Re: [Qemu-devel] QEMU/MIPS & dyntick kernel

[Dor Laor <dor.laor@gmail.com>]

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Paul Brook wrote:
>> There seem to have specific problems when using dynticks in Qemu. What I
>> can see is that it makes the PowerPC emulation quite unusable, at least
>> on my PC, which is an amd64 (with a fix CPU frequency), no matter if I
>> run 32 or 64 bits mode.
>>     
>
> I'd expect to see the same problems running a non-dynticks qemu on a heavily 
> loaded host, or a host that did not have /dev/rtc available.
>
> IIRC vanilla amd64-linux does not yet use the new kernel high resolution timer 
> infrastructure, so posix timers (as used by dynticks) have a fairly high 
> jitter+latency compared to /dev/rtc.
>
>   
As of linux 2.6.24 there is dyn-tick kernel support for 64 bits.
Expect for dyn-tick there is also the hpet option.
> The tradeoff is that on hosts that do implement high resolution timers (e.g. 
> i386) you get lower overhead and/or more accurate emulation. I don't think 
> there's any deterministic way of figuring out which is best. It may be 
> feasible to switch mechanisms dynamically if it's obvious one is sucking, but 
> I'm not sure how well that would work in practice.
>
> The only reliable solution is to isolate qemu from the host realtime 
> characteristics, though that has its own set of issues. I hope to have this 
> implemented fairly soon.
>
> Paul
>
>
>
>   


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