Messages in this thread | | | Date | Mon, 27 Feb 2012 15:12:22 -0800 | From | Chris Wright <> | Subject | Re: [PATCH 7/9] block: implement bio_associate_current() |
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* 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
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