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[dm-devel] Re: Too many I/O controller patches



On Tue, 05 Aug 2008 09:20:18 -0700
Dave Hansen <dave linux 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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