Re: clock_gettime

[Loïc Grenié <loic.grenie@gmail.com>]

2012/6/3 Bill Allombert <Bill.Allombert@math.u-bordeaux1.fr>:
> On Sun, Jun 03, 2012 at 01:19:57AM +0200, Loïc Grenié wrote:
>>    Hi pari developers,
>>
>>    a patch has just been submitted to allow for a different timing function:
>>   clock_gettime. The accuracy of timing in libpari is 1ms. Under linux,
>>   getrusage and times accuracy are limited by kernel HZ constant which means
>>   that accuracy is 10ms if HZ=100, 4ms if HZ=250 or 1ms if HZ=1000.
>>   clock_gettime however has the accuracy of the underlying hardware clock,
>>   but the system call is slightly more costly.
>>
>>     If you need better accuracy (and can spend ~15% more in clock_gettime than
>>   in getrusage) you can use clock_gettime. Otherwise just stick with the
>>   default.
>
> Two comments:
[snip]
> 2) on paridev at least the penalty is much larger that 15%:
> getrusage:
> ? for(i=1,10^7,gettime())
> ? ##
>  ***   last result computed in 808 ms.
> clock_gettime:
> ? for(i=1,10^7,gettime())
> ? ##
>  ***   last result computed in 2,262 ms.
>
> given the overhead of an empty loop is about 360ms, this give 448ms for
> getrusage against 1902ms, so this is four time slower (under GP, in C it should
> be worse).

    Indeed, sorry about that. I don't know how I've timed it.

     Thanks,

          Loïc