On 05/23/2010 06:31 PM, Michael S. Tsirkin wrote:
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
Ugh, can the timing be normalized per operation? This is unreadable.
So locked version seems to be faster than unlocked,
and share/unshare not to matter?
May be due to the processor using the LOCK operation as a hint to
reserve the cacheline for a bit.
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
That's expected. Hyperthread will be fastest (shared L1), shared L2/L3
will be slower, cross-socket will suck.
--
error compiling committee.c: too many arguments to function
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