From mboxrd@z Thu Jan 1 00:00:00 1970 From: Nick Piggin Date: Wed, 02 Sep 2009 17:12:13 +0000 Subject: Re: discontigmem vs sparsemem on ia64 platforms Message-Id: <20090902171213.GE28052@wotan.suse.de> List-Id: References: <20090902163810.GC28052@wotan.suse.de> In-Reply-To: <20090902163810.GC28052@wotan.suse.de> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: linux-ia64@vger.kernel.org On Wed, Sep 02, 2009 at 09:56:40AM -0700, Luck, Tony wrote: > > I wonder why you have DISCONTIGMEM_DEFAULT selected for GENERIC, SN2, and > ZX1? > > You don't explicitly disable SPARSEMEM for those platforms I think, so is > > there another reason than bugs for this? > > On ia64 DISCONTIGMEM was implemented first. SPARSEMEM came later, and > nobody has sent patches to switch the config defaults over to use it. > I'd love it if somebody came up with some definitive arguments for > why SPARSEMEM can replace DISCONTIGMEM so we could clear out some of > the clutter that has accumulated by having both implementations. Last > time I looked (a very long time ago) it looked like SPARSEMEM would > have difficulties handling the extreme levels of sparseness on the > SN2 platform (high order physical address bits are used to route > memory traffic to the right node ... and since SN2 supports a lot > of nodes, the bits in question are way up high). Not too sure. SPARSEMEM certainly can handle sparseness within a node better than DISCONTIGMEM I think (discontigmem is basically like a flat mem per-node so it gets redundant mem_map over holes within a node). I figured it was probably something to do with sn2, but I wondered about bugs in the other cases too. > > My followup question... I wonder why memory hotplug is disabled for > > discontigmem? It seems like there is code there to handle it (athough > > I don't know the memory hotplug code well so I didn't look too closely). > > Not at all sure of the history for this one. OK, I thought I'd ask here because some of the ia64 people are involved in developing mem hotplug. Thanks, Nick