Translation Look aside buffer
Drawbacks of Paging
How to decrease the page table size
How to decrease the effective access time
Locality of reference
In operating systems, the concept of locality of reference states that, instead of loading the entire process in the main memory, OS can load only those number of pages in the main memory that are frequently accessed by the CPU and along with that, the OS can also load only those page table entries which are corresponding to those many pages.
Translation look aside buffer (TLB)
A Translation look aside buffer can be defined as a memory cache which can be used to reduce the time taken to access the page table again and again.
It is a memory cache which is closer to the CPU and the time taken by CPU to access TLB is lesser then that taken to access main memory.
In other words, we can say that TLB is faster and smaller than the main memory but cheaper and bigger than the register.
TLB follows the concept of locality of reference which means that it contains only the entries of those many pages that are frequently accessed by the CPU.
In translation look aside buffers, there are tags and keys with the help of which, the mapping is done.
TLB hit is a condition where the desired entry is found in translation look aside buffer. If this happens then the CPU simply access the actual location in the main memory.
However, if the entry is not found in TLB (TLB miss) then CPU has to access page table in the main memory and then access the actual frame in the main memory.
Therefore, in the case of TLB hit, the effective access time will be lesser as compare to the case of TLB miss.
If the probability of TLB hit is P% (TLB hit rate) then the probability of TLB miss (TLB miss rate) will be (1-P) %.
Therefore, the effective access time can be defined as;
Where, p → TLB hit rate, t → time taken to access TLB, m → time taken to access main memory k = 1, if the single level paging has been implemented.
By the formula, we come to know that