On Wed, Aug 3, 2011 at 6:19 AM, Gui Jianfeng <guijianf...@cn.fujitsu.com> wrote: > When i did some io tests in Guest say dd like > dd if=/dev/zero oflag=direct of=/mnt/sdb1/date.img bs=4k count=262114 > > qemu will start one or several threads to perform IO requests. > > It seems qemu makes use of its own posixaio. I'm wondering how qemu > decides how many threads should be created to perform io operation?
Look at posix-aio-compat.c. If a new request comes in and there is no idle thread around, a new thread will be spawned up to a maximum limit. When you run a synchronous I/O workload like dd(1), expect to see only 1 or 2 aio worker threads. If the workload is multithreaded, expect to see many (depending on the guest's queue depth). Stefan
