Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S934146AbYB2Vep (ORCPT ); Fri, 29 Feb 2008 16:34:45 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1756584AbYB2Veg (ORCPT ); Fri, 29 Feb 2008 16:34:36 -0500 Received: from relay1.sgi.com ([192.48.171.29]:49191 "EHLO relay.sgi.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1753187AbYB2Vef (ORCPT ); Fri, 29 Feb 2008 16:34:35 -0500 Date: Fri, 29 Feb 2008 13:34:34 -0800 (PST) From: Christoph Lameter X-X-Sender: clameter@schroedinger.engr.sgi.com To: Andrea Arcangeli cc: Nick Piggin , akpm@linux-foundation.org, Robin Holt , Avi Kivity , Izik Eidus , kvm-devel@lists.sourceforge.net, Peter Zijlstra , general@lists.openfabrics.org, Steve Wise , Roland Dreier , Kanoj Sarcar , steiner@sgi.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org, daniel.blueman@quadrics.com Subject: Re: [patch 2/6] mmu_notifier: Callbacks to invalidate address ranges In-Reply-To: <20080229212327.GC8091@v2.random> Message-ID: References: <20080228005249.GF8091@v2.random> <20080228011020.GG8091@v2.random> <20080229005530.GO8091@v2.random> <20080229131302.GT8091@v2.random> <20080229201744.GB8091@v2.random> <20080229212327.GC8091@v2.random> MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2029 Lines: 43 On Fri, 29 Feb 2008, Andrea Arcangeli wrote: > On Fri, Feb 29, 2008 at 01:03:16PM -0800, Christoph Lameter wrote: > > That means we need both the anon_vma locks and the i_mmap_lock to become > > semaphores. I think semaphores are better than mutexes. Rik and Lee saw > > some performance improvements because list can be traversed in parallel > > when the anon_vma lock is switched to be a rw lock. > > The improvement was with a rw spinlock IIRC, so I don't see how it's > related to this. AFAICT The rw semaphore fastpath is similar in performance to a rw spinlock. > Perhaps the rwlock spinlock can be changed to a rw semaphore without > measurable overscheduling in the fast path. However theoretically Overscheduling? You mean overhead? > speaking the rw_lock spinlock is more efficient than a rw semaphore in > case of a little contention during the page fault fast path because > the critical section is just a list_add so it'd be overkill to > schedule while waiting. That's why currently it's a spinlock (or rw > spinlock). On the other hand a semaphore puts the process to sleep and may actually improve performance because there is less time spend in a busy loop. Other processes may do something useful and we stay off the contended cacheline reducing traffic on the interconnect. > preempt-rt runs quite a bit slower, or we could rip spinlocks out of > the kernel in the first place ;) The question is why that is the case and it seesm that there are issues with interrupt on/off that are important here and particularly significant with the SLAB allocator (significant hacks there to deal with that issue). The fastpath that we have in the works for SLUB may address a large part of that issue because it no longer relies on disabling interrupts. -- 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/