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





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