Messages in this thread | | | Date | Tue, 3 Feb 2015 21:24:56 +0530 | Subject | Re: [PATCH] lib/int_sqrt.c: Optimize square root function | From | Anshul Garg <> |
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On Tue, Feb 3, 2015 at 10:11 AM, Davidlohr Bueso <dave@stgolabs.net> wrote: > 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;
On doing this change also extra instruction(shown below) will execute if x is small(say 10000).
b = y + m; y >>= 1; if (x >= b) {
Although i understand your point that we can set value of m according to range of x. But in future also range of x can change if apart from memory some other module starts using this function more extensively. So in worst case it will be almost same as current implementation.
So i think its better to scale m according to x by doing while(m > x) m>>=2; rather than according to range of x.
Thanks Anshul Garg
>> >> (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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