Messages in this thread | | | Date | Tue, 28 Feb 2012 09:10:36 -0500 | From | Vivek Goyal <> | Subject | Re: [PATCH 7/9] block: implement bio_associate_current() |
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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.
Thanks Vivek
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