Why are Intel ditching Hyperthreading with Lion Cove and Lunar Lake?

Why Intel are saying goodbye to Hyperthreading with Lion Cove and Lunar Lake

Intel’s CPUs have featured hyperthreading for 20 years. The feature is undoubtedly a useful one, so why are Intel ditching it with Lion Cove? Thankfully, we were able to meet the minds behind Intel’s Lion Cove P-Core architecture,. They were happy to give us a rundown on why modern CPU design may remove hyperthreading (also known as SMT) from many future CPU designs. Now that we know, it’s time to pass this knowledge onto you.

We will start by talking about Intel’s design goals with Lion Cove. Simply put, Intel wants to maximise the single-threaded performance, performance per watt, and performance per area, of their new P-Core designs. These are the factors that Intel are considering when designing their newest cores. If a feature consumes too much power, takes up too much area, and doesn’t deliver enough performance to be worthwhile, it should be axed. Simply put, with Lion Cove, hyperthreading is on the chopping block.

Why do most Intel CPU cores feature hyperthreading?

With hyperthreading, Intel can increase the performance of their processors while also increasing their power efficiency. This is why hyperthreading has been a feature of Intel processors for two decades. By allowing a core to run two threads, a processors can make more use of its available resources. That said, changes to CPU design have made hyperthreading less useful.

How Hybrid CPU design changes things

Today’s Intel CPUs are made up to P-cores and E-cores, and threads need to be scheduled to run in an optimal way. For performance, an OS scheduler first places work on a P-core. Next, work is placed on an E-core. Finally, work is allocated to a P-core’s hyperthread.

If we focus on efficiency, work is first placed on E-cores, then P-cores and then P-core hyperthreads. In both scenarios, hyperthreads are used last.

Hybrid computing with both P-cores and E-cores have changed the paradigm. E-cores can now provide more performant and efficient multi-threaded acceleration than hyperthreading. When designing Lunar Lake and Lion Cove, Intel took a long look at hyperthreading, and it simply didn’t make sense anymore.

Yes, hyperthreading still makes sense in all P-core setups. That’s why we expect to continue seeing hyperthreading in future datacenter P-core only CPUs. Even so, it doesn’t make sense everywhere. Next we will explain why.

Why disable hyperthreading?

If we compare a P-core with Hyperthreading disabled and an efficiency optimised P-core without that lacks hyperthreading, we have a core that is smaller and more power efficient. Hyperthreading takes up space, and even when disabled, that feature consumes power.

If we do the same comparison, but enable hyperthreading on the first P-core, we see some interesting results. What we get is more performance per watt, but less performance per area. When it comes to performance/power/area, our new efficiency optimised P-core without hyperthreading still comes out ahead.

Hyperthreading wasn’t the only thing that was removed with Lion Cove. For this core design, Intel’s mantra was to “remove any transistor from the design that doesn’t directly contribute to productivity”. For a hybrid CPU design like Lunar Lake, hyper-threading didn’t make sense anymore. Intel could achieve more performance per watt and performance/power/area without it.

Does Intel Hyperthreading have a future?

The short answer to this question is yes. For P-core only designs where maximum performance matters, hyperthreading will continue to have a place in CPU design. For Lion Cove and Lunar Lake, it makes more sense to focus on single-threaded performance and rely on E-cores for added multi-threaded performance.

So why has Intel ditched hypertheading? The short answer is because it helps them create more efficient processors, and a longer answer is that hybrid CPU designs make hyperthreading a lot less useful than it once was. Intel are rethinking x86 CPU design, and that means that are changing the way that cores are created. For Lion Cove, that means that hyperthreading is going away, and more changes are coming with Intel’s future core designs.

You can join the discussion on Intel’s decision to drop hyperthreading on the OC3D Forums.

Mark Campbell

Mark Campbell

A Northern Irish father, husband, and techie that works to turn tea and coffee into articles when he isn’t painting his extensive minis collection or using things to make other things.

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