Re: [announce] KVM/NET, paravirtual network device

[Avi Kivity <avi-atKUWr5tajBWk0Htik3J/w@public.gmane.org>]

Andy Ross wrote:
> Avi Kivity wrote:
>  > Ingo Molnar wrote:
>  > >   | Qemu via RTL-8029 emulation            :    2.84 MB/sec |
>  > >   | KVM via RTL-8029 emulation             :    7.41 MB/sec |
>  > >   | KVM-para via KVM/NET device (mtu=1500) :  303.35 MB/sec |
>  >
>  > That's about a 5000% improvement :)
>  >
>  > Amazing, both the bandwidth results and the speed in which it was
>  > reached.  I'm really at a loss for words.
>
> Is maybe a better question to ask what it is that makes the Qemu
> hardware emulation so slow?
>   

To send a packet through ne2k, you need about 44 pio writes. at ~10us 
per pio write, that's about 2k packets/sec, or 3MB/sec.  RTL8029 is 
about twice as fast.

A pv interface can send multiple packets per hypercall.  Working 
backward from Ingo's numbers, 300 MB/sec = 200k packets/sec = 2 
packets/hypercall.  Or perhaps his hardware is faster and he does 1 
packet/hypercall.  Or maybe I need to take the memcpy time into account 
at this bandwidth :)

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.


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