Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1030397AbWARQiz (ORCPT ); Wed, 18 Jan 2006 11:38:55 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1030398AbWARQiy (ORCPT ); Wed, 18 Jan 2006 11:38:54 -0500 Received: from smtp.osdl.org ([65.172.181.4]:22711 "EHLO smtp.osdl.org") by vger.kernel.org with ESMTP id S1030397AbWARQix (ORCPT ); Wed, 18 Jan 2006 11:38:53 -0500 Date: Wed, 18 Jan 2006 08:38:44 -0800 (PST) From: Linus Torvalds To: Nick Piggin cc: Linux Memory Management , Linux Kernel Mailing List , Hugh Dickins , Andrew Morton , Andrea Arcangeli , David Miller Subject: Re: [patch 0/4] mm: de-skew page refcount In-Reply-To: <20060118024106.10241.69438.sendpatchset@linux.site> Message-ID: References: <20060118024106.10241.69438.sendpatchset@linux.site> MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1523 Lines: 42 On Wed, 18 Jan 2006, Nick Piggin wrote: > > The following patchset (against 2.6.16-rc1 + migrate race fixes) uses the new > atomic ops to do away with the offset page refcounting, and simplify the race > that it was designed to cover. > > This allows some nice optimisations Why? The real downside is that "atomic_inc_nonzero()" is a lot more expensive than checking for zero on x86 (and x86-64). The reason it's offset is that on architectures that automatically test the _result_ of an atomic op (ie x86[-64]), it's easy to see when something _becomes_ negative or _becomes_ zero, and that's what atomic_add_negative atomic_inc_and_test are optimized for (there's also "atomic_dec_and_test()" which reacts on the count becoming zero, but that doesn't have a pairing: there's no way to react to the count becoming one for the increment operation, so the "atomic_dec_and_test()" is used for things where zero means "free it"). Nothing else can be done that fast on x86. Everything else requires an insane "load, update, cmpxchg" sequence. So I disagree with this patch series. It has real downsides. There's a reason we have the offset. I suspect that whatever "nice optimizations" you have are quite doable without doing this count pessimization. Linus - 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/