Re: [PATCH] up to 50% faster sys_poll()

[Dan Kegel]

Manfred wrote:
> The improvement with large number of pipes probably comes from replacing
> __get_free_page() with kmalloc() - kmalloc is faster, especially on SMP.
> ...
> I expected 2 improvements:
> * poll with < 24 descriptors: around 800 cpu ticks faster, but that's
> just one or two microseconds.
> * if one of the first 8 descriptors has new data: add another ~200 cpu
> ticks.

OK, I reran with different numbers of fd's, again on a 650 MHz dual PIII SMP.

Times in microseconds to find 1 active pipe out of N total pipes:

                number of pipes
kernel        1      8      10000   30000
-----------------------------------------
2.4.0         7     10      14600   45843
2.4.0-pp      5      8      14321   44903

For small N, your patch makes poll() 2 microseconds faster uniformly.
That's a 20-30% speedup, not bad.
Anyone know if this would actually help real-life programs?

BTW, your change makes poll() slightly faster than select().

For large N, your patch speeds poll() up by 2-5%.  
(Increasing /proc/sys/fs/file-max to 70000 appears to reduce the speedup.)
I doubt that will have much effect on real-life programs.
- Dan