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Date:      14 Feb 2001 11:50:08 -0500
From:      Lowell Gilbert <lowell@world.std.com>
To:        freebsd-questions@freebsd.org
Subject:   tx underrun Re: (none)
Message-ID:  <44bss5qke7.fsf@lowellg.ne.mediaone.net>
In-Reply-To: Cy.Schubert@uumail.gov.bc.ca's message of "13 Feb 2001 17:40:33 %2B0100"
References:  <200102131639.f1DGdWj14713@cwsys.cwsent.com>

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Cy.Schubert@uumail.gov.bc.ca (Cy Schubert - ITSD Open Systems Group) writes:

> I came into work this morning and notice the following in my xconsole:
> 
> xl0: transmission error: 90
> xl0: tx underrun, increasing tx start threshold to 120 bytes
> 
> What would cause a tx underrun?  The only cause of any large amount of 
> traffic from my desktop system was a Veritas backup.

A tx underrun is caused by the computer not keeping up with the NIC
rather than the other way around, so it's basically a question of what
was keeping the computer from servicing the buffer-empty interrupts.
Any other interrupt that took too long being serviced could do that,
so it's hard to say what caused it in this case.

A tx underrun is not in itself a problem, however.  The messages are
important because they may help sometimes in tracking down other
problems, but a single underrun, which doesn't repeat with a larger
transmit buffer, is nothing to be concerned over.  It's virtually
unavoidable on slower PCs, depending on the type of NIC.


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