Re: Too many I/O controller patches

On Mon, 2008-08-04 at 22:44 +0200, Andrea Righi wrote:

Dave Hansen wrote:

quoted

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

quoted

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.

-- Dave