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

[Fam Zheng <famz@redhat.com>]

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/12h1QbecvxzlKxSFvGKYAzvAJ18kTW6AVTwDR6VA8hkw/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