Re: [Qemu-devel] [PATCH] posix-aio-compat: Fix idle_threads counter

[Stefan Hajnoczi <stefanha@gmail.com>]

On Tue, May 3, 2011 at 2:26 PM, Kevin Wolf <kwolf@redhat.com> wrote:
> A thread should only be counted as idle when it really is waiting for new
> requests. Without this patch, sometimes too few threads are started as busy
> threads are counted as idle.
>
> Not sure if it makes a difference in practice outside some artificial
> qemu-io/qemu-img tests, but I think the change makes sense in any case.
>
> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
> ---
>  posix-aio-compat.c |    6 ++----
>  1 files changed, 2 insertions(+), 4 deletions(-)

I think the critical change here is that idle_threads is not being
incremented by spawn_thread().  This means that we will keep spawning
threads as new requests come in and until the first thread goes idle.

Previously you could imagine a scenario where we spawn a thread but
don't schedule it yet.  Then we immediately submit more I/O and since
idle_threads was incremented we don't spawn additional threads to
handle the requests.

Are these the cases you were thinking about?

I like your patch because it reduces the number of places where we
mess with idle_threads.  I'd move the increment/decrement out of the
loop since the mutex is held here anyway so no one will test
idle_threads between loop iterations, but it's up to you if you want
to send a v2 or not.

Stefan