Caching Page Tables
Paging is a memory management scheme which allows physical address space of a process to be non-contiguous. The basic idea of paging is to break physical memory into fixed-size blocks called frames and logical memory into blocks of same size called pages. While executing the process the required pages of that process are loaded into available frames from their source that is disc or any backup storage device.
CPU generates logical address for the process which is divided into page number and page offset. The page table contains the base address of each page in physical memory. The base address combined with the page offset defines the physical memory address. The page number is used as an index into a page table. Page table is kept in main memory and a page table base register (PTBR) points to the page table.
To access a location x, find entry in the page table, using the value in the PTBR offset by the page number for x. The page table entry gives the frame number, which is combined with the page offset to produce the actual address. We can then access the required memory location. Thus to access a location x , two memory accesses are required reducing the speed of the operation.
A special, small, fast, lookup hardware cache called Translation Lookaside Buffer (TLB) is used to cache small number of entries from the page table. Each TLB entry consists of two parts: key(or tag) and value, here key is the page number and value is the frame number. All the entries of TLB are compared simultaneously with page number, the search is thus fast. A TLB typically contains 32–1024 entries. TLB is a hardware cache and modern computers implement it as a part of instruction pipeline thus causing no overhead of TLB search.
If a page number is not found in TLB , it is called as a TLB miss, the corresponding frame number is taken from the page table and TLB is updated. Now if TLB is already full, an entry of TLB needs to be replaced with this new entry. Various policies are used for such replacement like Least Recently Used (LRU) , round-robin or random replacement.
CPU today provides multi-level TLB. In case of L1 TLB miss, it is searched in L2 TLB and then in page table if L2 TLB is also a miss.The percentage of times that the page number of interest is found in the TLB is called the hit ratio (hit rate) and the percentage of times it is not found in the TLB is called the miss ratio (miss rate). Modern day systems have TLB miss rate of 0.1–1%, thus reducing overhead of accessing the page tables to a large extent.
Some TLBs also store Address-Space Identifiers(ASIDS) in each TLB entry. It uniquely identifies each process and when translation of logical address to physical address is done, only those TLB entries are checked which belong to that process, if no such entry is found it is considered TLB miss. This allows TLB to have entries for several processes simultaneously.If TLB does not allow ASIDS, then every time a context switch happens (change the executing process from one process to another), the TLB needs to be flushed or erased to ensure that next executing process do not use the translation information from the previous process.
Each TLB entry has a valid/invalid bit associated with it which signifies whether the TLB entry is valid or not. This is particularly useful when TLB is flushed, no actual deletion of TLB entry takes place, only all the entries of TLB are invalidated. Thus, before using the TLB entry for translation it is checked whether the entry is valid or not. Also, in case of TLB miss while updating a TLB entry, invalid entries are updated first.
There are several other bits in a TLB entry like global bit(G) which is used for pages that are shared globally across processes, several bits for ASID (ignored for shared pages), dirty bit (to determine if the page has to be written) etc. All these bits are used in a general TLB entry. However, the actual size of entry, various protection bits in TLB entry, number of levels in TLB, number of TLB entries in each level etc varies from architecture to architecture.
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