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

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Message-ID: <AANLkTimO+CG_iCHZ+9D1EtrZp+z0k0+xUN_WMsqU3NNk@mail.gmail.com>
Date:	Thu, 10 Feb 2011 11:06:37 -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 Wed, Feb 9, 2011 at 8:02 PM, Vivek Goyal <vgoyal@...hat.com> wrote:
> - What happens when cfqd->active_generation wraps over? Should we use
>  functions which take care of wrapping.

To be absolutely correct, you're right.  But even if the service tree
becomes completely idle every microsecond, we still have 1/2 million
years before the first wrap.

> - So when generation number changes you want to put the newly backlogged
>  group at the front of tree and not at the end of it? Though it kind
>  of make sense as any continuously backlogged groups will be on service
>  tree for long time and newly backlogged groups are either new IO
>  starting or some application which high think time and which does IO
>  one in a while and does not keep disk occupied for very long time. In
>  such cases it probably is not a bad idea to put newly backlogged
>  groups at the beginning of the tree.

Yes, that's it exactly.

>> @@ -906,6 +905,9 @@ cfq_group_service_tree_del(struct cfq_data *cfqd, struct cfq_group *cfqg)
>>       if (!RB_EMPTY_NODE(&cfqg->rb_node))
>>               cfq_rb_erase(&cfqg->rb_node, st);
>>       cfqg->saved_workload_slice = 0;
>> +     cfqg->generation_num = cfqd->active_generation;
>> +     if (RB_EMPTY_ROOT(&st->rb))
>> +             cfqd->active_generation++;
>
> I don't understand that what is the significance behind chaning generation
> number when tree is idle? When tree is idle does not mean that few
> recently deleted groups will not get backlogged soon?

It's a result of being both fair and work conserving.  If *all* the
queues are without requests, then whoever next has requests should get
to use the disk fully to be work-conserving.  But if the disk is
always kept busy (by anyone), then there is competition and we have to
provide fairness.

Chad
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