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

[Stefan Hajnoczi]

On Tue, Feb 28, 2012 at 5:01 PM, Chris Wright <chrisw@xxxxxxxxxx> wrote:
> * Vivek Goyal (vgoyal@xxxxxxxxxx) 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@xxxxxxxxxx>. 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?

What you say makes sense.  Today QEMU uses either Linux AIO or a
thread pool with preadv()/pwritev().  Vectored requests are passed to
the host kernel, they are no longer broken up into multiple
read()/write() calls.

QEMU takes requests from an emulated storage interface like virtio-blk
or IDE and issues them.  If the guest submits multiple requests at
once QEMU will try to issue them in parallel.

In the preadv()/pwritev() thread pool case QEMU makes no attempt at
binding sequential I/O patterns to the same worker thread.  Therefore
I think idle or anticipatory wait doesn't make sense for worker
threads - they have no state and simply grab the next available
request off a queue.  After 10 seconds of idle time a worker thread
will terminate.

I think it makes sense to use CLONE_IO for QEMU worker threads.

By the way, glibc isn't using CLONE_IO for its own POSIX aio
implementation either, so this flag really seems underused.

Stefan
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