From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754041Ab0EPQ4O (ORCPT ); Sun, 16 May 2010 12:56:14 -0400 Received: from mail-fx0-f46.google.com ([209.85.161.46]:57781 "EHLO mail-fx0-f46.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752434Ab0EPQ4M (ORCPT ); Sun, 16 May 2010 12:56:12 -0400 Message-ID: <4BF02402.8060204@colorfullife.com> Date: Sun, 16 May 2010 18:57:38 +0200 From: Manfred Spraul User-Agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.1.9) Gecko/20100330 Fedora/3.0.4-1.fc12 Thunderbird/3.0.4 MIME-Version: 1.0 To: Nick Piggin , Chris Mason CC: zach.brown@oracle.com, jens.axboe@oracle.com, linux-kernel@vger.kernel.org Subject: Re: [PATCH 1/2] ipc semaphores: reduce ipc_lock contention in semtimedop References: <1271098163-3663-1-git-send-email-chris.mason@oracle.com> <1271098163-3663-2-git-send-email-chris.mason@oracle.com> <4BC4A6B2.1090906@colorfullife.com> <20100413173941.GI13327@think> <20100413180945.GD5683@laptop> <20100413181937.GM13327@think> <20100413185756.GE5683@laptop> In-Reply-To: <20100413185756.GE5683@laptop> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 04/13/2010 08:57 PM, Nick Piggin wrote: > On Tue, Apr 13, 2010 at 02:19:37PM -0400, Chris Mason wrote: > >> I don't see anything in the docs about the FIFO order. I could add an >> extra sort on sequence number pretty easily, but is the starvation case >> really that bad? >> > Yes, because it's not just a theoretical livelock, it can be basically > a certainty, given the right pattern of semops. > > You could have two mostly-independent groups of processes, each taking > and releasing a different sem, which are always contended (eg. if it is > being used for a producer-consumer type situation, or even just mutual > exclusion with high contention). > > Then you could have some overall management process for example which > tries to take both sems. It will never get it. > > The management process won't get the sem on Linux either: Linux implements FIFO, but there is no protection at all against starvation. If I understand the benchmark numbers correctly, a 4-core, 2 GHz Phenom is able to do ~ 2 million semaphore operations per second in one semaphore array. That's the limit - cache line trashing on the sma structure prevent higher numbers. For a NUMA system, the limit is probably lower. Chris: Do you have an estimate how many semop() your app will perform in one array? Perhaps we should really remove the per-array list, sma->sem_perm.lock and sma->sem_otime. -- Manfred