Re: [Qemu-devel] [RFC PATCH 2/3] raw-posix: Convert Linux AIO submission to coroutines

[Kevin Wolf <kwolf@redhat.com>]

Am 28.11.2014 um 13:57 hat Kevin Wolf geschrieben:
> Am 27.11.2014 um 10:50 hat Peter Lieven geschrieben:
> > On 26.11.2014 15:46, Kevin Wolf wrote:
> > >This improves the performance of requests because an ACB doesn't need to
> > >be allocated on the heap any more. It also makes the code nicer and
> > >smaller.
> > >
> > >As a side effect, the codepath taken by aio=threads is changed to use
> > >paio_submit_co(). This doesn't change the performance at this point.
> > >
> > >Results of qemu-img bench -t none -c 10000000 [-n] /dev/loop0:
> > >
> > >       |      aio=native       |     aio=threads
> > >       | before   | with patch | before   | with patch
> > >------+----------+------------+----------+------------
> > >run 1 | 29.921s  | 26.932s    | 35.286s  | 35.447s
> > >run 2 | 29.793s  | 26.252s    | 35.276s  | 35.111s
> > >run 3 | 30.186s  | 27.114s    | 35.042s  | 34.921s
> > >run 4 | 30.425s  | 26.600s    | 35.169s  | 34.968s
> > >run 5 | 30.041s  | 26.263s    | 35.224s  | 35.000s
> > >
> > >TODO: Do some more serious benchmarking in VMs with less variance.
> > >Results of a quick fio run are vaguely positive.
> > 
> > I still see the main-loop spun warnings with this patches applied to master.
> > It wasn't there with the original patch from August.
> > 
> > ~/git/qemu$ ./qemu-img bench -t none -c 10000000 -n /dev/ram1
> > Sending 10000000 requests, 4096 bytes each, 64 in parallel
> > main-loop: WARNING: I/O thread spun for 1000 iterations
> > Run completed in 31.947 seconds.
> 
> Yes, I still need to bisect that. The 'qemu-img bench' numbers above are
> actually also from August, we have regressed meanwhile by about a
> second, and I also haven't found the reason for that yet.

Did the first part of this now. The commit that introduced the "spun"
message is 2cdff7f6 ('linux-aio: avoid deadlock in nested aio_poll()
calls').

The following patch doesn't make it go away completely, but I only see
it sometime during like every other run now, instead of immediately
after starting qemu-img bench. It's probably a (very) minor performance
optimisation, too.

Kevin


diff --git a/block/linux-aio.c b/block/linux-aio.c
index fd8f0e4..1a0ec62 100644
--- a/block/linux-aio.c
+++ b/block/linux-aio.c
@@ -136,6 +136,8 @@ static void qemu_laio_completion_bh(void *opaque)
 
         qemu_laio_process_completion(s, laiocb);
     }
+
+    qemu_bh_cancel(s->completion_bh);
 }
 
 static void qemu_laio_completion_cb(EventNotifier *e)
@@ -143,7 +145,7 @@ static void qemu_laio_completion_cb(EventNotifier *e)
     struct qemu_laio_state *s = container_of(e, struct qemu_laio_state, e);
 
     if (event_notifier_test_and_clear(&s->e)) {
-        qemu_bh_schedule(s->completion_bh);
+        qemu_laio_completion_bh(s);
     }
 }