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From Brian Pane <bri...@apache.org>
Subject Re: approximating division by a million Re: Why not POSIX time_t?
Date Tue, 16 Jul 2002 15:45:54 GMT
Ben Laurie wrote:

> Brian Pane wrote:
>
>> Building upon Cliff's formulas, here's another idea
>> for doing faster conversions of the current apr_time_t
>> format to seconds:
>>
>> What we want is t/1000000
>>
>> What we can do easily is t/1048576
>>
>> But what can we add to t/1048576 to approximate t/1000000?
>>
>> If I solve for 'C' in
>>   t/1000000 = t/1048576 + t/C
>> I get C= ~21,586,297
>> That's not a power of 2, but what if use 2^24 (~16M) as an
>> approximation:
>>
>> seconds = (t >> 20) + (t >> 24)
>>
>> That probably isn't accurate enough, but you get the basic idea:
>> sum a couple of t/(2^n) terms to approximate t/1000000.
>>
>> What do you think?
>
>
> I think you're all nuts. Are you seriously saying we compute time 
> stuff often enough to care how long it takes? 


Yes.  The product of:
     frequency of time manipulation * cost of time manipulation
is high enough to make 64-bit division one of the top 5 most
expensive functions in the httpd.  (See Bill Stoddard's dev@httpd
posts on performance profiling for some examples.)

This result doesn't mean that time manipulation is naturally
an expensive part of the httpd, though; rather, it means that
we're using a time representation that's mismatched to the
needs of the application.

--Brian



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