Re: [PATCH] Avoid preferential treatment of groups that aren't backlogged

[Vivek Goyal <vgoyal@redhat.com>]

On Fri, Feb 11, 2011 at 01:15:33PM -0500, Vivek Goyal wrote:
> On Thu, Feb 10, 2011 at 04:36:25PM -0800, Chad Talbott wrote:
> > On Thu, Feb 10, 2011 at 10:57 AM, Chad Talbott <ctalbott@google.com> wrote:
> > > On Wed, Feb 9, 2011 at 7:57 PM, Vivek Goyal <vgoyal@redhat.com> wrote:
> > >> If you ran different random readers in different groups of differnet
> > >> weight with group_isolation=1, then there is a case of having service
> > >> differentiation. In that case we will idle for 8ms on each group before
> > >> we expire the group. So in these test cases are low weight groups not
> > >> submitting IO with-in 8ms? Putting a random reader in separate group
> > >> with think time > 8, I think is going to hurt a lot because for every
> > >> single IO dispatched group is going to weight for 8ms before it is
> > >> expired.
> > >
> > > You're right about the behavior of group_idle.  We have more
> > > experience with earlier kernels (before group_idle).  With this patch
> > > we are able to achieve isolation without group_idle even with these
> > > large ratios.  (Without group_idle the random reader workloads will
> > > get marked seeky, and idling is disabled.  Without group_idle, we have
> > > to remember the vdisktime to get isolation.)
> > >
> > >> Can you run blktrace and verify what's happenig?
> > >
> > > I can run a blktrace, and I think it will show what you expect.
> > 
> > So, I ran the following two tests and took a blktrace.
> > 
> > 950 rdrand, 50 rdrand.delay10
> > weight 950 random reader with low think time vs weight 50 random
> > reader with 10ms think time
> > 
> > 950 rdrand, 50 rdrand.delay50 # 50ms think time
> > weight 950 random reader with low think time vs weight 50 random
> > reader with 50ms think time
> > 
> > I find that we are still idling for these random readers, even the one
> > with 50ms think time.  group_idle is 0 according to blktrace.
> > 
> > With this patch, both of these cases have correct isolation.  Without
> > this patch, the small weight reader is able to get more than its
> > share.
> > 
> > I think that idling for a random reader with a 50ms think time is
> > likely a bug, but a separate issue.
> 
> Thanks for checking this out. I agree that for a low weight random
> reader/writer which high think time, we need to remember the vdisktime
> otherwise it will showup as a fresh new candidate and get more done.
> 
> Having said that, one can say that random reader/writer doing small
> amount of IO should be able to get job done really fast and the one
> who are hogging the disk for long time, should get higher vdisktime.
> 
> So with this scheme, a random reader/writer shall have to be of higher
> weight to get the job done fast. A low weight reader/writer will still
> get higher vdisktime and get lesser share. I think it is reasonable.
> 
> And yes, even with group_idle=0 if we are idling on a 50ms thinktime
> random reader it sounds like a bug.

Thinking more about it, I think it must be happening because of the fact
that random IO goes on sync-noidle tree of group and there we idle on
whole tree. I think if you set slice_idle=0 along with group_idle=0, that
idling should go away.

Thanks
Vivek