On Mon, Mar 05, 2012 at 11:22:26AM -0800, Fengguang Wu wrote:
[..]
> > This is an interesting and complicated topic. As you mentioned we have had
> > tried to solve it but nothing has been merged yet. Personally, I am still
> > interested in having a discussion and see if we can come up with a way
> > forward.
>
> I'm interested, too. Here is my attempt on the problem a year ago:
>
> blk-cgroup: async write IO controller ("buffered write" would be more precise)
> https://github.com/fengguang/linux/commit/99b1ca4549a79af736ab03247805f6a9fc31ca2d
> https://lkml.org/lkml/2011/4/4/205
That was a proof of concept. Now we will need to provide actual user
visibale knobs and integrate with one of the existing controller (memcg
or blkcg).
[..]
> > Anyway, ideas to have better control of write rates are welcome. We have
> > seen issues wheren a virtual machine cloning operation is going on and
> > we also want a small direct write to be on disk and it can take a long
> > time with deadline. CFQ should still be fine as direct IO is synchronous
> > but deadline treats all WRITEs the same way.
> >
> > May be deadline should be modified to differentiate between SYNC and ASYNC
> > IO instead of READ/WRITE. Jens?
>
> In general users definitely need higher priorities for SYNC writes. It
> will also enable the "buffered write I/O controller" and "direct write
> I/O controller" to co-exist well and operate independently this way:
> the direct writes always enjoy higher priority than the flusher, but
> will be rate limited by the already upstreamed blk-cgroup I/O
> controller. The remaining disk bandwidth will be split among the
> buffered write tasks by another I/O controller operating at the
> balance_dirty_pages() level.
Ok, so differentiating IO among SYNC/ASYNC makes sense and it probably
will make sense in case of deadline too. (Until and unless there is a
reason to keep it existing way).
I am little vary of keeping "dirty rate limit" separate from rest of the
limits as configuration of groups becomes even harder. Once you put a
workload in a cgroup, now you need to configure multiple rate limits.
"reads and direct writes" limit + "buffered write rate limit". To add
to the confusion, it is not just direct write limit, it also is a limit
on writethrough writes where fsync writes will show up in the context
of writing thread.
But looks like we don't much choice. As buffered writes can be controlled
at two levels, we probably need two knobs. Also controlling writes while
entring cache limits will be global and not per device (unlinke currnet
per device limit in blkio controller). Having separate control for "dirty
rate limit" leaves the scope for implementing write control at device
level in the future (As some people prefer that). In possibly two
solutions can co-exist in future.
Assuming this means that we both agree that three should be some sort of
knob to control "dirty rate", question is where should it be. In memcg
or blkcg. Given the fact we are controlling the write to memory and
we are already planning to have per memcg dirty ratio and dirty bytes,
to me it will make more sense to integrate this new limit with memcg
instead of blkcg. Block layer does not even come into the picture at
that level hence implementing something in blkcg will be little out of
place?
Thanks
Vivek
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