On Tue, Sep 02, 2014 at 09:51:20AM -0400, Johannes Weiner wrote: > On Mon, Sep 01, 2014 at 01:55:51PM +0100, Mel Gorman wrote: > > Zones are allocated by the page allocator in either node or zone order. > > Node ordering is preferred in terms of locality and is applied automatically > > in one of three cases. > > > > 1. If a node has only low memory > > > > 2. If DMA/DMA32 is a high percentage of memory > > > > 3. If low memory on a single node is greater than 70% of the node size > > > > Otherwise zone ordering is used to preserve low memory. Unfortunately > > a consequence of this is that a machine with balanced NUMA nodes will > > experience different performance characteristics depending on which node > > they happen to start from. > > > > The point of zone ordering is to protect lower nodes for devices that require > > DMA/DMA32 memory. When NUMA was first introduced, this was critical as 32-bit > > NUMA machines commonly suffered from low memory exhaustion problems. On > > 64-bit machines the primary concern is devices that are 32-bit only which > > is less severe than the low memory exhaustion problem on 32-bit NUMA. It > > seems there are really few devices that depends on it. > > > > AGP -- I assume this is getting more rare but even then I think the allocations > > happen early in boot time where lowmem pressure is less of a problem > > > > DRM -- If the device is 32-bit only then there may be low pressure. I didn't > > evaluate these in detail but it looks like some of these are mobile > > graphics card. Not many NUMA laptops out there. DRM folk should know > > better though. > > > > Some TV cards -- Much demand for 32-bit capable TV cards on NUMA machines? > > > > B43 wireless card -- again not really a NUMA thing. > > > > I cannot find a good reason to incur a performance penalty on all 64-bit NUMA > > machines in case someone throws a brain damanged TV or graphics card in there. > > This patch defaults to node-ordering on 64-bit NUMA machines. I was tempted > > to make it default everywhere but I understand that some embedded arches may > > be using 32-bit NUMA where I cannot predict the consequences. > > This patch is a step in the right direction, but I'm not too fond of > further fragmenting this code and where it applies, while leaving all > the complexity from the heuristics and the zonelist building in, just > on spec. Could we at least remove the heuristics too? If anybody is > affected by this, they can always override the default on the cmdline. I see no problem with deleting the heuristics. Default node for 64-bit and default zone for 32-bit sound ok to you? -- Mel Gorman SUSE Labs -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>