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



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.

Paul


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