On Mon, Nov 14, 2016 at 04:29:49PM +0100, Paolo Bonzini wrote: > On 14/11/2016 16:26, Stefan Hajnoczi wrote: > > On Fri, Nov 11, 2016 at 01:59:25PM -0600, Karl Rister wrote: > >> QEMU_AIO_POLL_MAX_NS IOPs > >> unset 31,383 > >> 1 46,860 > >> 2 46,440 > >> 4 35,246 > >> 8 34,973 > >> 16 46,794 > >> 32 46,729 > >> 64 35,520 > >> 128 45,902 > > > > The environment variable is in nanoseconds. The range of values you > > tried are very small (all <1 usec). It would be interesting to try > > larger values in the ballpark of the latencies you have traced. For > > example 2000, 4000, 8000, 16000, and 32000 ns. > > > > Very interesting that QEMU_AIO_POLL_MAX_NS=1 performs so well without > > much CPU overhead. > > That basically means "avoid a syscall if you already know there's > something to do", so in retrospect it's not that surprising. Still > interesting though, and it means that the feature is useful even if you > don't have CPU to waste.
Can you spell out which syscall you mean? Reading the ioeventfd? The benchmark uses virtio-blk dataplane and iodepth=1 so there shouldn't be much IOThread event loop activity besides the single I/O request. The reason this puzzles me is that I wouldn't expect poll to succeed with QEMU_AIO_POLL_MAX_NS and iodepth=1. Thanks, Stefan
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