Re: [Qemu-devel] Very poor IO performance which looks like some design problem.

[Fam Zheng <famz@redhat.com>]

On Fri, 04/10 22:38, ein wrote:
> Qemu creates more than 70 threads and everyone of them tries to write to
> disk, which results in:
> 1. High I/O time.
> 2. Large latency.
> 2. Poor sequential read/write speeds.
> 
> When I limited number of cores, I guess I limited number of threads as
> well. That's why I got better numbers.
> 
> I've tried to combine AIO native and thread setting with deadline
> scheduler. Native AIO was much more worse.
> 
> The final question, is there any way to prevent Qemu for making so large
> number of processes when VM does only one sequential R/W operation?

aio=native will make QEMU only submit IO from the IO thread, so you shouldn't
see 70 threads with that. And that should usually be a better option for
performance.


Fam