Re: [PATCH 7/9] block: implement bio_associate_current()

[Chris Wright <chrisw@redhat.com>]

* Vivek Goyal (vgoyal@redhat.com) wrote:
> On Mon, Feb 20, 2012 at 08:59:22AM -0800, Tejun Heo wrote:
> > On Mon, Feb 20, 2012 at 09:22:33AM -0500, Vivek Goyal wrote:
> > > I guess you will first determine cfqq associated with cic and then do
> > > 
> > > cfqq->cfqg->blkg->blkcg == bio_blkcg(bio)
> > > 
> > > One can do that but still does not get rid of requirement of checking
> > > for CGRPOUP_CHANGED as not every bio will have cgroup information stored
> > > and you still will have to check whether submitting task has changed
> > > the cgroup since it last did IO.
> > 
> > Hmmm... but in that case task would be using a different blkg and the
> > test would still work, wouldn't it?
> 
> Oh.., forgot that bio_blkio_blkcg() returns the current tasks's blkcg if
> bio->blkcg is not set. So if a task's cgroup changes, bio_blkcg() will
> point to latest cgroup and cfqq->cfqg->blkg->blkcg will point to old
> cgroup and test will indicate the discrepancy. So yes, it should work
> for both the cases.
> 
> > 
> > > > blkcg doesn't allow that anyway (it tries but is racy) and I actually
> > > > was thinking about sending a RFC patch to kill CLONE_IO.
> > > 
> > > I thought CLONE_IO is useful and it allows threads to share IO context.
> > > qemu wanted to use it for its IO threads so that one virtual machine
> > > does not get higher share of disk by just craeting more threads. In fact
> > > if multiple threads are doing related IO, we would like them to use
> > > same io context.
> > 
> > I don't think that's true.  Think of any multithreaded server program
> > where each thread is working pretty much independently from others.
> 
> If threads are working pretty much independently, then one does not have
> to specify CLONE_IO.
> 
> In case of qemu IO threads, I have debugged issues where an big IO range
> is being splitted among its IO threads. Just do a sequential IO inside
> guest, and I was seeing that few sector IO comes from one process, next
> few sector come from other process and it goes on. A sequential range
> of IO is some split among a bunch of threads and that does not work
> well with CFQ if every IO is coming from its own IO context and IO
> context is not shared. After a bunch of IO from one io context, CFQ
> continues to idle on that io context thinking more IO will come soon.
> Next IO does come but from a different thread and differnet context.
> 
> CFQ now has employed some techniques to detect that case and try
> to do preemption and try to reduce idling in such cases. But sometimes
> these techniques work well and other times don't.  So to me, CLONE_IO
> can help in this case where application can specifically share
> IO context and CFQ does not have to do all the tricks.
> 
> That's a different thing that applications might not be making use
> of CLONE_IO.
> 
> > Virtualization *can* be a valid use case but are they actually using
> > it?  Aren't they better served by cgroup?
> 
> cgroup can be very heavy weight when hundred's of virtual machines
> are running. Why? because of idling. CFQ still has lots of tricks
> to do preemption and cut down on idling across io contexts, but
> across cgroup boundaries, isolation is much more stronger and very
> little preemption (if any) is allowed. I suspect in current
> implementation, if we create lots of blkio cgroup, it will be 
> bad for overall throughput of virtual machines (purely because of
> idling).
> 
> So I am not too excited about blkio cgroup solution because it might not
> scale well. (Until and unless we find a better algorithm to cut down
> on idling).
> 
> I am ccing Chris Wright <chrisw@redhat.com>. He might have thoughts
> on usage of CLONE_IO and qemu.

Vivek, you summed it up pretty well.  Also, for qemu, raw CLONE_IO is not
an option because threads are created via pthread (we had done some local
hacks to verify that CLONE_IO helped w/ the idling problem, and it did).

thanks,
-chris