Processor cache how much

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. The instruction cache deals with the information about the operation that the CPU must perform, while the data cache holds the data on which the operation is to be performed. L2 Level 2 cache is slower than the L1 cache but bigger in size.

Where an L1 cache may measure in kilobytes, modern L2 memory caches measure in megabytes. When it comes to speed, the L2 cache lags behind the L1 cache but is still much faster than your system RAM. The L1 memory cache is typically times faster than your RAM, while the L2 cache is around 25 times faster.

Onto the L3 Level 3 cache. Larger caches are both slower and more expensive. At six transistors per bit of SRAM 6T , cache is also expensive in terms of die size, and therefore dollar cost. The performance impact of adding a CPU cache is directly related to its efficiency or hit rate; repeated cache misses can have a catastrophic impact on CPU performance.

The following example is vastly simplified but should serve to illustrate the point. Imagine that a CPU has to load data from the L1 cache times in a row.

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.

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