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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>, "Hollis Blanchard" <hollisb@xxxxxxxxxx>, "Tian, Kevin" <kevin.tian@xxxxxxxxx>
Subject: RE: [Xen-devel] RE: [Patch] Fix IDLE issue with sedf scheduler on IA64
From: "Ian Pratt" <m+Ian.Pratt@xxxxxxxxxxxx>
Date: Thu, 14 Jul 2005 02:23:31 +0100
Cc: xen-devel@xxxxxxxxxxxxxxxxxxx, xen-ia64-devel@xxxxxxxxxxxxxxxxxxx
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Thread-topic: [Xen-devel] RE: [Patch] Fix IDLE issue with sedf scheduler on IA64
> 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.
> A related idea though for the scheduler experts to think about:
> Is it possible for idle to be an "alias" for domain0? 

If you really want to do something like this, it would be much better
just to detect a switch to the idle domain (on whatever CPU dom0 happens
to be running on) and load the register and mm state for dom0 and make
it appear to be the last domain that ran.  The lazy switching logic will
then take care of things.


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