Re: Re: cfq-iosched.c:Use cfqq->nr_sectors in charge the vdisktime
From: Lina Lu
Date: Thu Mar 31 2011 - 11:47:48 EST
On 2011-03-30 23:54:34, Vivek Goyal wrote:
> On Wed, Mar 30, 2011 at 11:23:30PM +0800, Lina Lu wrote:
> > Hi Vivek,
> > I find the weight policy can be more accuracy with cfqq->nr_sectors instead
> > of cfqq->slice_dispatch.
> > Today, I try to modify cfq_group_served(), and use "charge = cfqq->nr_sectors; "
> > instead of "charge = cfqq->slice_dispatch; " . The test result seens more accuracy.
> > Why you choose slice_dispatch here? Is the nr_sectors will lower the total performance?
>
> Lina,
>
> CFQ fundamentally allocates time slices hence accounting is done in time
> and not in terms of sectors. The other reason is that accounting in
> terms of time can be more accurate where some process is seeking all
> over the disk and doing little IO. If we account in terms of sectors
> then such seeky process will get much more share.
>
> > And in iops mod, if I try to apply weight policy on two IO processes with different
> > avgrq-sz, the test results will not exact match the weight value.
>
> IOPS mode kicks in when slice_idle=0. I suspect that group does not drive
> enough IO to remain on service tree hence gets deleted and hence loses
> share.
>
> Can you run a 20 sec backtrace and upload it somewhere.
>
Here is 20 sec backtrace:
http://www.fileden.com/files/2010/9/9/2965145/cfq_log.tar.gz
This time, I set two IO pid with weight 100, and the device is in iops_mod.
linux-kzr4:/home/blkio # cat tst1/blkio.weight
100
linux-kzr4:/home/blkio # cat tst2/blkio.weight
100
iostat:
Device: rrqm/s wrqm/s r/s w/s rMB/s wMB/s avgrq-sz avgqu-sz await svctm %util
dm-0 0.00 0.00 855.50 0.00 3.34 0.00 8.00 0.82 1.06 0.95 81.70
dm-1 0.00 0.00 844.00 0.00 26.38 0.00 64.00 0.83 0.98 0.98 82.60
Device: rrqm/s wrqm/s r/s w/s rMB/s wMB/s avgrq-sz avgqu-sz await svctm %util
dm-0 0.00 0.00 840.00 0.00 3.28 0.00 8.00 0.90 0.95 1.07 89.55
dm-1 0.00 0.00 794.00 0.00 24.81 0.00 64.00 0.87 1.10 1.10 87.00
Device: rrqm/s wrqm/s r/s w/s rMB/s wMB/s avgrq-sz avgqu-sz await svctm %util
dm-0 0.00 0.00 596.50 0.00 2.33 0.00 8.00 0.96 1.77 1.61 95.80
dm-1 0.00 0.00 626.00 0.00 19.56 0.00 64.00 0.94 1.48 1.50 93.70
Device: rrqm/s wrqm/s r/s w/s rMB/s wMB/s avgrq-sz avgqu-sz await svctm %util
dm-0 0.00 0.00 815.50 0.00 3.19 0.00 8.00 0.81 0.83 1.00 81.40
dm-1 0.00 0.00 828.50 0.00 25.89 0.00 64.00 0.77 0.95 0.93 77.45
Device: rrqm/s wrqm/s r/s w/s rMB/s wMB/s avgrq-sz avgqu-sz await svctm %util
dm-0 0.00 0.00 910.50 0.00 3.56 0.00 8.00 0.82 1.00 0.90 82.15
dm-1 0.00 0.00 845.00 0.00 26.41 0.00 64.00 0.81 0.96 0.96 80.95
Device: rrqm/s wrqm/s r/s w/s rMB/s wMB/s avgrq-sz avgqu-sz await svctm %util
dm-0 0.00 0.00 928.86 0.00 3.63 0.00 8.00 0.79 0.90 0.86 79.45
dm-1 0.00 0.00 848.26 0.00 26.51 0.00 64.00 0.65 0.77 0.77 65.17
>From the result, we can see that the iops match the weight value very well, but
the rMB/s are not the same as they has different avgrq-sz.
If I use the following patch, the rMB/s will be more accuracy.
--- block/cfq-iosched.c 2011-03-31 23:43:55.000000000 +0800
+++ block/cfq-iosched.c 2011-03-31 23:44:30.000000000 +0800
@@ -951,7 +951,7 @@
used_sl = charge = cfq_cfqq_slice_usage(cfqq);
if (iops_mode(cfqd))
- charge = cfqq->slice_dispatch;
+ charge = cfqq->nr_sectors;
else if (!cfq_cfqq_sync(cfqq) && !nr_sync)
charge = cfqq->allocated_slice;
Thanks
Lina
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