On 14/11/2016 18:06, Stefan Hajnoczi wrote: >>> > > 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?
I mean ppoll. If ppoll succeeds without ever going to sleep, you can achieve the same result with QEMU_AIO_POLL_MAX_NS=1, but cheaper. Paolo > 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.
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