linux-kernel - Re: i/o bandwidth controller infrastructure

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Date:	Tue, 17 Jun 2008 00:39:26 +0200 (MEST)
From:	Andrea Righi <righi.andrea@...il.com>
To:	Divyesh Shah <dpshah@...gle.com>
Cc:	balbir@...ux.vnet.ibm.com, menage@...gle.com,
	linux-kernel@...r.kernel.org, axboe@...nel.dk, matt@...ehost.com,
	roberto@...it.it, randy.dunlap@...cle.com,
	akpm@...ux-foundation.org
Subject: Re: i/o bandwidth controller infrastructure

Divyesh Shah wrote:
>> This is the core io-throttle kernel infrastructure. It creates the
>> basic
>> interfaces to cgroups and implements the I/O measurement and
>> throttling
>> functions.
> 
> I am not sure if throttling an application's cpu usage by explicitly  
> putting it to sleep
> in order to restrain it from making more IO requests is the way to go  
> here (though I can't think
> of anything better right now).
> With this bandwidth controller, a cpu-intensive job which otherwise  
> does not care about its IO
> performance needs to be pin-point accurate about IO bandwidth  
> required in order to not suffer
> from cpu-throttling. IMHO, if a cgroup is exceeding its limit for a  
> given resource, the throttling
> should be done _only_ for that resource.
> 
> -Divyesh

Divyesh,

I understand your point of view. It would be nice if we could just
"disable" the i/o for a cgroup that exceeds its limit, instead of
scheduling some sleep()s, so the tasks running in this cgroup would be
able to continue their non-i/o operations as usual.

However, how to do if the tasks continue to perform i/o ops under this
condition? we could just cache the i/o in memory and at the same time
reduce the i/o priority of those tasks' requests, but this would require
a lot of memory, more space in the page cache, and probably could lead
to potential OOM conditions. A safer approach IMHO is to force the tasks
to wait synchronously on each operation that directly or indirectly
generates i/o. The last one is the solution implemented by this
bandwidth controller.

We could collect additional statistics, or implement some heuristics to
predict the tasks' i/o patterns in order to not penalize cpu-bound jobs
too much, but the basic concept is the same.

Anyway, I agree there must be a better solution, but this is the best
I've found right now... nice ideas are welcome.

-Andrea
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