1. If I am correct, a page in a main memory is the smallest unit unit for transfer data between the main memory and an external storage device, such as a hard disk. A cache line of a main memory is the smallest unit for transfer data between the main memory and the cpu caches.

  2. I wonder if a page size is always or best to be a natural number of cache line size? If a cache line size is 64 byte, and a memory page size is 4KB, then each page has 4KB / 64 bytes == 64 cache lines in it.

  3. Are a page and a cache line both fixed objects in a memory? Or are they just any contiguous block of a memory of a certain size, which can start and float anywhere within the memory?

  4. Is it always that a cache line cannot span more than one pages, i.e. part of a cache line is in a page and the other part of the cache line is in another page?


  • 2
    The answers to your questions are highly CPU-architecture-dependent.
    – TMN
    Commented Jan 15, 2015 at 15:40

1 Answer 1

  1. A cache line is the smallest unit that you can touch physical memory with. Meaning when you read/write 1 byte, a full cache line containing it is read into the cpu cache and written back. Note that even instructions that bypass the cache to write (ephemeral streaming instructions) write in cache line sizes. Depending on the CPU, cache line sizes are typically 32/64/128 bytes. When Memory pages are written to disk, they are written in whole. This will happen when the memory pressure is too high or with hibernating processes (and possibly for other uses). They will also be read whole when needed again. This is mainly because the kernel has no way of knowing if the page is partially or fully used. Other external storage read/write operations can have arbitrary granularity (eg: fwrite(..)).

  2. The page size is CPU/GPU dependent. For most CPUs, page sizes will be at least 4KB and generally support a mix of 4KB/64KB/2MB/4MB/16MB/1GB (not necessarily all supported at the same time). The size will always be a power of two.

  3. Both a page and a cache line are contiguous and require to be aligned to their respective size. A 64 byte cache line is always 64 byte aligned and a 2MB page is always 2MB aligned.

  4. Because of #3, a cache line can never span 2 pages since page sizes will always be larger than a cache line and will always be a multiple of the size of a cache line.

See this excellent source for a wealth of information regarding memory: http://lwn.net/Articles/250967/

  • Is this answer still relevant in 2020? Have these sizes changed?
    – J'e
    Commented Aug 11, 2020 at 13:27
  • 3
    As far as I know, those values remain relevant today. All modern x86 and x64 CPUs use 64 bytes per cache line and 4KB remains the most common page size under Windows and Linux. For embedded devices, check your processor internals manual for details on what is supported. Commented Aug 12, 2020 at 14:18

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.