linux-kernel - Re: Too many I/O controller patches

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Date:	Wed, 6 Aug 2008 11:44:25 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Dave Hansen <dave@...ux.vnet.ibm.com>
Cc:	righi.andrea@...il.com, Satoshi UCHIDA <s-uchida@...jp.nec.com>,
	xen-devel@...ts.xensource.com,
	containers@...ts.linux-foundation.org,
	linux-kernel@...r.kernel.org, vtaras@...nvz.org,
	dm-devel@...hat.com, agk@...rceware.org,
	virtualization@...ts.linux-foundation.org, ngupta@...gle.com
Subject: Re: Too many I/O controller patches

On Tue, 05 Aug 2008 09:20:18 -0700
Dave Hansen <dave@...ux.vnet.ibm.com> wrote:

> On Tue, 2008-08-05 at 11:28 +0200, Andrea Righi wrote:
> > > Buffered write I/O is also related with cache system.
> > > We must consider this problem as I/O control.
> > 
> > Agree. At least, maybe we should consider if an IO controller could be
> > a valid solution also for these problems.
> 
> Isn't this one of the core points that we keep going back and forth
> over?  It seems like people are arguing in circles over this:
> 
> Do we:
> 	1. control potential memory usage by throttling I/O
> or
> 	2. Throttle I/O when memory is full
> 
> I might lean toward (1) if we didn't already have a memory controller.
> But, we have one, and it works.  Also, we *already* do (2) in the
> kernel, so it would seem to graft well onto existing mechanisms that we
> have.
> 
> I/O controllers should not worry about memory.  
I agree here ;)

>They're going to have a hard enough time getting the I/O part right. :)
> 
memcg have more problems now ;( 

Only a difficult thing to limit dirty-ratio in memcg is how-to-count dirty
pages. If I/O controller's hook helps, it's good.

My small concern is "What happens if we throttole I/O bandwidth too small
under some memcg." In such cgroup, we may see more OOMs because I/O will
not finish in time.
A system admin have to find some way to avoid this.

But please do I/O control first. Dirty-page control is related but different
layer's problem, I think.

Thanks,
-Kame

> Or, am I over-simplifying this?
> 



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