Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756204AbZDKPGY (ORCPT ); Sat, 11 Apr 2009 11:06:24 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1753557AbZDKPGK (ORCPT ); Sat, 11 Apr 2009 11:06:10 -0400 Received: from mail.vyatta.com ([76.74.103.46]:43345 "EHLO mail.vyatta.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752219AbZDKPGI (ORCPT ); Sat, 11 Apr 2009 11:06:08 -0400 Date: Sat, 11 Apr 2009 08:05:54 -0700 From: Stephen Hemminger To: Ingo Molnar Cc: "Paul E. McKenney" , Linus Torvalds , David Miller , Lai Jiangshan , jeff.chua.linux@gmail.com, dada1@cosmosbay.com, jengelh@medozas.de, kaber@trash.net, r000n@r000n.net, Linux Kernel Mailing List , netfilter-devel@vger.kernel.org, netdev@vger.kernel.org Subject: Re: iptables very slow after commit 784544739a25c30637397ace5489eeb6e15d7d49 Message-ID: <20090411080554.67a17410@nehalam> In-Reply-To: <20090411070854.GC11799@elte.hu> References: <20090410095246.4fdccb56@s6510> <20090410.182507.140306636.davem@davemloft.net> <20090411041533.GB6822@linux.vnet.ibm.com> <20090411070854.GC11799@elte.hu> Organization: Vyatta X-Mailer: Claws Mail 3.6.1 (GTK+ 2.16.0; x86_64-pc-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: 1563 Lines: 41 On Sat, 11 Apr 2009 09:08:54 +0200 Ingo Molnar wrote: > > * Paul E. McKenney wrote: > > > I will nevertheless suggest the following egregious hack to > > get a consistent sample of one counter for some other CPU: > > > > a. Disable interrupts > > b. Atomically exchange the bottom 32 bits of the > > counter with the value zero. > > c. Atomically exchange the top 32 bits of the counter > > with the value zero. > > d. Concatenate the values obtained in (b) and (c), which > > is the snapshot value. > > Note, i have recently implemented full atomic64_t support on 32-bit > x86, for the perfcounters code, based on the CMPXCHG8B instruction. > > Which, while not the lightest of instructions, is still much better > than the sequence above. > > So i think a better approach would be to also add a dumb generic > implementation for atomic64_t (using a global lock or so), and then > generic code could just assume that atomic64_t always exists. > > It is far nicer - and faster as well - as the hack above, even on > 32-bit x86. > > Ingo The iptables counters are write mostly, read rarely so they don't fit the seq counter or atomic use case. Also, it is important to get a consistent snapshot of the whole set not just each individual counter. -- 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/