Instances Of L3 Cache (Quad, Hexa & Octa Core Processors)

Nature of L3 Cache

L3 Cache is pooled and shared by all of the cores (in most instances) unlike the L1 and L2 cache that are dedicated to each core separately.

While L3 cache is the largest of the four well known caches, it is also the slowest. In order to ensure that each core uses the L3 cache closest to it physically, algorithms are used.

L3 is essential for carrying out specific functions separate from the L1, L2 and L4 caches.

How Do You Find Out about the The Cache Configuration in Your Processor

There are countless professional websites out there that review all the latest processors in depth.

Websites like notebookcheck.net have all the information regarding the processors including information on their micro architecture.

Alternately, you can visit the manufacturers websites. They usually have article detailing the micro architecture of their processors.

Related: Components Of A Processor (CPUs Explained For The ALU)

Instances of L3 Cache in Quad Core Processors

Quad core processors from both AMD and Intel use a single instance of L3 cache.

Die Map of Intel Ivy Bridge Quad Core Processor

This is a die chart for an Ivy Bridge Quad Core processor. While it is a relatively old CPU, the same architecture is used for the current Quad Core processors.

As you can see here, the four Cores share a pool of L3 cache. However, an algorithm makes sure that each core accesses the L3 cache directly below it.

Therefore, the instances where Core 0 has to access the L3 cache next to Core 3 is quite rare, though, not impossible.

Instances of L3 Cahce in Hexa Core Processors

Hexa core processors have currently become famous with the Intel 8th Generation Coffee Lake Processors.

For example, the latest high end 8th generation Intel Core i7 processors are all hexa core processors with hyper-threading. 

Related: How Many Threads Can A Quad Core Processor Handle At Once?

Block Diagram of Intel Hexa-Core Coffee Lake Processors

This block diagram shows the working of the L3 Cache in a much more detailed manner.

As you can see the brownish blocks next to each core are the L3 Cache. They are all connected together via a Ring Bus. Therefore, while technically each core can access all L3 Cache blocks via the Ring route, they tend to only access the ones next to them as much as possible.

Again, here we can see that the Hexa Core processors from Intel also SHARE the L3 Cache. Therefore, there is only one instance.

Instances of L3 Cache in Octa Core Processors

This is where the distinction between AMD Ryzen and Intel Core processor becomes way more visible.

While the Octa Core Intel processor still use a Shared L3 Cache, the Octa Core Ryzen processors have TWO instances of L3 Cache.

Die Map of Intel Octa-Core Haswell-E Processors

Haswell-E was the code-name for the fifth generation Intel Core series. While they are a bit older, Intel still uses the same architecture for l3 Cache.

As you can see in this image, all of the 8 cores are sharing the same L3 Cache.

AMD’s Case: CPU Complex

The Octa Core processors, Such as the Ryzen 7 from the 1st and 2nd generation, use a somewhat different architecture for L3 as compared to the Intel processors.

AMD uses what they call a CPU Complex or CCX as a unit to define 4 cores connected to an L3 Cache.

While a Quad Core processor from AMD Ryzen shares its L3 cache with all of its cores, the Octa Core does not.

This image should illustrate this further

As you can see, this is an Octa-Core processor that utilizes two CPU Complex (CCX) units. Each CCX has four cores and its own pool of L3 shared memory.

The Cores from CCX0 cannot access the L3 cache of the CCX1 and vice versa.

Conclusion

To answer the question: how many instances of L3 Cache in Quad Core, Hexa Core and Octa Core Processors?

The answer is that it depends entirely on the architecture of the processor. Generally, there is only a single instance, or a single pool, of L3 cache in a processor.

However, there can be more instances and separate pools as can be seen in the AMD ZEN microarchitecture.