On Mon, Nov 13, 2023 at 10:19:48AM +0000, Ryan Roberts wrote: > On 13/11/2023 05:18, Matthew Wilcox wrote: > > My hope is to abolish the 64kB page size configuration. ie instead of > > using the mixture of page sizes that you currently are -- 64k and > > 1M (right? Order-0, and order-4) > > Not quite; the contpte-size for a 64K page size is 2M/order-5. (and yes, it is > 64K/order-4 for a 4K page size, and 2M/order-7 for a 16K page size. I agree that > intuitively you would expect the order to remain constant, but it doesn't). > > The "recommend" setting above will actually enable order-3 as well even though > there is no HW benefit to this. So the full set of available memory sizes here is: > > 64K/order-0, 512K/order-3, 2M/order-5, 512M/order-13 > > > , that 4k, 64k and 2MB (order-0, > > order-4 and order-9) will provide better performance. > > > > Have you run any experiements with a 4kB page size? > > Agree that would be interesting with 64K small-sized THP enabled. And I'd love > to get to a world were we universally deal in variable sized chunks of memory, > aligned on 4K boundaries. > > In my experience though, there are still some performance benefits to 64K base > page vs 4K+contpte; the page tables are more cache efficient for the former case > - 64K of memory is described by 8 bytes in the former vs 8x16=128 bytes in the > latter. In practice the HW will still only read 8 bytes in the latter but that's > taking up a full cache line vs the former where a single cache line stores 8x > 64K entries. This is going to depend on your workload though -- if you're using more 2MB than 64kB, you get to elide a layer of page table with 4k base, rather than taking up 4 cache lines with a 64k base.
