Re: [Qemu-devel] Combining synchronous and asynchronous IO

[Sergio Lopez <slp@redhat.com>]

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Kevin Wolf <kwolf@redhat.com> writes:

> Am 15.03.2019 um 16:33 hat Sergio Lopez geschrieben:
>> 
>> Stefan Hajnoczi writes:
>> 
>> > On Thu, Mar 14, 2019 at 06:31:34PM +0100, Sergio Lopez wrote:
>> >> Our current AIO path does a great job at unloading the work from the VM,
>> >> and combined with IOThreads provides a good performance in most
>> >> scenarios. But it also comes with its costs, in both a longer execution
>> >> path and the need of the intervention of the scheduler at various
>> >> points.
>> >> 
>> >> There's one particular workload that suffers from this cost, and that's
>> >> when you have just 1 or 2 cores on the Guest issuing synchronous
>> >> requests. This happens to be a pretty common workload for some DBs and,
>> >> in a general sense, on small VMs.
>> >> 
>> >> I did a quick'n'dirty implementation on top of virtio-blk to get some
>> >> numbers. This comes from a VM with 4 CPUs running on an idle server,
>> >> with a secondary virtio-blk disk backed by a null_blk device with a
>> >> simulated latency of 30us.
>> >
>> > Can you describe the implementation in more detail?  Does "synchronous"
>> > mean that hw/block/virtio_blk.c makes a blocking preadv()/pwritev() call
>> > instead of calling blk_aio_preadv/pwritev()?  If so, then you are also
>> > bypassing the QEMU block layer (coroutines, request tracking, etc) and
>> > that might explain some of the latency.
>> 
>> The first implementation, the one I've used for getting these numbers,
>> it's just preadv/pwrite from virtio_blk.c, as you correctly guessed. I
>> know it's unfair, but I wanted to take a look at the best possible
>> scenario, and then measure the cost of the other layers.
>> 
>> I'm working now on writing non-coroutine counterparts for
>> blk_co_[preadv|pwrite], so we have SIO without bypassing the block layer.
>
> Maybe try to keep the change local to file-posix.c? I think you would
> only have to modify raw_thread_pool_submit() so that it doesn't go
> through the thread pool, but just calls func directly.
>
> I don't think avoiding coroutines is possible without bypassing the block
> layer altogether because everything is really expecting to be run in
> coroutine context.

Turns out what I initially thought was a cost induced by the AIO nature
of our block layer, it's actually a bug in which polling mode works
against aio=threads, delaying the execution of the request completions.

This has been fixed by Paolo's "aio-posix: ensure poll mode is left when
aio_notify is called":

https://lists.gnu.org/archive/html/qemu-devel/2019-04/msg01426.html

So we can throw away the idea of combining synchronous and asynchronous
requests, as it doesn't provide a significant improvement that would
justify the added complexity.

Thanks,
Sergio.

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