Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751098AbaFDSFz (ORCPT ); Wed, 4 Jun 2014 14:05:55 -0400 Received: from www.linutronix.de ([62.245.132.108]:40502 "EHLO Galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750750AbaFDSFy (ORCPT ); Wed, 4 Jun 2014 14:05:54 -0400 Date: Wed, 4 Jun 2014 20:05:47 +0200 (CEST) From: Thomas Gleixner To: Doug Anderson cc: Kukjin Kim , Tomasz Figa , Daniel Lezcano , Vincent Guittot , Chirantan Ekbote , David Riley , olof@lixom.net, linux-samsung-soc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org Subject: Re: [PATCH 3/3] clocksource: exynos_mct: Optimize register reads with ldmia In-Reply-To: <1401903034-20074-3-git-send-email-dianders@chromium.org> Message-ID: References: <1401903034-20074-1-git-send-email-dianders@chromium.org> <1401903034-20074-3-git-send-email-dianders@chromium.org> User-Agent: Alpine 2.10 (DEB 1266 2009-07-14) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII X-Linutronix-Spam-Score: -1.0 X-Linutronix-Spam-Level: - X-Linutronix-Spam-Status: No , -1.0 points, 5.0 required, ALL_TRUSTED=-1,SHORTCIRCUIT=-0.0001 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, 4 Jun 2014, Doug Anderson wrote: > As we saw in (clocksource: exynos_mct: cache mct upper count), the > time spent reading the MCT shows up fairly high in real-world > profiles. That means that it's worth some optimization. > > We get a roughly 10% speedup in userspace gettimeofday() by using an > ldmia to read the two halfs of the MCT. That seems like a worthwhile > thing to do. > > Before: 1173084 us for 1000000 gettimeofday in userspace > After: 1045674 us for 1000000 gettimeofday in userspace > > NOTE: we could actually do better than this if we really wanted to. > Technically we could register the clocksource as a 32-bit timer and > only use the "lower" half. Doing so brings us down to 1014429 us for > 1000000 gettimeofday in userspace (and doesn't even require assembly > code). That would be an alternative to this change. I was about to ask exactly that question: What's the advantage of the 64 bit dance there? AFAICT nothing. Thanks, tglx -- 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/