Re: [PATCH] lib/int_sqrt.c: Optimize square root function

[Davidlohr Bueso <dave@stgolabs.net>]

On Mon, 2015-02-02 at 11:13 -0800, Linus Torvalds wrote:
> IOW, instead of
> 
>   m = 1UL << (BITS_PER_LONG - 2);
> 
> perhaps something like
> 
>    m = 1UL << (BITS_PER_LONG/2- 2);
>    if (m < x)
>        m <<= BITS_PER_LONG/2;
> 
> (assuming gcc can change that code into a "cmov") might cut down the
> "lots of empty loops" case in half for small values of 'x'.

Makes a lot of sense.

> There's probably some other better cheap initial guess value estimator.

Just to get a feeling for the normal arg range in the non-driver parts
that use this thing:

fs/ceph/super.h:        congestion_kb = (16*int_sqrt(totalram_pages)) << (PAGE_SHIFT-10);
fs/nfs/write.c: nfs_congestion_kb = (16*int_sqrt(totalram_pages)) << (PAGE_SHIFT-10);
fs/nfsd/nfscache.c:     limit = (16 * int_sqrt(low_pages)) << (PAGE_SHIFT-10);
kernel/rcu/tree_plugin.h:               ls = int_sqrt(nr_cpu_ids);
mm/memcontrol.c:                inactive_ratio = int_sqrt(10 * gb);
mm/page_alloc.c:                ratio = int_sqrt(10 * gb);
mm/page_alloc.c:        new_min_free_kbytes = int_sqrt(lowmem_kbytes * 16);

So mostly values that scale according to mem or cpu.