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[Xen-devel] Interrupt Coalescence

To: xen-devel@xxxxxxxxxxxxxxxxxxxxx
Subject: [Xen-devel] Interrupt Coalescence
From: Håvard Bjerke <havarbj@xxxxxxxxxxx>
Date: Thu, 11 Nov 2004 18:13:49 +0100
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I've found that Interrupt Coalescence (IC) has a detrimental effect on some 
benchmarks with the e1000 driver. So I tried disabling it with the argument 

This had the wanted effect in native linux, ie. full speed, 116 MB/s. However, 
in Xen0, disabling IC has the effect of severely reducing bandwidth down to 
around 1 MB/s.

Turning of IC has the effect that an interrupt is generated from the NIC for 
each received or sent MTU (1500 bytes). On a gigabit network this means that an 
interrupt may be generated, according to 
[http://www.pam2004.org/papers/265.pdf], every 12 us.

I looked at #interrupts generated with and without IC, and they are ~ 1100 and 
~ 400 respectively when sending a 2 MB message with ttcp.

So I'm trying to explain what's happening. Is Xen slower in handling the 
interrupts than native? The CPU is an Intel P4 @ 3400 MHz. Does it need more 
than 12 us to do a context switch when using Xen?


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