Re: [PATCH 4/5] kvm: qemu: Use vringfd to eliminate copies

[Anthony Liguori <aliguori@us.ibm.com>]

Rusty Russell wrote:
> On Sunday 15 June 2008 09:28:34 Anthony Liguori wrote:
>   
>> Have you benchmarked the driver?  I wasn't seeing great performance
>> myself although I think that was due to some bugs in the vringfd code.
>>     
>
> Yeah, every time I get close to benchmarking I find another bug :(  But I've 
> spent some time optimising the normal lguest net device, so we'll have a fair 
> comparison.
>
> In theory vringfd will get us zero copy from guest sendfile out to external 
> machines.  For anything else we're doing a copy anyway, so avoiding copying 
> has no great benefit.
>   

There's nothing that prevents zero-copy to be implemented for tun 
without vringfd.  In fact, I seem to recall that your earlier patches 
implemented zero-copy :-)

I like the vringfd model and I think it's a good way to move forward.  
My concern is that it introduces an extra syscall in the TX path.  Right 
now, we do a single write call whereas with vringfd we need to insert 
the TX packet into the queue, do a notify, and then wait for indication 
that the TX has succeeded.

I know we'll win with TSO but we don't need vringfd for TSO.  The jury's 
still out IMHO as to whether we should do vringfd or just try to merge 
TSO tun patches.

Regards,

Anthony Liguori

> The interface is still worthwhile to provide zero-copy receive on intelligent 
> or bound NICs, but that's science fiction at the moment...
>
> Cheers,
> Rusty.
>