How does l3 cache work

The L1 cache has a 1ns access latency and a percent hit rate. It, therefore, takes our CPU nanoseconds to perform this operation. Haswell-E die shot click to zoom in. The repetitive structures in the middle of the chip are 20MB of shared L3 cache. Now, assume the cache has a 99 percent hit rate, but the data the CPU actually needs for its th access is sitting in L2, with a cycle 10ns access latency.

That means it takes the CPU 99 nanoseconds to perform the first 99 reads and 10 nanoseconds to perform the th. A 1 percent reduction in hit rate has just slowed the CPU down by 10 percent. If the data has been evicted from the cache and is sitting in main memory, with an access latency of ns, the performance difference between a 95 and 97 percent hit rate could nearly double the total time needed to execute the code.

A cache is contended when two different threads are writing and overwriting data in the same memory space. It hurts the performance of both threads — each core is forced to spend time writing its own preferred data into the L1, only for the other core promptly overwrite that information. Later Ryzen CPUs do not share cache in this fashion and do not suffer from this problem.

This graph shows how the hit rate of the Opteron an original Bulldozer processor dropped off when both cores were active, in at least some tests. Zen 2 does not have these kinds of weaknesses today, and the overall cache and memory performance of Zen and Zen 2 is much better than the older Piledriver architecture.

These tiny cache pools operate under the same general principles as L1 and L2, but represent an even-smaller pool of memory that the CPU can access at even lower latencies than L1. Often companies will adjust these capabilities against each other. In contrast to the L1 and L2 caches, both of which are typically fixed and vary only very slightly and mostly for budget parts both AMD and Intel offer different chips with significantly different amounts of L3.

Intel typically sells at least a few Xeons with lower core counts, higher frequencies, and a higher L3 cache-per-CPU ratio. Home Computing L2 vs. This site may earn affiliate commissions from the links on this page. The more cache there is, the more data can be stored closer to the CPU. Not a lot of physical space is allocated for cache.

There is more space for RAM, which is usually larger and less expensive. Necessary Necessary. Necessary cookies are absolutely essential for the website to function properly.

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