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SubjectRe: [patch] x86: unify/rewrite SMP TSC sync code
Arjan van de Ven wrote:
> it's the cost of a syscall (1000 cycles?) plus what it takes to get a
> reasonable time estimate. Assuming your kernel has enough time support
> AND your tsc is reasonably ok, it'll be using that. If it's NOT using
> that then that's a pretty good sign that you can't also use it in
> userspace....
>

I wrote a quick and dirty program that I've attached to test the cost
difference between RDTSC and gettimeofday (gtod), the results:

wink@winkc2d1:~/linux/linux-2.6/test/rdtsc-pref$ time ./rdtsc-pref 100000000
rdtsc: average ticks= 65
gtod: average ticks= 222
gtod_us: average ticks= 232

real 0m36.002s
user 0m35.997s
sys 0m0.000s

About a 3.5x cost difference, still for most of my uses gtod was not as costly
as I had supposed.

But, there are other uses that it wouldn't be acceptable. For instance, I
have used a memory mapped time stamp counter in an embedded ARM based
system for instrumenting the interrupt service routine, syscalls
and task switches. For this type of instrumentation a gtod type call wouldn't
have been suitable.

Anyway for x86_64 systems, if I can use a memory mapped HPET counter, I might be
able to have my cake and eat it too. One counter that can be used inside and
outside the kernel that is cheap, precise and accurate, nirvana! We'll have to see.

BTW my system is a 2.4ghz Core 2 duo running 2.6.19-rc6 with HPET enabled,
in the attachment I've included my config file.

Cheers,

Wink

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