On 11/09/2016 06:13 PM, Stefan Hajnoczi wrote: > Recent performance investigation work done by Karl Rister shows that the > guest->host notification takes around 20 us. This is more than the "overhead" > of QEMU itself (e.g. block layer). > > One way to avoid the costly exit is to use polling instead of notification. > The main drawback of polling is that it consumes CPU resources. In order to > benefit performance the host must have extra CPU cycles available on physical > CPUs that aren't used by the guest. > > This is an experimental AioContext polling implementation. It adds a polling > callback into the event loop. Polling functions are implemented for > virtio-blk > virtqueue guest->host kick and Linux AIO completion. > > The QEMU_AIO_POLL_MAX_NS environment variable sets the number of nanoseconds > to > poll before entering the usual blocking poll(2) syscall. Try setting this > variable to the time from old request completion to new virtqueue kick. > > By default no polling is done. The QEMU_AIO_POLL_MAX_NS must be set to get > any > polling! > > Karl: I hope you can try this patch series with several QEMU_AIO_POLL_MAX_NS > values. If you don't find a good value we should double-check the tracing > data > to see if this experimental code can be improved. > > Stefan Hajnoczi (3): > aio-posix: add aio_set_poll_handler() > virtio: poll virtqueues for new buffers > linux-aio: poll ring for completions > > aio-posix.c | 133 > ++++++++++++++++++++++++++++++++++++++++++++++++++++ > block/linux-aio.c | 17 +++++++ > hw/virtio/virtio.c | 19 ++++++++ > include/block/aio.h | 16 +++++++ > 4 files changed, 185 insertions(+)
Hmm, I see all affected threads using more CPU power, but the performance numbers are somewhat inconclusive on s390. I have no proper test setup (only a shared LPAR), but all numbers are in the same ballpark of 3-5Gbyte/sec for 5 disks for 4k random reads with iodepth=8. What I find interesting is that the guest still does a huge amount of exits for the guest->host notifications. I think if we could combine this with some notification suppression, then things could be even more interesting. Christian
