Re: [Qemu-devel] [patch] option -no-tsc for i386 with speedstep

[Taras Glek <taras.judge@shaw.ca>]

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Filip Navara wrote:

> Jonas Maebe wrote:
>
>> On 25 Apr 2005, at 22:17, Massimo Dal Zotto wrote:
>>
>>> In the meantime until we find a better solution could you give
>>> us some explanation on why using a microseconds clock from
>>> gettimeofday instead of rdtsc the guest os clock runs always
>>> 20% slower?
>>
>>
>> Because a system call (which gettimeofday is) is very slow (two
>> context switches).
>
>
> I remember there was a Linux patch which changed the gettimeofday
> function to read the info from user mode shared page (which is
> updated by the kernel) and thus avoided the syscall overhead. Not
> sure if/when it was integrated into the official sources...
>
> (Once again Windows NT were ahead of time with this idea
> implemented way earlier ;-)

Another *solution* is to use kernel mode linux =). See
http://web.yl.is.s.u-tokyo.ac.jp/~tosh/kml/
It makes already low Linux syscall overhead much lower.

Tara
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