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

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

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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