Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933871AbbBCErd (ORCPT ); Mon, 2 Feb 2015 23:47:33 -0500 Received: from smtp2.provo.novell.com ([137.65.250.81]:44878 "EHLO smtp2.provo.novell.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932733AbbBCEra (ORCPT ); Mon, 2 Feb 2015 23:47:30 -0500 Message-ID: <1422938843.2293.4.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 20:47:23 -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: 1625 Lines: 42 On Mon, 2015-02-02 at 11:00 -0800, Linus Torvalds wrote: > Hmm. I don't disagree, but would like some more feedback. > > Davidlohr - you were the person to touch this function last (commit > 30493cc9dddb: "lib/int_sqrt.c: optimize square root algorithm"), and > you did so for performance reasons. And in fact, when you did that, > you removed that initial loop: > > - one = 1UL << (BITS_PER_LONG - 2); > - while (one > op) > - one >>= 2; > > but I'm not sure that was actually all that conscious, I think the > real optimization was the changes inside the loop to make the final > real loop faster and simpler. I missed that. And yes, the real optimization should be in the loop. > > Also, you had performance numbers, so presumably a test harness for it > all. It probably depends a lot on the actual distribution of argument > values, of course, but it would be good to accompany the patch with > actual real numbers like lasty time. Aha. In my case I recall I ran a usersapce program using each function from 1 to a million, and throwing perf at it for 10 times. > (I'm also not entirely sure what uses int_sqrt() that ends up being so > performance-critical, so it would be good to document that too, since > that probably also matters for the "what's the normal argument range" > question..) It's not a big deal afaik. Thanks, Davidlohr -- 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/