From mboxrd@z Thu Jan 1 00:00:00 1970 From: Waiman Long Subject: Re: [RFC PATCH-tip v2 5/6] locking/rwsem: Change RWSEM_WAITING_BIAS for better disambiguation Date: Wed, 15 Jun 2016 15:31:51 -0400 Message-ID: <5761AD27.9020106@hpe.com> References: <1465944489-43440-1-git-send-email-Waiman.Long@hpe.com> <1465944489-43440-6-git-send-email-Waiman.Long@hpe.com> <20160615174348.GT30921@twins.programming.kicks-ass.net> Mime-Version: 1.0 Content-Type: text/plain; charset="ISO-8859-1"; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20160615174348.GT30921@twins.programming.kicks-ass.net> Sender: linux-alpha-owner@vger.kernel.org To: Peter Zijlstra Cc: Ingo Molnar , linux-kernel@vger.kernel.org, x86@kernel.org, linux-alpha@vger.kernel.org, linux-ia64@vger.kernel.org, linux-s390@vger.kernel.org, linux-arch@vger.kernel.org, Davidlohr Bueso , Jason Low , Dave Chinner , Scott J Norton , Douglas Hatch List-Id: linux-arch.vger.kernel.org On 06/15/2016 01:43 PM, Peter Zijlstra wrote: > On Tue, Jun 14, 2016 at 06:48:08PM -0400, Waiman Long wrote: >> even the reduced maximum of about 16k (32-bit) or 1G (64-bit) should >> be more than enough for the foreseeable future. > So what happens if I manage to create 16k+ threads on my 32bit kernel > and get them all to do mmap() or so at the same time. > > That doesn't seem too far fetched. > > Then again, with double that (the current limit) that doesn't seem > impossible either. To hit the limit, we need to have all the threads calling down_write() at exactly the same instance in time which, I think, is pretty hard to do. Also, I don't believe you will ever see a 16k-cpu massive SMP system running on 32-bit kernel. I can imagine such a system running on 64-bit kernel, but certainly not 32-bit. Cheers, Longman From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-bn1bon0135.outbound.protection.outlook.com ([157.56.111.135]:29856 "EHLO na01-bn1-obe.outbound.protection.outlook.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1751867AbcFOTcC (ORCPT ); Wed, 15 Jun 2016 15:32:02 -0400 Message-ID: <5761AD27.9020106@hpe.com> Date: Wed, 15 Jun 2016 15:31:51 -0400 From: Waiman Long MIME-Version: 1.0 Subject: Re: [RFC PATCH-tip v2 5/6] locking/rwsem: Change RWSEM_WAITING_BIAS for better disambiguation References: <1465944489-43440-1-git-send-email-Waiman.Long@hpe.com> <1465944489-43440-6-git-send-email-Waiman.Long@hpe.com> <20160615174348.GT30921@twins.programming.kicks-ass.net> In-Reply-To: <20160615174348.GT30921@twins.programming.kicks-ass.net> Content-Type: text/plain; charset="ISO-8859-1"; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-arch-owner@vger.kernel.org List-ID: To: Peter Zijlstra Cc: Ingo Molnar , linux-kernel@vger.kernel.org, x86@kernel.org, linux-alpha@vger.kernel.org, linux-ia64@vger.kernel.org, linux-s390@vger.kernel.org, linux-arch@vger.kernel.org, Davidlohr Bueso , Jason Low , Dave Chinner , Scott J Norton , Douglas Hatch Message-ID: <20160615193151.H7iv7I9R9rOlzzUL4qPIu_w68wtNc36ep3_g5CoDd9I@z> On 06/15/2016 01:43 PM, Peter Zijlstra wrote: > On Tue, Jun 14, 2016 at 06:48:08PM -0400, Waiman Long wrote: >> even the reduced maximum of about 16k (32-bit) or 1G (64-bit) should >> be more than enough for the foreseeable future. > So what happens if I manage to create 16k+ threads on my 32bit kernel > and get them all to do mmap() or so at the same time. > > That doesn't seem too far fetched. > > Then again, with double that (the current limit) that doesn't seem > impossible either. To hit the limit, we need to have all the threads calling down_write() at exactly the same instance in time which, I think, is pretty hard to do. Also, I don't believe you will ever see a 16k-cpu massive SMP system running on 32-bit kernel. I can imagine such a system running on 64-bit kernel, but certainly not 32-bit. Cheers, Longman