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

[Balbir Singh <balbir linux vnet ibm com>]

KAMEZAWA Hiroyuki wrote:
> 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.

Yes, please solve the I/O control problem first.

-- 
	Warm Regards,
	Balbir Singh
	Linux Technology Center
	IBM, ISTL