Question related to processor's IPC

[Hellowalkman]

Does IPC ( Instructions Per Cycle ) only include Single-threaded performance or does it also include performance gained through Simultaneous Multi-threading ????

 

[Eximo]

Depends on the exact test/benchmark. There are tests for both. But generally it is the Instructions Per Cycle of the entire CPU running at its rated frequency.

 

[blazorthon]

When most people are talking about IPC on Tom's and other such sites, they are usually referring to IPC on a per-core basis. Technically, they are usually using the term incorrectly (whether on a per core basis or for the whole CPU), but that's another matter entirely.

As for "simultaneous multi-threading", I assume you mean Hyper-Threading and similar technologies and no, that is usually not included, especially since mostly just i3s, mobile i5s, and i7s support it nowadays (for the consumer market). It is not included because anything that can take advantage of it needs to be able to use at least two cores/threads simultaneously with good performance scaling, so it isn't part of single-threaded performance.

 

[Hellowalkman]

When most people are talking about IPC on Tom's and other such sites, they are usually referring to IPC on a per-core basis. Technically, they are usually using the term incorrectly (whether on a per core basis or for the whole CPU), but that's another matter entirely.

As for "simultaneous multi-threading", I assume you mean Hyper-Threading and similar technologies and no, that is usually not included, especially since mostly just i3s, mobile i5s, and i7s support it nowadays (for the consumer market). It is not included because anything that can take advantage of it needs to be able to use at least two cores/threads simultaneously with good performance scaling, so it isn't part of single-threaded performance.

My question is related to Zen since it supposedly has 40% higher IPC over Excavator and we know Zen is going to have SMT .

So does AMD mean per core performance jump over Excavator or per thread ??

because if it is per core then single-threaded performance jump is not going to be as big ..

 

[blazorthon]

tl;dr : SMT should not be counted for the IPC gain.

40% higher IPC in that sense means 40% higher performance per core per clock. So, compare say a 4GHz FX-83xx CPU to an 8-core Zen CPU also at 4GHz. Theoretically, the Zen CPU would have 40% higher single-threaded performance and more than 40% higher multi-threaded performance with SMT. Assuming similar performance scaling to Hyper-Threading, we can assume the SMT in Zen would get about up to 30% more multi-threaded performance compared to it being disabled, so that 40% comes out to about 82% higher multi-threaded performance. That's a bigger leap than Intel's made sense Core 2 coming from Netburst about a decade ago and the multi-threaded performance improvement would be huge by nearly doubling.

Some other examples: Let's compare an 8-core FX to a six-core Zen, also with both at 4GHz. The Zen CPU would still have 40% higher single-threaded performance, but multi-threaded performance would be only 37% faster. Now let's throw Intel into those comparisons. Skylake has probably about 60% or so higher performance per core per clock compared to FX-x3xx CPUs such as the FX-8370. Let's say Zen holds similar frequencies to the FX-8350. A quad-core Zen CPU would then have single-threaded performance fairly similar to the i5-6400 or i5-6500 and multi-threaded performance something like 30% higher. Of course, that's based on a little speculation about Zen's frequencies, power consumption, and SMT scaling and we really don't know what they'll be, if it even will hold true to the claimed 40% IPC increase, or how that'll translate to real-world performance, but it's the best guesswork we can really do and it explains why Zen is supposed to be a winner for AMD.

Basically, Zen should have performance per clock for each core similar to either Sandy or Ivy, making it relatively competitive on that front. What its power consumption and frequency curves will end up being is anybodies guess. The single-threaded performance jump should be large according to AMD, but take that with a grain of salt based on how much hype they had about Bulldozer and how that ended up. I'd like for Zen to be everything it is supposedly going to be this time around.

SMT support should work out similar to Intel's. A quad core Zen CPU should perform like an i5 or an i7 depending on SMT being disabled or enabled.

Realistically, IPC refers to the cores handling 40% more instructions per clock and that in a way includes SMT, but SMT is not what contributes to that 40% more, yet it also benefits from it. Basically, the core gets 40% more IPC, THEN SMT is also thrown on to increase resource utilization.

Remember this: Going from Core 2 to Nehalem, you got more performance per clock for single-threaded software and you got Hyper-Threading (on dual core models and on i7s). Supposedly, Zen should be a similar sort of deal, granted its performance improvements aren't all for the same reasons.