Re: [PATCH] sysctl: vfs_cache_divisor

[Kyle Moffett]

On Mar 23, 2007, at 16:59:02, H. Peter Anvin wrote:
> Randy Dunlap wrote:
> > Hi,
> > That makes a lot of sense to me.  It gives us finer-grained  
> > control without having to support fixed-point data.  I've been  
> > working on the fixed-point data patch, but I'm going to give this  
> > method some time also, to see how it looks in code (instead of  
> > just thinking about it).
>
> Well, to be specific, it actually is the same thing with different  
> syntax (38.55 or 3855/100).

Not quite; if you're dealing with numbers near the limit of the range  
of "int" then IIRC fixed point is harder to get both good accuracy  
and good overflow behavior compared to a user-set fraction:

Assuming "fraction" is a reduce (num, den) pair, then to compute  
"result = value * fraction" with optimal accuracy:

/* On input: reduce num/den but retain old values for display  
purposes */
gcd = euclid(&inputnum, &inputden);
num = inputnum/gcd;
den = inputden/gcd;
valmax = UINT_MAX / num;

/* For computation: */
result = (val > valmax) ? (val/den)*num : (val*num)/den;

Doing that with fixed point is possible, but a bit more tricky, and  
you lose accuracy for something like "1/3", which isn't quite equal  
to "0.3333".  Note that if you aren't really careful with your fixed- 
point then "0.3333 * 3" ends up being 0:  (3333 * 3) / 10000 ==  
9999 / 10000 == 0.  Rounding is possible but it's better to just let  
them specify the exact fraction directly and avoid the confusion.

Cheers,
Kyle Moffett

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