From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 65502C433EF for ; Tue, 26 Oct 2021 19:14:56 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 45F8F610A0 for ; Tue, 26 Oct 2021 19:14:56 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S235804AbhJZTRT (ORCPT ); Tue, 26 Oct 2021 15:17:19 -0400 Received: from mail.kernel.org ([198.145.29.99]:47746 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230184AbhJZTRS (ORCPT ); Tue, 26 Oct 2021 15:17:18 -0400 Received: from gandalf.local.home (cpe-66-24-58-225.stny.res.rr.com [66.24.58.225]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 897D16108D; Tue, 26 Oct 2021 19:14:53 +0000 (UTC) Date: Tue, 26 Oct 2021 15:14:51 -0400 From: Steven Rostedt To: Kalesh Singh Cc: surenb@google.com, hridya@google.com, namhyung@kernel.org, kernel-team@android.com, Jonathan Corbet , Ingo Molnar , Shuah Khan , Masami Hiramatsu , Tom Zanussi , linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org Subject: Re: [PATCH v4 6/8] tracing/histogram: Optimize division by a power of 2 Message-ID: <20211026151451.7f3e09a4@gandalf.local.home> In-Reply-To: <20211025200852.3002369-7-kaleshsingh@google.com> References: <20211025200852.3002369-1-kaleshsingh@google.com> <20211025200852.3002369-7-kaleshsingh@google.com> X-Mailer: Claws Mail 3.17.8 (GTK+ 2.24.33; x86_64-pc-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-doc@vger.kernel.org On Mon, 25 Oct 2021 13:08:38 -0700 Kalesh Singh wrote: > == Results == > > Divisor is a power of 2 (divisor == 32): > > test_hist_field_div_not_optimized | 8,717,091 cpu-cycles > test_hist_field_div_optimized | 1,643,137 cpu-cycles > > If the divisor is a power of 2, the optimized version is ~5.3x faster. > > Divisor is not a power of 2 (divisor == 33): > > test_hist_field_div_not_optimized | 4,444,324 cpu-cycles > test_hist_field_div_optimized | 5,497,958 cpu-cycles To optimize this even more, if the divisor is constant, we could make a separate function to not do the branch, and just shift or divide. And even if it is not a power of 2, for constants, we could implement a multiplication and shift, and guarantee an accuracy up to a defined max. If div is a constant, then we can calculate the mult and shift, and max dividend. Let's use 20 for shift. // This works best for small divisors if (div > max_div) { // only do a real division return; } shift = 20; mult = ((1 << shift) + div - 1) / div; delta = mult * div - (1 << shift); if (!delta) { /* div is a power of 2 */ max = -1; return; } max = (1 << shift) / delta; We would of course need to use 64 bit operations (maybe only do this for 64 bit machines). And perhaps even use bigger shift values to get a bigger max. Then we could do: if (val1 < max) return (val1 * mult) >> shift; -- Steve