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Re: [Qemu-block] [Qemu-devel] Question about QEMU's threading model and


From: Fam Zheng
Subject: Re: [Qemu-block] [Qemu-devel] Question about QEMU's threading model and stacking multiple block drivers
Date: Thu, 9 Feb 2017 09:27:39 +0800
User-agent: Mutt/1.7.1 (2016-10-04)

On Wed, 02/08 11:00, Adrian Suarez wrote:
> >
> > Do you only start submitting request to B (step 3) after the fast device
> > I/O
> > completes (step 2.a)? The fact that they are serialized incurs extra
> > latency.
> > Have you tried to do 2 and 3 in parallel with AIO?
> 
> 
> In step 2, we perform an asynchronous call to the fast device, supplying a
> callback that calls aio_bh_schedule_oneshot() to schedule the completion in
> the AioContext of the block driver. Step 3 uses bdrv_aio_writev(), but I'm
> not sure if this is actually causing the write to be performed
> synchronously to the backing device. What I'm expecting is that
> bdrv_aio_writev() issues the write and then yields so that we don't
> serialize all writes to the backing device.

OK, what I'm wondering is why call bdrv_aio_writev() in a BH instead of right
away. IOW, have you traced how much time is spent before even calling
bdrv_aio_writev()?

> 
> Thanks,
> Adrian
> 
> On Wed, Feb 8, 2017 at 6:30 AM, Fam Zheng <address@hidden> wrote:
> 
> > On Wed, 02/08 14:59, Max Reitz wrote:
> > > CC-ing qemu-block, Stefan, Fam.
> > >
> > >
> > > On 08.02.2017 03:38, Adrian Suarez wrote:
> > > > We’ve implemented a block driver that exposes storage to QEMU VMs. Our
> > > > block driver (O) is interposing on writes to some other type of storage
> > > > (B). O performs low latency replication and then asynchronously issues
> > the
> > > > write to the backing block driver, B, using bdrv_aio_writev(). Our
> > problem
> > > > is that the write latencies seen by the workload in the guest should be
> > > > those imposed by O plus the guest I/O and QEMU stack (around 25us total
> > > > based on our measurements), but we’re actually seeing much higher
> > latencies
> > > > (around 120us). We suspect that this is due to the backing block
> > driver B’s
> > > > coroutines blocking our coroutines. The sequence of events is as
> > follows
> > > > (see diagram:
> > > > https://docs.google.com/drawings/d/12h1QbecvxzlKxSFvGKYAzvAJ18kTW
> > 6AVTwDR6VA8hkw/pub?w=576&h=565
> >
> > I cannot open this, so just trying to understand from steps below..
> >
> > > > ):
> > > >
> > > > 1. Write is issued to our block driver O using the asynchronous
> > interface
> > > > for QEMU block driver.
> > > > 2. Write is replicated to a fast device asynchronously.
> > > > 2.a. In a different thread, the fast device invokes a callback on
> > > > completion that causes a coroutine to be scheduled to run in the QEMU
> > > > iothread that acknowledges completion of the write to the guest OS.
> > > > 2.b. The coroutine scheduled in (2.a) is executed.
> > > > 3. Write is issued asynchronously to the backing block driver, B.
> > > > 3.a. The backing block driver, B, invokes the completion function
> > supplied
> > > > by us, which frees any memory associated with the write (e.g. copies
> > of IO
> > > > vectors).
> >
> > Do you only start submitting request to B (step 3) after the fast device
> > I/O
> > completes (step 2.a)? The fact that they are serialized incurs extra
> > latency.
> > Have you tried to do 2 and 3 in parallel with AIO?
> >
> > > >
> > > > Steps (1), (2), and (3) are performed in the same coroutine (our
> > driver's
> > > > bdrv_aio_writev() implementation). (2.a) is executed in a thread that
> > is
> > > > part of our transport library linked by O, and (2.b) and (3.a) are
> > executed
> > > > as coroutines in the QEMU iothread.
> > > >
> > > > We've tried improving the performance by using separate iothreads for
> > the
> > > > two devices, but this only shaved about lowered the latency to around
> > 100us
> > > > and caused stability issues. What's the best way to create a separate
> > > > iothread for the backing driver to do all of its work in?
> > >
> > > I don't think it's possible to use different AioContexts for
> > > BlockDriverStates in the same BDS chain, at least not currently. But
> > > others may know more about this.
> >
> > This may change in the future but currently all the BDSes in a chain need
> > to
> > stay on the same AioContext.
> >
> > Fam
> >



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