Re: NetBSD 5.1 TCP performance issue (lots of ACK)

[Thor Lancelot Simon <tls%panix.com@localhost>]

On Thu, Oct 27, 2011 at 07:55:16PM +0200, Manuel Bouyer wrote:
> On Thu, Oct 27, 2011 at 12:44:22PM -0400, Greg Troxel wrote:
> > 
> > one has to wonder how bpf works when tso is enabled.  are the packets
> > read back from the card, or are they also created in software and then
> > discarded?
> 
> The mbuf passed to the adapter is also passed to bpf_mtap(),
> so bpf_mtap() gets the tso packet and not the on-wire packets when
> tso is in use.
> 
> AFAIK bpf_mtap() doesn't do any attempts at doing something special
> with TSO mbufs it could get. So I guess tcpdump is getting the TSO
> buffer ...

...which should frequently exceed the MTU, shouldn't it?  So I would
actually expect tcpdump to misbehave.

Thor