All about AMD’s revolutionary V-Cache for Ryzen

AMD’s mic-drop moment at Computex was the news of its closely guarded “V-Cache” technology, which would enable chip stacking on Ryzen CPUs.

PCWorld interviewed AMD’s Sam Naffziger about V-Cache. We boiled down that conversation into a baker’s dozen of key things you need to know. 

amd computex 2021 show keynote 3d chiplet technology page 06 AMD

The V-Cache in the CPU above sits above the existing CPU’s L3 cache. Additional silicon next to it is used to stiffen the die and convey heat to the heat spreader.

What is V-Cache?

V-Cache uses TSMC’s SoIC (System on Integrated Chips) chip-stacking technology to add 64MB of SRAM L3 cache to the compute dies of existing Zen 3-based Ryzen CPUs. That will basically triple the amount of cache to an insane 192MB of L3 for the 12-core and 16-core versions of AMD’s CPUs.

V-Cache will come to Zen 3-based Ryzen CPUs later this year

There was a little confusion as to which CPUs would get V-Cache, and when, as AMD CEO Lisa Su held up a prototype Ryzen with the technology. The company has since confirmed that yes, V-Cache will come to high-end, Zen 3-based Ryzen CPUs at the end of the year.

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AMD previously used TSV (Through-Silicon Via) technology in its original Vega GPU’s HBM memory. As you can see, it channels through the silicon itself to connect to the next layer down.

How is V-Cache connected?

The individual 64MB SRAM cache will be stacked on each compute die over the existing L3 cache and be connected using a technology called Through-Silicon Via, which connects stacked chips using a tunneling system. This isn’t AMD’s first experience with TSV: The original Vega GPU with its HBM memory also used TSV technology.

Latency shouldn’t be an issue

One concern with the large amount of L3 is latency, or delay, as the CPU fetches instructions or data from the cache. AMD’s Naffziger said that shouldn’t be a problem  with V-Cache, because the TSV construction offers a more direct route to the cache.

How fast will V-Cache be?

AMD is touting a jaw-dropping 2TBps and beyond of bandwidth for V-Cache. That’s insanely fast. As a comparison, Intel’s 2013-era Core i7-4770R “Crystal Well” chip, which featured an impressive 128MB of eDRAM L4, offered about 100GBps of bandwidth at the time.

Can you stack more V-Cache on top of each other?

Although that may not happen with the first Ryzen chips with V-Cache, there’s nothing that would prevent the stacking of V-Cache.

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