Re: [Qemu-devel] [patch] non-blocking disk IO

[John Coiner <jcoiner@stanfordalumni.org>]

Jens Axboe wrote:
  > Why not use aio for this instead, seems like a better fit than spawning
> a thread per block device? That would still require a thread for
> handling completions, but you could easily just use a single completion
> thread for all devices for this as it would not need to do any real
> work.

Pthreads vs. AIO is an open question.

The pthreads implementation works with all disk image formats, without 
modifying any format-specific code. To use AIO, each image format driver 
would have to be modified.

QEMU already uses pthreads, therefore using pthreads does not add a new 
dependency on the host platform. (Or so I thought. The build on Windows 
broke anyway. Haha... my bad...)

My questions are:
  * Which QEMU host platforms support Posix AIO?
  * Which host platform has the slowest pthreads library, and how many 
cycles will it waste on pthreads overhead?
  * What is the likely performance benefit of using AIO?

When I have time, I'll hack AIO into the "raw" image driver, and take 
some benchmarks on a linux host.

Thanks.

-John