Yesterday we mentioned how caches work, what the distinction is between L1 and L2, and the assorted design parts that decide how briskly (and the way efficient) a CPU’s cache is. Right this moment, we’re going to take one step additional and discover the distinction between L2 and L3 caches.
At its easiest stage, an L3 cache is only a bigger, slower model of the L2 cache. Again when most chips had been single-core processors, this was usually true. The primary L3 caches had been truly constructed on the motherboard itself, linked to the CPU through the bottom bus. When AMD launched its K6-III processor household, many current K6/Ok-2 motherboards might settle for a K6-III as properly. Sometimes these boards had 512Ok-2MB of L2 cache — when a K6-III, with its built-in L2 cache was inserted, these slower, motherboard-based caches grew to become L3 as an alternative.
By the flip of the century, slapping a further L3 cache on a chip had grow to be a straightforward method to enhance efficiency — Intel’s first consumer-oriented Pentium four “Excessive Version” was a repurposed Gallatin Xeon with a 2MB L3 on-die. Including that cache was adequate to purchase the Pentium four EE a 10-20 % efficiency enhance over the usual Northwood line.
Cache and the multi-core curveball
As multicore processors grew to become extra widespread, L3 cache began showing extra continuously on shopper . These chips, like Intel’s Nehalem and AMD’s K10 (Barcelona) used L3 as greater than only a bigger, slower backstop for L2. Along with this operate, the L3 cache is commonly shared between the entire processors on a single piece of silicon. That’s in distinction to the L1 and L2 caches, each of which are typically non-public and devoted to the wants of every specific core. (AMD’s Bulldozer design is an exception to this — Bulldozer, Piledriver, and Steamroller all share a standard L1 instruction cache between the 2 cores in every module).
Intel’s Haswell-E, for instance, has eight separate cores that every one again as much as a standard L3 cache.
Non-public L1/L2 caches and a shared L3 is hardly the one option to design a cache hierarchy, nevertheless it’s a standard method that a number of distributors have adopted. Giving every particular person core a devoted L1 and L2 cuts entry latencies and reduces the prospect of cache rivalry — that means two completely different cores gained’t overwrite important information that the opposite put in a location in favor of their very own workload. The widespread L3 cache is slower however a lot bigger, which implies it might probably retailer information for all of the cores directly. Subtle algorithms are used to make sure that Core zero tends to retailer info closest to itself, whereas Core 7 throughout the die additionally places crucial information nearer to itself.
In contrast to the L1 and L2, that are practically all the time CPU-focused and personal, the L3 may also be shared with different units or capabilities. Intel’s Sandy Bridge CPUs shared an 8MB L3 cache with the on-die graphics core (Ivy Bridge gave the GPU its personal devoted slice of L3 cache in lieu of sharing the whole 8MB).
In distinction to the L1 and L2 caches, each of that are sometimes mounted and differ solely very barely (and principally for price range elements) each AMD and Intel supply completely different chips with considerably completely different quantities of L3. Intel sometimes sells a minimum of just a few Xeons with decrease core counts, larger frequencies, and the next L3 cache-per-CPU ratio. Intel’s Core i7 processors have maintained an 8MB L3 for the reason that debut of Nehalem in 2008 (roughly 2MB of L3 for each CPU core) however the highest-end elements are sometimes pegged at 2.5MB of cache per CPU core.
Right this moment, the L3 is characterised as a pool of quick reminiscence widespread to all of the CPUs on an SoC. It’s usually gated independently from the remainder of the CPU core and might be dynamically partitioned to steadiness entry pace, energy consumption, and storage capability. Whereas not practically as quick as L1 or L2, it’s usually extra versatile and performs an important function in managing inter-core communication. With Intel having already added L4 to its Skylake chips, it’s doable we’ll see the L3 take a extra simplified function — with a few of its capabilities and capabilities shifting over to the newer, bigger pool of cache.