- Buffer Cache
The buffer cache contains data buffers that are used by the block device drivers.
These buffers are of fixed sizes (for example 512 bytes) and contain blocks of information that have either been read from a block device or are being written to it. A block device is one that can only be accessed by reading and writing fixed sized blocks of data. All hard disks are block devices.
The buffer cache is indexed via the device identifier and the desired block number and is used to quickly find a block of data. Block devices are only ever accessed via the buffer cache. If data can be found in the buffer cache then it does not need to be read from the physical block device, for example a hard disk, and access to it is much faster.
- Page Cache
This is used to speed up access to images and data on disk.
It is used to cache the logical contents of a file a page at a time and is accessed via the file and offset within the file. As pages are read into memory from disk, they are cached in the page cache.
- Swap Cache
Only modified (or dirty) pages are saved in the swap file.
So long as these pages are not modified after they have been written to the swap file then the next time the page is swapped out there is no need to write it to the swap file as the page is already in the swap file. Instead the page can simply be discarded. In a heavily swapping system this saves many unnecessary and costly disk operations.
- Hardware Caches
One commonly implemented hardware cache is in the processor; a cache of Page Table Entries. In this case, the processor does not always read the page table directly but instead caches translations for pages as it needs them. These are the Translation Look-aside Buffers and contain cached copies of the page table entries from one or more processes in the system.
When the reference to the virtual address is made, the processor will attempt to find a matching TLB entry. If it finds one, it can directly translate the virtual address into a physical one and perform the correct operation on the data. If the processor cannot find a matching TLB entry then it must get the operating system to help. It does this by signalling the operating system that a TLB miss has occurred. A system specific mechanism is used to deliver that exception to the operating system code that can fix things up. The operating system generates a new TLB entry for the address mapping. When the exception has been cleared, the processor will make another attempt to translate the virtual address. This time it will work because there is now a valid entry in the TLB for that address.
The drawback of using caches, hardware or otherwise, is that in order to save effort Linux must use more time and space maintaining these caches and, if the caches become corrupted, the system will crash.