Re: [Xen-devel] Interrupt Coalescence

[Håvard Bjerke <havarbj@xxxxxxxxxxx>]

On Fri, Nov 12, 2004 at 08:33:26AM -0000, Ian Pratt wrote:
> > 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 "InterruptThrottleRate=0,0".
> > 
> > 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.
> 
> I'd like to get to the bottom of this. A couple of people have reported odd 
> performance anomalies with e1000's, but ours seem to work just fine.
> 
> Are you results measured from dom0 or from another domain? If the latter, 
> what's the CPU assignment?
> 

Single CPU using dom0

> What happenes if you boot with 'igrnorebiostables' on the Xen command line 
> (this forces uniprocessor)?  This will use the legacy PIC rather than the 
> IOAPIC.
> 

I've already found that 'ignorebiostables' solves the IOAPIC problem, giving 
equal performance in native and dom0.

Whether with or without 'ignorebiostables', disabling IC leads to dramatically 
reduced bandwidth in dom0 but not in native.

> The dramatic reduction in bandwidth you're seeing with coallescing on is 
> probably our best chance of tracking down this bug (everyone else was seeing 
> more subtle effects).
> 
> Ian
> 
> > 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?
> > 
> > 
> > Håvard
> > 
> > 
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