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Re: [Xen-devel] RE: [Patch] Fix IDLE issue with sedf scheduler on IA64

To: "Magenheimer, Dan (HP Labs Fort Collins)" <dan.magenheimer@xxxxxx>
Subject: Re: [Xen-devel] RE: [Patch] Fix IDLE issue with sedf scheduler on IA64
From: Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>
Date: Wed, 13 Jul 2005 16:14:38 +0100
Cc: "Tian, Kevin" <kevin.tian@xxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxx, xen-ia64-devel@xxxxxxxxxxxxxxxxxxx
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On 13 Jul 2005, at 16:05, Magenheimer, Dan (HP Labs Fort Collins) wrote:

Neat, but doesn't this only solve half the problem?  Idle is now
an "impostor" for the last runnable domain.  Generally the machine
goes idle because all domains are waiting for a device interrupt.
Since (in general) all device interrupts go through domain0,
a context switch is still necessary from idle=last_runnable_domain
to domain0 to process the device interrupt, then back to domU to
process the virtual interrupt.

In a I/O bound system, interrupt latency still seems to be
twice what it could be.

If you really care about i/o throughput you should probably be prepared to burn a core or a hyperthread on your driver domain (probably dom0) as otherwise the domu<->dom0 context switches are a serious overhead.

Mostly, if context switch times are a bottleneck limit then you are cpu bound anyway, and there will be no idle time to optimise usage of.

 -- Keir

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