From mboxrd@z Thu Jan 1 00:00:00 1970 Date: Fri, 1 Feb 2008 00:41:01 +0100 From: Andrea Arcangeli Subject: Re: [PATCH] mmu notifiers #v5 Message-ID: <20080131234101.GS7185@v2.random> References: <20080131045750.855008281@sgi.com> <20080131171806.GN7185@v2.random> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Sender: owner-linux-mm@kvack.org Return-Path: To: Christoph Lameter Cc: Robin Holt , Avi Kivity , Izik Eidus , kvm-devel@lists.sourceforge.net, Peter Zijlstra , steiner@sgi.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org, daniel.blueman@quadrics.com List-ID: On Thu, Jan 31, 2008 at 03:09:55PM -0800, Christoph Lameter wrote: > On Thu, 31 Jan 2008, Christoph Lameter wrote: > > > > pagefault against the main linux page fault, given we already have all > > > needed serialization out of the PT lock. XPMEM is forced to do that > > > > pt lock cannot serialize with invalidate_range since it is split. A range > > requires locking for a series of ptes not only individual ones. > > Hmmm.. May be okay after all. I see that you are only doing it on the pte > level. This means the range callbacks are taking down a max of 512 > entries. So you have a callback for each pmd. A callback for 2M of memory? Exactly. The point of _pages is to reduce of an order of magnitude (512, or 1024 times) the number of needed invalidate_page calls in a few places where it's a strightforward optimization for both KVM and GRU. Thanks to the PT lock this remains a totally obviously safe design and it requires zero additional locking anywhere (nor linux VM, nor in the mmu notifier methods, nor in the KVM/GRU page fault). Sure you can do invalidate_range_start/end for more than 2M(/4M on 32bit) max virtual ranges. But my approach that averages the fixed mmu_lock cost already over 512(/1024) ptes will make any larger "range" improvement not strongly measurable anymore given to do that you have to add locking as well and _surely_ decrease the GRU scalability with tons of threads and tons of cpus potentially making GRU a lot slower _especially_ on your numa systems. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org