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Re: [patch] netconsole - log kernel messages over the network. 2.4.10.

To: jamal <hadi@xxxxxxxxxx>
Subject: Re: [patch] netconsole - log kernel messages over the network. 2.4.10.
From: Ingo Molnar <mingo@xxxxxxx>
Date: Thu, 27 Sep 2001 07:45:51 +0200 (CEST)
Cc: <linux-kernel@xxxxxxxxxxxxxxx>, <linux-net@xxxxxxxxxxxxxxx>, <netdev@xxxxxxxxxxx>
In-reply-to: <Pine.GSO.4.30.0109261713500.6825-100000@xxxxxxxxxxxxxxxx>
Reply-to: <mingo@xxxxxxx>
Sender: owner-netdev@xxxxxxxxxxx
On Wed, 26 Sep 2001, jamal wrote:

> Is there any reason you are not using dev_queue_xmit()? (side
> benefits, you could hack this into using scatter gather schemes etc)

the reason is latency of messages, and reliability of sends. If we are in
the middle of a *nasty* crash that takes down the system down 10
microseconds later, then i wanted to make 100% sure the message gets out
and the bug can be debugged. The VGA console and the serial logging
console have such properties - messages are instantaneous. Networking is
very asynchron in nature, so a new network driver extension was needed to
achieve this goal.

There are crashes where there are no new IRQs processed after the crash,
so there is no chance for dev_queue_xmit to do any useful send. We must be
able to send packets even if the output interface's hardware queues are
full - and dev_queue_xmit relies on future device interrupts to send
queued packets.

this is not just theory - about 10% of the crashes i experience (during
development...) are such, and the majority of the 'hard to fix' crashes
have such properties, especially on busy remote servers.

        Ingo


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