Re: Too many I/O controller patches

Hi, Andrea,

I'm working with Ryo on dm-ioband and other stuff.

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On Mon, 2008-08-04 at 20:22 +0200, Andrea Righi wrote:

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But I'm not yet convinced that limiting the IO writes at the device
mapper layer is the best solution. IMHO it would be better to throttle
applications' writes when they're dirtying pages in the page cache (the
io-throttle way), because when the IO requests arrive to the device
mapper it's too late (we would only have a lot of dirty pages that are
waiting to be flushed to the limited block devices, and maybe this could
lead to OOM conditions). IOW dm-ioband is doing this at the wrong level
(at least for my requirements). Ryo, correct me if I'm wrong or if I've
not understood the dm-ioband approach.

The avoid-lots-of-page-dirtying problem sounds like a hard one.  But, if
you look at this in combination with the memory controller, they would
make a great team.

The memory controller keeps you from dirtying more than your limit of
pages (and pinning too much memory) even if the dm layer is doing the
throttling and itself can't throttle the memory usage.

mmh... but in this way we would just move the OOM inside the cgroup,
that is a nice improvement, but the main problem is not resolved...

The concept of dm-ioband includes it should be used with cgroup memory
controller as well as the bio cgroup. The memory controller is supposed
to control memory allocation and dirty-page ratio inside each cgroup.

Some guys of cgroup memory controller team just started to implement
the latter mechanism. They try to make each cgroup have a threshold
to limit the number of dirty pages in the group.

I feel this is good approach since each functions can work independently.

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.

I guess it would make the memory controller team guys happier if you
can help them design their dirty-page ratio controlling functionality
much cooler and more generic. I think their goal is almost the same
as yours.

Limiting read operations is a lot more easy, because they're always
synchronized with i/o requests.

-Andrea

Thank you,
Hirokazu Takahashi.