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

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Message-ID: <4BF951BE.1010402@redhat.com>
Date:	Sun, 23 May 2010 19:03:10 +0300
From:	Avi Kivity <avi@...hat.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
CC:	Rusty Russell <rusty@...tcorp.com.au>,
	linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org, kvm@...r.kernel.org,
	qemu-devel@...gnu.org
Subject: Re: [Qemu-devel] [PATCH RFC] virtio: put last seen used index into
 ring itself

On 05/23/2010 06:51 PM, Michael S. Tsirkin wrote:
>>
>>> 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.
>>      
> Maybe we should use atomics on index then?
>    

This should only be helpful if you access the cacheline several times in 
a row.  That's not the case in virtio (or here).

I think the problem is that LOCKSHARE and SHARE are not symmetric, so 
they can't be directly compared.

> OK, after adding mb in code patch will be sent separately,
> the test works for my workstation. locked is still fastest,
> unshared sometimes shows wins and sometimes loses over shared.
>
> [root@...19 ~]# ./cachebounce share 0 1
> CPU 0: share cacheline: 6638521 usec
> CPU 1: share cacheline: 6638478 usec
>    

66 ns? nice.

> [root@...19 ~]# ./cachebounce share 0 2
> CPU 0: share cacheline: 14529198 usec
> CPU 2: share cacheline: 14529156 usec
>    

140 ns, not too bad.  I hope I'm not misinterpreting the results.

-- 
error compiling committee.c: too many arguments to function

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