Re: Alternative implementation of the generic __ffs

["Alexander van Heukelum" <heukelum@fastmail.fm>]

On Sat, 19 Apr 2008 20:06:57 -0700, "Joe Perches" <joe@perches.com>
said:
> On Sun, 2008-04-20 at 01:29 +0300, Matti Aarnio wrote:
> > I am curious, why not take the code already in glibc ffs() for ARM ?
> > That is, if the ffs() is all that important detail in kernel ?

Hi,

The glibc version is based on a table-lookup. This makes it
behave differently in hot and cold cache situations. That's
fine if __ffs is used in tight loops, but in the kernel such
use of __ffs is avoided because it might be slow. I added it
to the benchmark, but it would need testing for the cold
cache case too.

As for the importance of __ffs in the kernel: as far as I
know the hot-spots in the kernel using __ffs are the
schedular (sched_find_first_bit) and the cpu mask walking
code (for_each_cpu_mask).

Greetings,
    Alexander

> Here's test results with the glibc ffs implementation.
> (small const is still using slower add rather than or)

Added, thanks.

> $ gcc -Os -fomit-frame-pointer ffs.c
> $ ./a.out
> Original:       3155 tics,  8331 tics
> New:            4211 tics,  8793 tics
> Smallest:       4019 tics,  7754 tics
> Small const:    3552 tics,  6308 tics
> glibc:          2816 tics,  6911 tics
> Empty loop:     1516 tics,  2244 tics
> 
> $ gcc -O2 -fomit-frame-pointer ffs.c
> $ ./a.out
> Original:       3155 tics,  7828 tics
> New:            4792 tics,  8825 tics
> Smallest:       4401 tics,  7155 tics
> Small const:    3539 tics,  5805 tics
> glibc:          2720 tics,  7061 tics
> Empty loop:     1516 tics,  2148 tics
> 
> $ gcc -O3 -fomit-frame-pointer ffs.c
> $ ./a.out
> Original:       3080 tics,  7706 tics
> New:            4721 tics,  8663 tics
> Smallest:       4334 tics,  7116 tics
> Small const:    3466 tics,  5672 tics
> glibc:          2649 tics,  6939 tics
> Empty loop:     1444 tics,  2012 tics
> 
> 
-- 
  Alexander van Heukelum
  heukelum@fastmail.fm

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