[Leaplist] need some jiffies

[Andrew]

On Oct 9, 2007, at 8:01 PM, John Simpson wrote:

> On 2007-10-09, at 1052, Richard F. Ostrow Jr. wrote:
>>
>> Hmm... the last time I'd heard of the word "jiffies" was back when  
>> I was
>> coding on the C64 when I was 12 years old. I believe the  
>> definition back
>> then was more linked to machine cycles than any measurement of  
>> time... so
>> a jiffy on one machine will be different than a jiffy on another
>> machine(hence a "jiffy" is not equal to x microseconds).
>
> that's exactly what it is under linux. the kernel programs the  
> timer chip to send interrupts 100 times per second. (for old-school  
> DOS hackers, this is the same timer chip that DOS programs for 18.2  
> times per second. it uses hardware interrupt line 0, which maps to  
> the processor's INT 0x08.)

Something to be aware of is that you cannot absolutely count on it  
being 100 times per second.  On the Alpha, it has always been 1000  
times per second, and the IA-64 port followed that model.  IIRC, Red  
Hat increased the timer resolution in their kernels to 1000 per  
second as well, for better performance characteristics.  Also, I have  
not been closely following the "tickless kernel" work, so there may  
be some additional caveats that come into play there.