Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933756AbbBCEly (ORCPT ); Mon, 2 Feb 2015 23:41:54 -0500 Received: from smtp2.provo.novell.com ([137.65.250.81]:35140 "EHLO smtp2.provo.novell.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755945AbbBCElv (ORCPT ); Mon, 2 Feb 2015 23:41:51 -0500 Message-ID: <1422938501.2293.1.camel@stgolabs.net> Subject: Re: [PATCH] lib/int_sqrt.c: Optimize square root function From: Davidlohr Bueso To: Linus Torvalds Cc: Anshul Garg , Linux Kernel Mailing List , anshul.g@samsung.com Date: Mon, 02 Feb 2015 20:41:41 -0800 In-Reply-To: References: <1422897162-111998-1-git-send-email-aksgarg1989@gmail.com> Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.12.7 Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1386 Lines: 38 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. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/