Re: [Qemu-devel] [PATCH RFC] virtio: put last seen used index into ring itself

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, May 20, 2010 at 02:38:16PM +0930, Rusty Russell wrote:
> On Thu, 20 May 2010 02:31:50 pm Rusty Russell wrote:
> > On Wed, 19 May 2010 05:36:42 pm Avi Kivity wrote:
> > > > Note that this is a exclusive->shared->exclusive bounce only, too.
> > > >    
> > > 
> > > A bounce is a bounce.
> > 
> > I tried to measure this to show that you were wrong, but I was only able
> > to show that you're right.  How annoying.  Test code below.
> 
> This time for sure!


What do you see?
On my laptop:
	[mst@tuck testring]$ ./rusty1 share 0 1
	CPU 1: share cacheline: 2820410 usec
	CPU 0: share cacheline: 2823441 usec
	[mst@tuck testring]$ ./rusty1 unshare 0 1
	CPU 0: unshare cacheline: 2783014 usec
	CPU 1: unshare cacheline: 2782951 usec
	[mst@tuck testring]$ ./rusty1 lockshare 0 1
	CPU 1: lockshare cacheline: 1888495 usec
	CPU 0: lockshare cacheline: 1888544 usec
	[mst@tuck testring]$ ./rusty1 lockunshare 0 1
	CPU 0: lockunshare cacheline: 1889854 usec
	CPU 1: lockunshare cacheline: 1889804 usec
So locked version seems to be faster than unlocked,
and share/unshare not to matter?

same on a workstation:
[root@qus19 ~]# ./rusty1 unshare 0 1
CPU 0: unshare cacheline: 6037002 usec
CPU 1: unshare cacheline: 6036977 usec
[root@qus19 ~]# ./rusty1 lockunshare 0 1
CPU 1: lockunshare cacheline: 5734362 usec
CPU 0: lockunshare cacheline: 5734389 usec
[root@qus19 ~]# ./rusty1 lockshare 0 1
CPU 1: lockshare cacheline: 5733537 usec
CPU 0: lockshare cacheline: 5733564 usec

using another pair of CPUs gives a more drastic
results:

[root@qus19 ~]# ./rusty1 lockshare 0 2
CPU 2: lockshare cacheline: 4226990 usec
CPU 0: lockshare cacheline: 4227038 usec
[root@qus19 ~]# ./rusty1 lockunshare 0 2
CPU 0: lockunshare cacheline: 4226707 usec
CPU 2: lockunshare cacheline: 4226662 usec
[root@qus19 ~]# ./rusty1 unshare 0 2
CPU 0: unshare cacheline: 14815048 usec
CPU 2: unshare cacheline: 14815006 usec


The share test seems to never finish on the
workstation. I am debugging this.

-- 
MST