L3 Cache Explained [CPUs]

With AMD highlighting the performance impact of L3 cache through their 3D V-cache design, there’s no better time to learn about this important component of CPUs. This article will explain the L3 cache and its working. 

Key Takeaways

• L3 cache is the third type of CPU cache. It’s much larger (and capacious) but slower than the L2 cache. It is also further away from the CPU cores than the L2 cache. 
• Like L1 and L2 cache, it reduces the latency of CPU-DRAM operations. It also speeds up the lower-level caches.  
• It is in the form of a pool shared by multiple cores, while in the case of higher-level caches (L1 and L2), each core has a fixed capacity of them.
• About capacity, L3 cache varies from 4 MB in entry-level CPUs to 128 MB in enthusiast-grade CPUs. 

CPU Cache

Before diving into the main topic, you’ll need to know what CPU cache is in general. A CPU cache is a temporary form of storage that is much faster than system memory or DRAM. Thus, the CPU cache reduces the latency of CPU-DRAM operations as it’s much closer to the CPU than DRAM is.

A CPU cache is made up of extremely fast memory called SRAM or Static RAM. This speed comes at a cost, as it’s much more expensive than DRAM. There are three CPU cache levels, one of which is L3. 

L3 Cache

L3 cache is the third level or “type” of CPU cache that is much larger (but slower) than L1 and L2 cache. At the same, it’s further away from the CPU cores than the higher-level caches. 

Each core has a specific amount of L1 and L2 cache memory. Meanwhile, the L3 cache is in the form of a pool shared by multiple cores (usually up to 4). This is visualized in the diagram above. 

Although the L3 cache is the slowest of the three cache levels, it is still way faster than system memory. In terms of size, the L3 cache varies from 4 MB of entry-level CPUs to 128 MB of enthusiast gaming CPUs (such as the Ryzen 9 7950X3D). 

How Does L3 Cache Work?

The CPU stores instructions it needs quick access to in the cache. Depending on the specific instructions, they may be stored in the L3 cache. This traffic of instructions moves from DRAM to the CPU cache through the system bus for this purpose.

When the CPU needs to process these instructions, it looks for them in the L1 cache. If it fails to find the instructions in the L1 cache, it searches in the L2 cache. If it does not find the instructions in the L2 cache, it will look for them in the L3 cache.

Assuming the CPU finds the necessary instructions in the L3 cache, it will use them to carry out a specific task. This is called a cache hit. It’s worth mentioning that if the CPU does not find the instructions in the L3 cache, it proceeds to look in the system memory. 

Why Companies Are Focusing On More Cache

CPU manufacturers in Intel and AMD are actively focusing on fitting more cache into their CPUs, along with more cores and higher clock speeds. This is because the cache is a very important operator in your CPU, allowing the CPU to process workloads much faster. The more cache there is, the more room the CPU has to store instructions for quick access. In general, more cache is always better than less cache.

AMD’s revolutionary 3D V-cache technology vertically stacks L3 cache on their CPU dies. This allows much more L3 cache to be stacked into a single die. The performance upgrade brought by this seemingly simple move is impressive. For example, Ryzen 7 5800X3D outperformed the Ryzen 9 5900X by about 10% despite having lower clock speeds and fewer cores. 

Final Words

The L3 cache is a vital component of your CPU. It drastically reduces operation times by acting as a bridge between the CPU and the system memory. Still, the L3 cache shouldn’t be your primary focus when looking for a CPU. Clock speeds and core/thread counts are much more important factors to consider.