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

[Balbir Singh <balbir linux vnet ibm com>]

Paul Menage wrote:
> On Mon, Aug 4, 2008 at 1:44 PM, Andrea Righi <righi andrea gmail com> wrote:
>> A safer approach IMHO is to force the tasks to wait synchronously on
>> each operation that directly or indirectly generates i/o.
>>
>> In particular the solution used by the io-throttle controller to limit
>> the dirty-ratio in memory is to impose a sleep via
>> schedule_timeout_killable() in balance_dirty_pages() when a generic
>> process exceeds the limits defined for the belonging cgroup.
>>
>> Limiting read operations is a lot more easy, because they're always
>> synchronized with i/o requests.
> 
> I think that you're conflating two issues:
> 
> - controlling how much dirty memory a cgroup can have at any given
> time (since dirty memory is much harder/slower to reclaim than clean
> memory)
> 
> - controlling how much effect a cgroup can have on a given I/O device.
> 
> By controlling the rate at which a task can generate dirty pages,
> you're not really limiting either of these. I think you'd have to set
> your I/O limits artificially low to prevent a case of a process
> writing a large data file and then doing fsync() on it, which would
> then hit the disk with the entire file at once, and blow away any QoS
> guarantees for other groups.
> 
> As Dave suggested, I think it would make more sense to have your
> page-dirtying throttle points hook into the memory controller instead,
> and allow the memory controller to track/limit dirty pages for a
> cgroup, and potentially do throttling as part of that.

Yes, that would be nicer. The IO controller should control both read and write
and dirty pages is mostly related to writes.

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