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

[Chris Wright <chrisw@redhat.com>]

* Vivek Goyal (vgoyal@redhat.com) wrote:
> On Mon, Feb 27, 2012 at 03:12:22PM -0800, Chris Wright wrote:
> 
> [..]
> > > > > > 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).
> 
> Chris,
> 
> Just to make sure I understand it right I am thinking loud.
> 
> That means CLONE_IO is useful and ideally qemu would like to make use of it
> but beacuse pthread interface does not support it, it is not used as of
> today.

It depends on the block I/O model in qemu.  It can use either a pthread
pool or native aio.  All of the CFQ + idling problems we encountered were
w/ a pthread pool (and IIRC, those would have predated preadv/pwritev,
so the actual i/o vector from the guest was pulled apart and submitted as
a bunch of discrete pread/pwrite requests), however in many cases simply
using aio may be the better sol'n.  Stefan, Kevin, have any thoughts re:
qemu block I/O and making use of CLONE_IO in a modern qemu?

thanks,
-chris