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
