L4 Cache
In this article, we have explored the idea of L4 cache which is a relatively rarely used cache technology and is known to improve Graphics performance of system. L4 cache was first introduced into production by Intel in 2013.
Table of contents:
- What is L4 Cache?
- L4 cache in real systems
- Difference between L4 cache and L1, L2 & L3 cache
- Conclusion
Pre-requisites:
What is L4 Cache?
L4 Cache is a level 4 cache and is an uncommon cache technology which is used in specific systems like Intel Broadwell i7-5775C. L4 cache comes after L3 cache.
Key points about L4 Cache:
- L4 cache is made up of DRAM or eDRAM. SRAM is the common choice of other caches.
- L4 is larger than L1, L2 and L3 cache.
- L4 cache is slower than L1, L2 and L3 cache.
- L4 cache is accessed by both CPU and GPU.
L4 cache was first introduced by Intel through Haswell microarchitecture in 4th June, 2013. It had 128MB of L4 cache and was made up of embedded Dynamic Random Access Memory (eDRAM). It was linked to L3 cache and can be used by both CPU and on-die GPU.
Following diagram illustrates the positioning of L4 cache in the system:
L4 cache improves the Graphics performance of the system while there is no much difference in the overall System performance.
L4 cache in real systems
L4 cache size | |
|---|---|
| Size in MB (Mega Byte) | |
| Intel SkyLake 6785R | 128 MB |
| Intel SkyLake 6660U Mobile | 64 MB |
| Intel KabyLake 7560U | 64 MB |
L4 cache is made up of eDRAM which is mainly used by GPUs. CPU performance do not benefit much from introduction of L4 cache. Hence, L4 cache gives a major boost to Graphics Performance. eDRAM is expensive as well.
For all these reasons, L4 cache is not popular today and is used in specific processors only and these processors are mainly targetted towards low-powered laptops or mobile devices. Hence, L4 cache is not available in all Intel processors.
Difference between L4 cache and L1, L2 & L3 cache
Difference between L4 cache and L1, L2 & L3 cache are as follows:
L1 vs L2 vs L3 vs L4 cache | |
|---|---|
| Differences between L1, L2, L3 and L4 cache | |
| Speed | L1 > L2 > L3 > L4 |
| Size | L4 > L3 > L2 > L1 |
| Sharing | L4 is shared by L3 caches L3 is shared among CPU cores L1, L2 is not shared. |
| Data, Instruction cache | L1 is both data and instruction. L2, L3, L4 is only data cache |
| Location | L1 is in CPU core L2: in chip, outside CPU core L3 outside chip L4 outside chip |
| Performance (CPU, GPU) | L1, L2, L3 improves CPU performance L4 improves GPU performance |
Conclusion
With this article at OpenGenus, you have a good idea of the importance of L4 cache and how does it compare with other cache systems like L1, L2 and L3 cache.