linux-kernel - Re: Re: cfq-iosched.c:Use cfqq->nr

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Date:	Thu, 31 Mar 2011 15:46:56 -0400
From:	Vivek Goyal <vgoyal@...hat.com>
To:	Lina Lu <lulina_nuaa@...mail.com>
Cc:	linux kernel mailing list <linux-kernel@...r.kernel.org>
Subject: Re: Re: cfq-iosched.c:Use cfqq->nr_sectors in charge the vdisktime

On Thu, Mar 31, 2011 at 11:46:37PM +0800, Lina Lu wrote:
> 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.  

How did you put device in iops mode? What's the device you are using and
what kind of configuration dm-0 and dm-1 are in.

> 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;

In IOPS mode we calculate the number of IOPS (that is number of requests
dispatched) and not number of sectors. nr_sectors is more of getting
the equal bandwidth even when we are operating at different request sizes.
So instead of operating in iops mode, if you operate in regular time
based mode, you should get better results.

Why are you not using regular time based fairness mode?

Thanks
Vivek 
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