On Wed, Sep 13, 2006 at 10:23:10AM -0700, Christoph Lameter wrote: > On Wed, 13 Sep 2006, Jes Sorensen wrote: > > > > There was the floppy driver and one type of USB stick that I noticed > > > during the work on the project. But other drivers may depend also depend > > > indirectly on DMA functionality and may also be disabled. > > > > Ok, USB should ring alarm bells, floppy I think is less relevant these > > days :) > > If you want all drivers then you must of course have ZONE_DMA. > Distributions that want to cover all drivers will have it on by default > and ZONE_DMA is available by default. > > However, if you want to create a lean and mean kernel then you can switch > ZONE_DMA off and if there is just one zone left then the VM can > optimized much better because loops are avoided and some macros > become constant etc etc. > > Some architectures never need ZONE_DMA because all hardware supports DMA > to all of memory. SGI Altix is one example. Carrying an additional > useless zone around unecessarily bloats the kernel both in term of code > and data. Data is a particular issue since zones contain per cpu elements. > For a 1k cpu 1k node configuration this saves around 1 million per cpu > structures (one zone per node with 1k per cpu pagesets). > Most distros release GENERIC kernels for IA64. If _any_ IA64 platform requires ZONE_DMA, then it must be configured ON. Two questions: - will any IA64 platform require that ZONE_DMA be enabled (I think the answer is "yes") - if ZONE_DMA is enabled, ALTIX will still use only 1 zone. In your statement above, you say that disabling ZONE_DMA save 1M cpu structures. If ZONE_DMA is enabled, will these 1M structure be allocated on SN even though they are not needed? -- jack - To unsubscribe from this list: send the line "unsubscribe linux-ia64" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
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