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Re: do_gettimeofday

To: Steve Modica <modica@xxxxxxx>
Subject: Re: do_gettimeofday
From: Andi Kleen <ak@xxxxxxx>
Date: Thu, 2 Oct 2003 21:29:42 +0200
Cc: netdev@xxxxxxxxxxx
In-reply-to: <3F7C6F3B.6070502@sgi.com>
References: <3F7C6F3B.6070502@sgi.com>
Sender: netdev-bounce@xxxxxxxxxxx
On Thu, Oct 02, 2003 at 01:32:27PM -0500, Steve Modica wrote:
> We've been doing some experiments here with large numbers of adapters on a 
> 64p Linux system. 
> When running 8 threads and 8 cpus, the do_gettimeofday code starts to use a 
> lot of time.

That's a known problem. The funny thing is that the only users
of this time stamp is SO_TIMESTAMP, which is rarely used (except tcpdump)
and something in DECnet. IMHO the right fix is to add a global
counter that counts all all sockets that use SO_TIMESTAMP and when it's
zero never call it. Decnet could be probably fixed to just use jiffies
like TCP does.

Drawback is that when you enable SO_TIMESTAMP there is a small time window
when the packets are not time stamped yet. The socket layer can just fill
in the current time though.

-Andi

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