[RFC][patch] time applet execution speed

[Bernhard Fischer]

On Mon, Sep 26, 2005 at 10:10:10AM +0200, Bernhard Fischer wrote:
>On Sun, Sep 25, 2005 at 05:14:22PM -0500, Rob Landley wrote:
>>On Sunday 25 September 2005 15:49, Bernhard Fischer wrote:
>>> On Sat, Sep 24, 2005 at 01:33:18PM -0500, Rob Landley wrote:
>>> >Does rdtsc work happily across scheduler invocations?
>>>
>>> I don't see why it shouldn't. rdtsc and the PPC variant are reading
>>> a CPU built-in timer source. It will of course wrap every now and
>
>>Actually, what I meant was more along the lines of if an interrupt comes in, 
>>that time is credited to this process, and if we're scheduled away and 
>>something else runs for 1/10 of a second is _that_ credited to this process, 
>>and isn't it even theoretically possible that the other process could reset 
>>the timer?
>
>The TSC is set to 0 if it wraps (on a 3GHz CPU approximately every 190
>years) or if the given CPU is reset. The CPU reset is imho not a
>concern as you have to set the CPU offline before you replace it
>anyways.
>
>rdtsc();schedule();rdtsc(); gives the total elapsed time.
>
>This implies that on a loaded system, the measured time applet takes to
>run from start to it's end will be greater than on an idle system. It's
>just a wallclock timer, which is a bit more accurate (253668511.pdf,
>IA-32 Architecture Software Developer's Manual Vol.3, Sect. 15.8).
>
>I do not know how many percent of the machines running busybox support
>PMC (performance monitor counters, section 15.10.2 in abovementioned
>doc).
>>
>>Or is the timer value a register that gets saved in our register profile when 
>>we schedule?  (In which case migrating from CPU to CPU shouldn't be too bad 
>>anyway.)
>
>It's a CPU-local, monotonic counter and AFAIK is not saved in the flags.

Is there any current interrest in adding this?

I certainly like it and since it can be configured out completely, it
would not hurt, so i would like to apply it unless there are major
objections..

Comments?

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