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Subject: RE: Problem with e100 driver and latency on different packet sizes
Date: Thu, 15 May 2003 10:12:34 -0700
Message-ID: <C6F5CF431189FA4CBAEC9E7DD5441E010107D6D0@orsmsx402.jf.intel.com>
Thread-Topic: Problem with e100 driver and latency on different packet sizes
Thread-Index: AcMa8valj1p1ADDeSQuVea4hTsN+CAAEV4Fg
From: "Feldman, Scott" <scott.feldman@intel.com>
To: "Corey Minyard" <cminyard@mvista.com>,
       "LKML" <linux-kernel@vger.kernel.org>
Sender: linux-kernel-owner@vger.kernel.org
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> I've attached a small program to measure latency of 
> round-trip time on UDP.  If I send 85-byte packets between 
> two of my machines, I get 170us round-trip latency.  If I 
> send 86-byte packets, I get 1329us latency. 
> This seems quite odd.  If I test on the eepro100 driver, I 
> get expected linear increase in round-trip time as the packet 
> size increases, and it never gets close to 1300us.

This sounds like a side-effect of the "CPU Cycle Saver" feature to
bundle Rx packets per one interrupt.  See
Documentation/networking/e100.txt.  I haven't tried your setup, but I
would guess that you can play around with the BundleSmallFr module
parameter, or better yet, if you want the lowest latencies, turn off CPU
Saver (ucode=0).

CPU Saver trades latency for reduced interrupts, resulting in CPU
savings, hence the name.

Hope this helps.

-scott
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