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Hirokazu Takahashi 08-05-2008 06:16 AM

Too many I/O controller patches
 
Hi, Andrea,

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

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

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Hirokazu Takahashi 08-05-2008 06:28 AM

Too many I/O controller patches
 
Hi,

> > >> 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...
> >
> > A safer approach IMHO is to force the tasks to wait synchronously on
> > each operation that directly or indirectly generates i/o.
>
> Fine in theory, hard in practice. :)
>
> I think the best we can hope for is to keep parity with what happens in
> the rest of the kernel. We already have a problem today with people
> mmap()'ing lots of memory and dirtying it all at once. Adding a i/o
> bandwidth controller or a memory controller isn't really going to fix
> that. I think it is outside the scope of the i/o (and memory)
> controllers until we solve it generically, first.

Yes, that's right. This should be solved.

But there is a good thing when you use a memory controller.
A problem occurred in a certain cgroup will be confined in its cgroup.
I think this is a great point, don't you think?

Thank you,
Hirokazu Takahashi.




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