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SubjectRe: Alternative implementation of the generic __ffs
Date
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

--
http://www.fastmail.fm - A no graphics, no pop-ups email service



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