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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