Messages in this thread | | | Date | Fri, 29 Jul 2011 22:45:46 +0800 | From | Liu Yuan <> | Subject | Re: [RFC PATCH]vhost-blk: In-kernel accelerator for virtio block device |
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On 07/29/2011 08:50 PM, Stefan Hajnoczi wrote: > I hit a weirdness yesterday, just want to mention it in case you notice it too. > > When running vanilla qemu-kvm I forgot to use aio=native. When I > compared the results against virtio-blk-data-plane (which *always* > uses Linux AIO) I was surprised to find average 4k read latency was > lower and the standard deviation was also lower. > > So from now on I will run tests both with and without aio=native. > aio=native should be faster and if I can reproduce the reverse I'll > try to figure out why. > > Stefan On my laptop, I don't meet this weirdo. the emulated POSIX AIO is much worse than the Linux AIO as expected. If iodepth goes deeper, the gap gets wider.
If not set aio=none, qemu uses emulated posix aio interface to do the IO. I peek at the posix-aio-compat.c,it uses thread pool and sync preadv/pwritev to emulate the AIO behaviour. The sync IO interface would even cause much poorer performance for random rw, since io-scheduler would possibly never get a chance to merge the requests stream. (blk_finish_plug->queue_unplugged->__blk_run_queue)
Yuan
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