Translation lookaside buffer

Flushing the TLB

One other wrinkle occurs with TLBs. We saw that if a MMU keeps the page table in registers, then every time a context switch occurs, the OS must copy the new process's page table into the registers.

Similarly, when a context switch occurs when a TLB is being used, the OS must take care that the MMU is using only PTEs from the new process, not entries from the old process (because any two processes have different page tables, and if one was not careful, the MMU might be using cached PTEs from the old process instead of the new one.

1. Flush the TLB on every context switch. This ensures that all the PTEs for the new process will be fetched initially from page tables in memory (but thereafter they will be cached). Each PTE in the TLB has a 'valid' bit. 'Invalid' lines in the cache are ignored. Flushing the cache means invalidating all the entries in the cache.

   

2. Include a 'context number' along with every PTE in the TLB. The OS allocates every process a unique context ID, which is stored in a special MMU register while the process is running. The context ID is implicitly added to the front of every page number when the comparison is made with the entries in the TLB. Only PTEs with a context ID matching the process's context ID are matched. For example, processes 1 & 2. Process 1 accesses page 4, and the PTE for page 4 is stored in the TLB. Context switch. Thread in process 2 accesses its page 4. MMU sees that the PTEs have different context numbers and treats them as separate pages.

   

PTEs in the TLB must sometimes be invalidated for other reasons. For example, when a page is swapped out of memory, to disk, its PTE in the TLB must be marked as invalid. In fact in any case where a page table entry is deleted or modified, the entry must be removed from the TLB if it is there, forcing the MMU to read the updated entry from the page table next time the page is accessed.

In the case of the SPARC, if a process were terminated, all the PTEs with its context number would be flushed, to allow the context number to be reused for another, new process.