Some processors use an inclusive cache design meaning data stored in the L1 cache is also duplicated in the L2 cache while others are exclusive meaning the two caches never share data. This chart shows the relationship between an L1 cache with a constant hit rate, but a larger L2 cache. Note that the total hit rate goes up sharply as the size of the L2 increases.
A larger, slower, cheaper L2 can provide all the benefits of a large L1, but without the die size and power consumption penalty.
Most modern L1 cache rates have hit rates far above the theoretical 50 percent shown here — Intel and AMD both typically field cache hit rates of 95 percent or higher. The next important topic is the set-associativity. The tag RAM is a record of all the memory locations that can map to any given block of cache.
If a cache is fully associative, it means that any block of RAM data can be stored in any block of cache. The advantage of such a system is that the hit rate is high, but the search time is extremely long — the CPU has to look through its entire cache to find out if the data is present before searching main memory. At the opposite end of the spectrum, we have direct-mapped caches.
A direct-mapped cache is a cache where each cache block can contain one and only one block of main memory. This type of cache can be searched extremely quickly, but since it maps to memory locations, it has a low hit rate. In between these two extremes are n- way associative caches.
An eight-way associative cache means that each block of main memory could be in one of eight cache blocks. The next two slides show how hit rate improves with set associativity. Keep in mind that things like hit rate are highly particular — different applications will have different hit rates.
So why add continually larger caches in the first place? Because each additional memory pool pushes back the need to access main memory and can improve performance in specific cases. Each stair step represents a new level of cache. Larger caches are both slower and more expensive. In the early days of computing, processor speed and memory speed were low. However, during the s, processor speeds began to increase—rapidly.
There is primary storage, like a hard disk or SSD, which stores the bulk of the data—the operating system and programs. This is much faster than the primary storage but is only a short-term storage medium.
Your computer and the programs on it use RAM to store frequently accessed data, helping to keep actions on your computer nice and fast. Computer memory has a hierarchy based upon its operational speed. The CPU cache stands at the top of this hierarchy, being the fastest. It is also the closest to where the central processing occurs, being a part of the CPU itself.
Programs and apps on your computer are designed as a set of instructions that the CPU interprets and runs. When you run a program, the instructions make their way from the primary storage your hard drive to the CPU. This is where the memory hierarchy comes into play. CPUs these days are capable of carrying out a gigantic number of instructions per second. The memory cache then carries out the back and forth of data within the CPU.
Memory hierarchy exists within the CPU cache, too. The memory hierarchy is again according to the speed and, thus, the size of the cache. L1 Level 1 cache is the fastest memory that is present in a computer system. In terms of priority of access, the L1 cache has the data the CPU is most likely to need while completing a certain task.
The size of the L1 cache depends on the CPU. There is no "standard" L1 cache size, so you must check the CPU specs to determine the exact L1 memory cache size before purchasing. The L1 cache is usually split into two sections: the instruction cache and the data cache. There are various ways of implementing cache hierarchies.
Most PC systems have processors with a small first-level cache L1, up to kB , which is often divided into a data cache and an instruction cache. L2 caches can work exclusively or inclusively, which means that they either store a copy of the L1 contents - or they don't. AMD will soon offer a third cache level, which will be used as a shared cache memory for the AMD Phenom processors with up to four cores. The same is anticipated for Intel's Nehalem processor architecture, which will replace Core 2.
L1 cache has always been on the processor, while first L2 caches were implemented onto motherboards, as it was the case with many DX computers and Pentium machines. Simple SRAM chips static RAM were used as first-cache memory; pipelined burst cache took over soon Pentium until on-chip and on-die caches became possible.
The Pentium Pro at to MHz was the first processor to host kB L2 cache memory inside the CPU, making it the largest ceramic package ever on desktops or workstations. Integrated L2 cache resulted in considerably improved performance across virtually all applications.
0コメント