 5.48209 s, 59.8 MB/s
> root@quadstation # echo 0 >/sys/block/sda/queue/iosched/slice_idle
> root@quadstation # echo 1 >/proc/sys/vm/drop_caches
> root@quadstation # dd if=/dev/sda of=/dev/null bs=64k count=5000
> 5000+0 records in
> 5000+0 records out
> 327680000 bytes (328 MB) copied, 2.93932 s, 111 MB/s
> root@quadstation # hdparm -Tt /dev/sda
>  Timing cached reads:   7264 MB in  2.00 seconds = 3633.82 MB/sec
>  Timing buffered disk reads:  322 MB in  3.01 seconds = 107.00 MB/se
> root@quadstation # echo 0 >/sys/block/sda/queue/iosched/slice_idle
> root@quadstation # echo 1 >/proc/sys/vm/drop_caches
> root@quadstation # hdparm -Tt /dev/sda
>  Timing cached reads:   15268 MB in  2.00 seconds = 7643.54 MB/sec
>  Timing buffered disk reads:  328 MB in  3.01 seconds = 108.85 MB/sec
> 
> 
> To be sure, i did it all again:
> noop:
> root@quadstation # echo 1 >/proc/sys/vm/drop_caches
> root@quadstation # dd if=/dev/sda of=/dev/null bs=64k count=5000
> 5000+0 records in
> 5000+0 records out
> 327680000 bytes (328 MB) copied, 2.85503 s, 115 MB/s
> root@quadstation # echo 1 >/proc/sys/vm/drop_caches
> root@quadstation # hdparm -tT /dev/sda
>  Timing cached reads:   14076 MB in  2.00 seconds = 7045.78 MB/sec
>  Timing buffered disk reads:  328 MB in  3.01 seconds = 109.12 MB/sec
> 
> anticipatory:
> root@quadstation # echo 1 >/proc/sys/vm/drop_caches
> root@quadstation # dd if=/dev/sda of=/dev/null bs=64k count=5000
> 5000+0 records in
> 5000+0 records out
> 327680000 bytes (328 MB) copied, 2.96948 s, 110 MB/s
> root@quadstation # echo 1 >/proc/sys/vm/drop_caches
> root@quadstation # hdparm -tT /dev/sda
>  Timing cached reads:   13424 MB in  2.00 seconds = 6719.29 MB/sec
>  Timing buffered disk reads:  328 MB in  3.01 seconds = 109.13 MB/sec
> 
> cfq:
> root@quadstation # echo 1 >/proc/sys/vm/drop_caches
> root@quadstation # dd if=/dev/sda of=/dev/null bs=64k count=5000
> 5000+0 records in
> 5000+0 records out
> 327680000 bytes (328 MB) copied, 5.25252 s, 62.4 MB/s
> root@quadstation # echo 1 >/proc/sys/vm/drop_caches
> root@quadstation # hdparm -tT /dev/sda
>  Timing cached reads:   13434 MB in  2.00 seconds = 6723.59 MB/sec
>  Timing buffered disk reads:  188 MB in  3.00 seconds =  62.57 MB/sec
> 
> Thisd would appear to be quite a considerable performance difference.

Indeed, that is of course a bug. The initial mail here mentions this as
a regression - which kernel was the last that worked ok?

If someone would send me a blktrace of such a slow run, that would be
nice. Basically just do a blktrace /dev/sda (or whatever device) while
doing the hdparm, preferably storing output files on a difference
device. Then send the raw sda.blktrace.* files to me. Thanks!

-- 
Jens Axboe

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