What is TLB in computer architecture, and how is it surprisingly related to cabrio cars? It will be a simple question after you read the article, but we can give you a little hint: It is all about speed, speed, and speed. Despite processing fewer data, similar to cabrios’ two seats, TLB ensures quick and simple access to the page table like a beautiful road ride at midnight.
Every memory visit by a program takes at least twice as long since page tables are kept in main memory, requiring two memory accesses—one to get the physical address and the other to get the contents. We may use both temporal and spatial locality to prevent this by building a unique cache just for storing recently used translations. That’s the point that TLB (Translation lookaside buffer) steps on the gas.
What is TLB in computer architecture?
The most recent translations from virtual memory to physical memory are kept in a memory cache, and it’s called a translation look-aside buffer (TLB).
The search begins in the CPU when the software references a virtual memory address. Instruction caches are examined first. The system must look for the memory’s physical address if the necessary memory is not already in these extremely quick caches. The place in physical memory is now quickly referenced using the TLB.
The function of TLB is to speed up access to a user’s memory location. It is referred to as an address-translation cache.
Translation lookaside buffer is a component of the chip’s memory management unit (MMU). A TLB may be located between the CPU and the CPU cache or between the several levels of the multi-level cache.
One or more TLBs are typically present in the memory-management hardware of desktop, laptop, and server CPUs. They are almost always present in processors that use paged or segmented virtual memory.
The TLB serves as a page table cache for entries that only correspond to physical pages. A portion of the page table’s virtual-to-physical page mappings is also present in the TLB. Blue represents the TLB mappings.
The tag field is necessary because the TLB acts as a cache. The page table must be looked at if the TLB does not include a matching entry for a given page. The page table either provides the page with a physical page number (which can subsequently be used to create a TLB entry) or signals that the page is stored on disk, in which case a page fault happens.
How is the TLB tag calculated?
The number of address bits less the number of index bits less the number of offset bits would be the tag size in a data cache (within the cache block).
Multitasking and programming flaws can harm TLB performance. Cache thrash is the term used to describe this performance decline. Cache thrash results from an ongoing computer process that stalls out because of resource conflicts or excessive resource usage.
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TLB miss handling
What happens if the virtual page number does not match a TLB entry?
This occurrence is known as a TLB miss, and depending on the CPU architecture, one of two approaches is taken:
- Hardware TLB miss handling: In this instance, the CPU proceeds to walk the page table in search of the appropriate PTE. The CPU adds the updated translation to the TLB if the PTE can be located and is flagged as a present. If this doesn’t happen, the CPU raises a page fault and gives the operating system control.
- Software TLB miss handling: The CPU in this situation merely raises a TLB miss fault. The operating system detects the fault and invokes the TLB miss handler. The new translation is subsequently placed in the TLB once the miss handler walks the page table in software and finds a matching Pte that is tagged present. Control is passed to the page fault handler if the PTE cannot be located.
The end consequence of a TLB miss handling is a page-table walk, and if a Pte marked present can be found, the TLB is updated with the new translation, regardless of whether the miss handling is done in hardware or software.
You can analyze average L2 miss rates and more in this study.
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Most RISC architectures (like Alpha) and CISC architectures (like IA-32) handle TLB misses using the software. Although less versatile, a hardware solution is frequently faster. In fact, if the hardware does not adequately meet the operating system’s requirements, the performance benefit could be lost.
Typical TLB
How many entries are in TLB? Let’s find out!
TLB performance levels:
- Size: 12 bits – 4,096 entries
- Hit time: 0.5 – 1 clock cycle
- Miss penalty: 10 – 100 clock cycles
- Miss rate: 0.01 – 1% (20–40% for sparse/graph applications)
Effective memory access time (EMAT)
- EMAT = h(c+m) + (1-h)(c+2m)
- h = Hit ratio of TLB
- m = Memory access time
- c = TLB access time
By using the formula, we can determine that:
- If the TLB hit rate increases, the effective access time will also decrease.
- Multilevel paging will result in a longer effective access time.
How is the TLB tag calculated?
The number of address bits less the number of index bits less the number of offset bits would be the tag size in a data cache (within the cache block).
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Advantages and disadvantages of TLB
Like everything else, translation lookaside buffer has both advantages and disadvantages.
Advantages of TLB
- Improved speed.
- Easy access to the page table.
- Cheap.
- Smaller than the main memory.
Disadvantages of TLB
- Wasting primary RAM.
- Size of a page table.
- A single word read from the main memory by the CPU will take longer.
- Multitasking and programming flaws.
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TLB vs page table: The difference
The page table links every virtual page to the corresponding physical frame. The TLB accomplishes the same thing, except it only includes a portion of the page table. So you can ask, “What is the function of TLB if the page table does the same thing with more data?” For the same reason that people want a new cabrio car, speed even there is less room.
Why is TLB faster than the page table?
A cache that keeps (presumably) recently utilized pages is called the TLB. According to the principle of locality, the pages mentioned in the TLB will probably be used again soon. This is the fundamental concept behind all caching. Finding the page’s address in the TLB is quick and easy when these pages are needed again. Walking the length of the page table to locate the required page’s address can be slow because the page table itself can be quite large.
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TLB vs cache: Are TLB and cache the same?
What’s the difference between CPU Cache and TLB, especially when people generally suggest that TLB is also a cache type? Both CPU Cache and TLB are hardware components utilized in microprocessors, but there are some differences:
| CPU Cache | TLB |
| Central Processing Unit Cache is referred to as CPU cache. | Translation Lookaside Buffer is known as TLB. |
| Hardware cache | Memory cache |
| Data access from the main memory takes less time on average thanks to it. | It is used to shorten the time it takes for a user to reach a memory location from our computer’s main memory. |
| Being closer to the processor core, it keeps duplicates of the data from frequently accessed main memory locations. | The majority of our machines have multiple TLBs in MMH (memory management hardware) |
