Difference between revisions of "Page size"
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* SPECbench numbers have been shown to be boosted significantly by 16kB pages and yet somewhat more by 64kB pages.
== address space ==
Revision as of 19:27, 27 September 2012
Since a while the Linux/MIPS kernel support page sizes other than 4kB.
- The R6000 processor only supports 16kB page size.
- The R8000 processor supports 8kB page size as it's smallest pagesize. It's also the only MIPS processor to support 8kB page size.
- Large compute workloads benefit sometimes dramatically from the increased TLB reach. Downside: the memory overhead can be significant and at times prohibitive.
- Large pagesize eleminates cache aliases and the overhead and potencial for bugs associated with them.
- SPECbench numbers have been shown to be boosted significantly by 16kB pages and yet somewhat more by 64kB pages.
Addressable virtual address space
|Page Size||1 Level||2 Level||3 Levels||4 Levels|
|4kB||21 bits / 2MB||30 bits / 1GB||39 bits / 512GB||48 bits / 256TB|
|8kB||23 bits / 8MB||33 bits / 8GB||43 bits / 8TB||53 bits / 8PB|
|16kB||25 bits / 32MB||36 bits / 64GB||47 bits / 128TB||58 bits / 256PB|
|32kB||27 bits / 128MB||39 bits / 512GB||51 bits / 2PB||64 bits / 4EB|
|64kB||29 bits / 512MB||42 bits / 4TB||55 bits / 32PB||68 bits / 256EB|
This table is mostly meant to show how the numbers work out. In reality there are a few constraints, so it should be taken with a grain of salt:
- For 3 level page tableswith 4kB pages Linux uses two pages for the page directory so the actual virtual address space capacity is 40 bits / 1TB.
- The largest virtual address space supported by any MIPS processor is 48bits on the old and rather odd R8000 processor. Typical 64-bit MIPS processors support 36-bit, 40-bit or 44-bit address spaces but other sizes are permitted by the MIPS64 architecture.
- Address spaces of less than 2GB are not likely to be very useful either.
- Only 3-level tables are supported by 64-bit MIPS kernels.
- As of 2.6.18 16kB pagesize is supported for 64-bit kernels but experimental for the 32-bit kernel. 64kB is experimental.
Hugetlbfs is a special magic filesystem. Any file mmapped from hugetlbfs will use huge pages. The problem with hugetlbfs is that it software will need to be modified to use it and memory to be used for huge pages with hugetlbfs needs to be put aside manually.
Cavium's SDK is able to put away with the need to modify software by using an special preload library that wraps malloc. This method works for most but not all applications.
Transparent Huge Pages
Transparent Huge Pages allows normal unmodified software to use huge pages. Other than possibly having to enable THP support there is no administrative activity required. The price for this is a slight overhead.