Re: [RFC] writeback and cgroup

On Thu, Apr 05, 2012 at 09:38:54AM -0700, Tejun Heo wrote:

Hey, Vivek.

On Wed, Apr 04, 2012 at 11:49:09AM -0700, Tejun Heo wrote:

quoted

quoted

I am not sure what are you trying to say here. But primarily blk-throttle
will throttle read and direct IO. Buffered writes will go to root cgroup
which is typically unthrottled.

Ooh, my bad then.  Anyways, then the same applies to blk-throttle.
Our current implementation essentially collapses at the face of
write-heavy workload.

I went through the code and couldn't find where blk-throttle is
discriminating async IOs.  Were you saying that blk-throttle currently
doesn't throttle because those IOs aren't associated with the dirtying
task?

Yes that's what I meant. Currently most of the async IO will come from
flusher thread which is in root cgroup. So all the async IO will be in
root group and we typically keep root group unthrottled. Sorry for the
confusion here.

If so, note that it's different from cfq which explicitly
assigns all async IOs when choosing cfqq even if we fix tagging.

Yes. So if we can properly account for submitter, and for blk-throttle,
async IO will go in right cgroup. Unlike CFQ, there is no hard coded logic
to keep async IO in a particular group. It is just a matter of getting
the right cgroup information.

Thanks
Vivek