vr(4) performance

[Pyun YongHyeon]

On Thu, Nov 02, 2006 at 03:27:46PM -0800, Sam Leffler wrote:
 > Devon H. O'Dell wrote:
 > > Hey all,
 > > 
 > > So, vr(4) kind of sucks, and it seems like this is mostly due to the
 > > fact that we call m_defrag() on every mbuf that we send through it.
 > > This seems to really screw performance on outgoing packets (something
 > > like 33% the output efficiency of fxp(4), if I'm understanding this
 > > all correctly).
 > > 
 > > I'm sort of wondering if anybody has attempted to address this before
 > > and if there's a way to possibly mitigate this behavior. I know Bill
 > > Paul's comments say ``Unfortunately, FreeBSD FreeBSD doesn't guarantee
 > > that mbufs will be filled in starting at longword boundaries, so we
 > > have to do a buffer copy before transmission.'' -- since it's been a
 > > long day, and I'm about to go home to grab a pizza and stop thinking
 > > about code, would anybody mind offering suggestions as to either:
 > > 
 > > a) Pros and cons of guaranteeing that they're filled in aligned (and
 > > possibly hints on doing it), or
 > > b) Possible workarounds / hacks to do this faster for vr(4)
 > > 
 > > Any input is appreciated! (Except ``vr(4) is lol'')
 > 
 > m_defrag is ~10x slower than it needs to be.  I proposed changes to
 > address this a while back but eventually gave up and put driver-specific
 > code in ath.  You can look there or I can send you some patches to
 > m_defrag to try out in vr.
 > 

Because the purpose of m_defrag(9) in vr(4) is to guarantee longword
aligned mbufs I'm not sure ath_defrag can be used here. If memory
serve me right ath_defrag would not change the first mbuf address
in a chain. If the first mbuf is not aligned on longword boundary
it wouldn't work I guess. Of course we can check the first mbuf in
the chain before calling super-fast ath_defrag, I guess.

-- 
Regards,
Pyun YongHyeon