tcp_output starving -- is due to mbuf get delay?

Mike Silbersack silby at
Thu Apr 10 14:45:18 PDT 2003

On Thu, 10 Apr 2003, Borje Josefsson wrote:

> > > Have you tried running kernel profiling yet?  It would be interesting to
> > > see which functions are using up the largest amount of time.
> Could do that if I knew how... Not before the weekend though, right now
> I'm at the longue at the airport...

I believe that the manpages regarding how to set it up were pretty useful,
it didn't take me long to get it operational last time I tried.  However,
that was a while ago, so I can't give you any helpful tips.

> If everything is OK (which it apparently isn't), top will show free CPU,
> and netstat should show a *very* steady packet flow (around 90kpps if You
> have MTU 1500). Any packet loss is fatal for this speed, so if there is a
> way (as indicated by Mike above) to not restarting with windowsize from
> scratch that will make recovery much better.

Well, the packet loss I pointed out would be due to the ifqueue
overflowing, which could concieveably happen even if the actual network
wasn't congested.  I don't have the equipment to create such a situation,
but it sounds like you might, in which case adding a debug printf or a
counter to see if it's happening might be advantageous.

Mike "Silby" Silbersack

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