Re: [Qemu-devel] [patch] Poor user-net performance.

[Juergen Lock]

On Sun, Dec 18, 2005 at 11:28:27PM +0000, Paul Brook wrote:
> Dan mentioned he was seeing very poor network performance when using 
> user-net.  
> It turns out this is because packets were getting dropped (the NIC buffers 
> were full), and the slirp code doesn't throttle properly when this occurs. 
> You end up with a bunch of packets getting sent to the guest, half of which 
> get dropped, then a delay until slirp retransmits.
> 
> This is much more noticeable on Arm because the NIC only has a small memory 
> buffer.
> 
> Teaching the slirp code how to do tcp backoff/window scaling is probably a 
> fair amount of work.  The easy alternative is to throttle the slirp output 
> when we know the card's buffers are full. slirp has a convenient hook for 
> this, so it's a matter of adding the hooks to the hardware emulation.
> 
> This increases arm user-net performance from painfully slow (~200kbit) to 
> guest CPU bound (~20Mbit).
> 
Hmm is the patch supposed to work for i386 guests too?  I get no dhcp
response and network unreachable when i try to ssh to the host from the
guest after manually ifconfig'ing, seem like the ne2k doesn't receive
any packets.  (FreeBSD/i386 host, linux i386 guest - kanotix livecd iso).
The same works normally without the patch.