Level 3 or L3 cache is specialized memory that works hand-in-hand with L1 and L2 cache to improve computer performance. L1, L2 and L3 cache are computer processing unit (CPU) caches, verses other types of caches in the system such as hard disk cache. CPU cache caters to the needs of the microprocessor by anticipating data requests so that processing instructions are provided without delay. CPU cache is faster than random access memory (RAM), and is designed to prevent bottlenecks in performance.
When a request is made of the system the CPU requires instructions for executing that request. The CPU works many times faster than system RAM, so to cut down on delays, L1 cache has bits of data at the ready that it anticipates will be needed. L1 cache is very small, which allows it to be very fast. If the instructions aren’t present in L1 cache, the CPU checks L2, a slightly larger pool of cache, with a little longer latency. With each cache miss it looks to the next level of cache. L3 cache can be far larger than L1 and L2, and even though it’s also slower, it’s still a lot faster than fetching from RAM.
Assuming the needed instructions are found in L3 cache (a cache hit), bits of data might be evicted from L1 cache to hold the new instructions in case they’re needed again. L3 cache can then remove that line of instructions since it now resides in another cache (referred to as exclusive cache), or it might hang on to a copy (referred to as inclusive cache), depending on the design of the CPU.
For example, in November 2008 AMD® released their quad-core Shanghai chip. Each core has its own L1 and L2 caches, but the cores share a common L3 cache. L3 keeps copies of requested items in case a different core makes a subsequent request.
The architecture for multi-level cache continues to evolve. L1 cache used to be external to the CPU, built into the motherboard, but now both L1 and L2 caches are commonly incorporated into the CPU die. L3 cache has typically been built into the motherboard, but some CPU models are already incorporating L3 cache. The advantage of having on-board cache is that it’s faster, more efficient and less expensive than placing separate cache on the motherboard.
Fetching instructions from cache is faster than calling upon system RAM, and a good cache design greatly improves system performance. Cache design and strategy will be different on various motherboards and CPUs, but all else being equal, more cache is better.
