lkml.org 
[lkml]   [2010]   [Jul]   [16]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
From
SubjectRe: [PATCH] [RFC] CFQ: Make prio_trees per cfq group basis to improve IO performance
Date
Vivek Goyal <vgoyal@redhat.com> writes:

> On Fri, Jul 16, 2010 at 05:21:00PM +0800, Gui Jianfeng wrote:
>> Currently, prio_trees is global, and we rely on cfqq_close() to search
>> a coorperator. If the returned cfqq and the active cfqq don't belong to
>> the same group, coorperator searching fails. Actually, that's not the case.
>> Even if cfqq_close() returns a cfqq which belong to another cfq group,
>> it's still likely that a coorperator(same cfqg) resides in prio_trees.
>> This patch introduces per cfq group prio_trees that should solve the above
>> issue.
>>
>
> Hi Gui,
>
> I am not sure I understand the issue here. So are you saying that once
> we find a cfqq which is close but belongs to a different group we reject
> it. But there could be another cfqq in the same group which is not as
> close but still close enough.
>
> For example, assume there are two queues q1 and q2 and in group and third
> queue q3 in group B. Assume q1 is active queue and we are searching for
> cooperator. If cooperator code finds q3 as closest then we will not pick
> this queue as it belongs to a different group. But it could happen that
> q2 is also close enough and we never considered that possibility.
>
> If yes, then its a good theoritical concern but I am worried practically
> how often does it happen. Do you have any workload which suffers because
> of this?

That was my reading. It also means that, in the case that we have
cgroups in use, each rb tree will be smaller.

> I am not too inclined to push more complexity in CFQ until and unless we
> have a good use case.

I don't think this adds complexity, does it? It simply moves the
priority trees up a level, which is arguably where they belong.

>> +static struct cfq_queue *
>> +cfq_prio_tree_lookup(struct cfq_group *cfqg, struct rb_root *root,
>> + sector_t sector, struct rb_node **ret_parent,
>> + struct rb_node ***rb_link)
>> +{

You can get rid of the cfqg argument. I know you're just keeping with
the prior model (where cfqd was passed in and not used), but let's kill
it.

Cheers,
Jeff


\
 
 \ /
  Last update: 2010-07-16 16:25    [W:0.049 / U:1.256 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site