On Tue, 2014-06-24 at 15:14 +0100, Catalin Marinas wrote: > On Mon, Jun 23, 2014 at 02:17:03PM +0100, Mark Salter wrote: > > On Mon, 2014-06-23 at 12:09 +0100, Catalin Marinas wrote: > > > My proposal (in the absence of any kind of description) is to still > > > create a ZONE_DMA if we have DMA memory below 32-bit, otherwise just add > > > everything (>32-bit) to ZONE_DMA. Basically an extension from your CMA > > > patch, make dma_phys_limit static in that file and set it to > > > memblock_end_of_DRAM() if no 32-bit DMA. Re-use it in the > > > zone_sizes_init() function for ZONE_DMA (maybe with a pr_info for no > > > 32-bit only DMA zone). > > > > There's a performance issue with all memory being in ZONE_DMA. It means > > all normal allocations will fail on ZONE_NORMAL and then have to fall > > back to ZONE_DMA. It would be better to put some percentage of memory > > in ZONE_DMA. > > Is the performance penalty real or just theoretical? I haven't run any > benchmarks myself. > It is real insofar as you must eat cycles eliminating ZONE_NORMAL from consideration in the page allocation hot path. How much that really costs, I don't know. But it seems like it could be easily avoided by limiting ZONE_DMA size. Is there any reason it needs to be larger than 4GiB?
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