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Date:      Sun, 13 Apr 2003 20:51:20 -0500 (CDT)
From:      Mike Silbersack <silby@silby.com>
To:        Hiten Pandya <hiten@unixdaemons.com>
Cc:        freebsd-hackers@freebsd.org
Subject:   Re: tcp_output starving -- is due to mbuf get delay?
Message-ID:  <20030413204451.V93049@odysseus.silby.com>
In-Reply-To: <20030412195711.GA30459@unixdaemons.com>
References:  <200304101311.h3ADBgjY022790@samson.dc.luth.se>    <20030412195711.GA30459@unixdaemons.com>

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On Sat, 12 Apr 2003, Hiten Pandya wrote:

> > That tcp_quench knocks the window size back to one packet, if I'm not
> > mistaken.  You might want to put a counter there and see if that's
> > happening frequently to you; if so, it might explain some loss of
> > performance.
>
> Maybe something like this:
>
> Cheers.
>
>   -- Hiten

As Jayanth pointed out, ip_output already keeps statistics on when it
returns ENOBUFS, so adding this additional variable wouldn't be of much
benefit.

For reference (from netstat -s):

        0 output packets dropped due to no bufs, etc.

Although, I think ip_output could be improved.  It precalculates the
expected # of packets to be sent, and returns ENOBUFS early if its
estimate looks bad.  However, if it gets to the point where if_output is
called, and if_output then fails, no statistic is kept.  So, it does look
like Hiten's patch could be useful, as well as another counter tracking
if_output failures.

But I'm too busy to worry about them for now. :)

Mike "Silby" Silbersack



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