Messages in this thread | | | Subject | Re: memory hotremove prototype, take 3 | Date | Tue, 9 Dec 2003 16:45:10 -0800 | From | "Luck, Tony" <> |
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> If your target is NUMA, then you really, really need CONFIG_NONLINEAR. > We don't support multiple pgdats per node, nor do I wish to, as it'll > make an unholy mess ;-). With CONFIG_NONLINEAR, the discontiguities > within a node are buried down further, so we have much less complexity > to deal with from the main VM. The abstraction also keeps the poor > VM engineers trying to read / write the code saner via simplicity ;-) > > WRT generic discontigmem support (not NUMA), doing that via pgdats > should really go away, as there's no real difference between the > chunks of physical memory as far as the page allocator is concerned. > The plan is to use Daniel's nonlinear stuff to replace that, and keep > the pgdats strictly for NUMA. Same would apply to hotpluggable zones - > I'd hate to end up with 512 pgdats of stuff that are really all the > same memory types underneath.
I guess this all depends on whether you allow bits of memory on nodes to be hot-plugged ... or insist on the whole node being added/removed in one fell swoop. I'd expect the latter to be a more common model, and in that case the "pgdat-for-the-node" is the same as the "pgdat-for-the-hot-plug-zone", so you don't have a proliferation of pgdats to support hotplug.
> The real issue you have is the mapping of the struct pages - if we can > achieve a non-contig mapping of the mem_map / lmem_map array, we should > be able to take memory on and offline reasonably easy. If you're willing > for a first implementation to pre-allocate the struct page array for > every possible virtual address, it makes life a lot easier.
On 64-bit systems with CONFIG_VIRTUAL_MEMMAP, this would be trivial, and avoids the need for the extra level of indirection in the psection[] and vection[] arrays in CONFIG_NONLINEAR (ok ... it doesn't really get rid of the indirection, as the page table lookup to access the virtual mem_map effectively ends up doing the same thing).
> Adding the other layer of indirection for access the struct page array > should fix up most of that, and is very easily abstracted out via the > pfn_to_page macros and friends. I ripped out all the direct references > to mem_map indexing already in 2.6, so it should all be nicely > abstracted out.
I did go back and look at the CONFIG_NONLINEAR patch again, and I still can't see how to make it useful on 64-bit machines. Jack Steiner asked a bunch of questions on how it would work for an architecture like the SGI: http://marc.theaimsgroup.com/?l=lse-tech&m=101828803506249&w=2 I don't remember seeing any answers on the list. Assuming he were to use a section size of 64MB (a convenient number for ia64) he'd end up with psection[]/vsection[] tables with 8 million entries each (@ 4 bytes/entry -> 64MB for the pair).
-Tony Luck
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