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

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Date:	Thu, 10 Feb 2011 16:36:25 -0800
From:	Chad Talbott <ctalbott@...gle.com>
To:	Vivek Goyal <vgoyal@...hat.com>
Cc:	jaxboe@...ionio.com, guijianfeng@...fujitsu.com, mrubin@...gle.com,
	teravest@...gle.com, jmoyer@...hat.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Avoid preferential treatment of groups that aren't backlogged

On Thu, Feb 10, 2011 at 10:57 AM, Chad Talbott <ctalbott@...gle.com> wrote:
> On Wed, Feb 9, 2011 at 7:57 PM, Vivek Goyal <vgoyal@...hat.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.

Chad
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