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

From: Paul Brook
Subject: Re: [Qemu-devel] QEMU/MIPS & dyntick kernel
Date: Mon, 15 Oct 2007 02:23:50 +0100
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> 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.

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.


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