HEADS UP: zerocopy bpf commits impending

Robert Watson rwatson at FreeBSD.org
Mon Mar 24 07:15:53 PDT 2008

On Mon, 24 Mar 2008, Christian S.J. Peron wrote:

> I just want everyone to know that I have completed the zerocopy bpf commit. 
> Please be on the "lookout" for any strange bpf related issues.
> For people that want to test the new zerocopy bpf implementation, a patch 
> can be found here:
> http://people.freebsd.org/~csjp/pcap.1206364304.diff
> Any comments, patches etc can be sent to Robert Watson (rwatson@) or myself.

FYI, right now there is a known issue in which only one of the two BPF buffers 
can be owned by user processes at a time.  As a result, when acking one 
buffer, it's almost always the case that userspace will enter select() even 
though another buffer is essentially ready, leading to a system call being 
generated for each buffer even though that's undesirable.  I'm working on some 
changing allowing both buffers to be owned by userspace at a time, but it will 
be a couple of weeks before that enters CVS.  I believe that the current 
libpcap patches should keep working with that fine, although of course, we'll 
see. :-)  The bpf.4 documentation is very careful to warn that applications 
should not assume that there are any invariants about the number of buffers 
assigned to userspace at a time.

Robert N M Watson
Computer Laboratory
University of Cambridge

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