Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933187AbbBCGjT (ORCPT ); Tue, 3 Feb 2015 01:39:19 -0500 Received: from mail.linuxfoundation.org ([140.211.169.12]:39392 "EHLO mail.linuxfoundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932187AbbBCGjR (ORCPT ); Tue, 3 Feb 2015 01:39:17 -0500 Date: Mon, 2 Feb 2015 22:38:51 -0800 From: Andrew Morton To: "Wang, Yalin" Cc: "'Kirill A. Shutemov'" , "'arnd@arndb.de'" , "'linux-arch@vger.kernel.org'" , "'linux-kernel@vger.kernel.org'" , "'linux@arm.linux.org.uk'" , "'linux-arm-kernel@lists.infradead.org'" Subject: Re: [RFC] change non-atomic bitops method Message-Id: <20150202223851.f30768d0.akpm@linux-foundation.org> In-Reply-To: <35FD53F367049845BC99AC72306C23D1044A02027E0C@CNBJMBX05.corpusers.net> References: <35FD53F367049845BC99AC72306C23D1044A02027E0A@CNBJMBX05.corpusers.net> <20150202152909.13bfd11f192fb0268b2ab4bf@linux-foundation.org> <20150203011730.GA15653@node.dhcp.inet.fi> <35FD53F367049845BC99AC72306C23D1044A02027E0B@CNBJMBX05.corpusers.net> <35FD53F367049845BC99AC72306C23D1044A02027E0C@CNBJMBX05.corpusers.net> X-Mailer: Sylpheed 2.7.1 (GTK+ 2.18.9; x86_64-redhat-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1169 Lines: 54 On Tue, 3 Feb 2015 13:42:45 +0800 "Wang, Yalin" wrote: > > ... > > #ifdef CHECK_BEFORE_SET > if (p[i] != times) > #endif > > ... > > ---- > One run on CPU0, reader thread run on CPU1, > Test result: > sudo ./cache_test > reader:8.426228173 > 8.672198335 > > With -DCHECK_BEFORE_SET > sudo ./cache_test_check > reader:7.537036819 > 10.799746531 > You aren't measuring the right thing. You should compare if (p[i] != x) p[i] = x; versus p[i] = x; and you should do this for two cases: a) p[i] == x b) p[i] != x The first code sequence will be slower when (p[i] != x) and faster when (p[i] == x). Next, we should instrument the kernel to work out the frequency of set_bit on an already-set bit. It is only with both these ratios that we can work out whether the patch is a net gain. My suspicion is that set_bit on an already-set bit is so rare that the patch will be a loss. -- 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/