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IPC

IPC?

  • Iunno. It measures performance...

  • Instructions per clock.

  • Instructions per core.

  • Other.

Results are only viewable after voting.
R

richaron

Golden Member
Hi all,

As a reader of ~technical~ forums, and just out of curiosity, I'm interested in seeing what people mean when they use the abbreviation "IPC". I'm wondering if we even mean the same thing whilst trying to have a discussion.

So, being honest & non-judgemental, what's the first thing you think when reading "IPC"?
 
Thanks Nosta tho I think you may have missed the point of this thread, clearly there are many people who could vote on any option above. With language meanings are fluid, there's an argument that something means "exactly what I thought it meant when I wrote it".

Even what you consider the "wrong" meaning can be perfectly acceptable, even "right" meaning amongst other groups of people. I was simply wondering where we are with IPC.
 
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richaron said:
Even what you consider the "wrong" meaning can be perfectly acceptable, even "right" meaning amongst other groups of people. I was simply wondering where we are with IPC.
Click to expand...

IPC is instructions per clock, period. What the hell would, "Instructions per core," even mean, and why would it be a relevant indicator of anything?
I know, let's start a thread as to whether "hp" means "horsepower," or, "hydrogen atoms per car."
 
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It's not a matter of opinion, it's a matter of being correct or incorrect. It's "instructions per clock".
 
Whatever the acronym actually stands for it's generally used to indicate single thread performance. Instructions per cycle is just part of what gives that performance, and even that is a vague term - is that peak, average? What type of instructions must be included? - any, some specific mix, etc.
 
Dribble said:
Whatever the acronym actually stands for it's generally used to indicate single thread performance. Instructions per cycle is just part of what gives that performance, and even that is a vague term - is that peak, average? What type of instructions must be included? - any, some specific mix, etc.
Click to expand...

exactly, IPC is a vague term in practice.
Are there any de facto standard how to measure IPC out there?
 
I think most people simply use it as the average single threaded performance per clock cycle. If Haswell is 10% faster than Sandy Bridge on average at the same clock speed, it has a 10% higher IPC.
 
teejee said:
exactly, IPC is a vague term in practice.
Are there any de facto standard how to measure IPC out there?
Click to expand...

Not any especially good ones. IPC differs massively depending on what code you use. If you have lots of memory stalls in your code, its IPC will be much lower than something that is just banging on straight line arithmetic with a working set that fits into L2.
 
NTMBK said:
Not any especially good ones. IPC differs massively depending on what code you use. If you have lots of memory stalls in your code, its IPC will be much lower than something that is just banging on straight line arithmetic with a working set that fits into L2.
Click to expand...

Branch on condition jumps also tend to destroy the IPC unless there is a really good branch predictor that predicts the right outcome correctly each time.
 
So how will we figure out how to turn this topic, which is clearly impossible to disagree upon, into a massive debate?
 
NTMBK said:
...clock cycle.
Click to expand...
Interestingly, in "Computer Architecture: A Quantitative Approach", H&P define IPC as Instruction Per Clock and CPI as Clock cycle Per Instruction. Oh well it doesn't matter it all means the same 😛

ThatsABigOne said:
Branch on condition jumps also tend to destroy the IPC unless there is a really good branch predictor that predicts the right outcome correctly each time.
Click to expand...
Modern branch predictors are typically well above 80% of correct predictions (and in fact above 90% in non pathological programs). The issue with branch misprediction is that if you get it wrong, you have to cancel everything you did speculatively after the mispredicted branch, while you can still execute instructions safely under a load miss.
 
Well I doubt anyone will chime in voting for "core" at this stage, but it's nice there is a consensus here at least. Quite a few times I've seen a higher frequency chip recommended because of "IPC".

I can pretend this was a public education thread, or a comment on the irony of people getting angsty over the suggestion of a bastardised term (whilst using English), but mostly I was curious/concerned as to what people thought. So thanks.

Edit: FWIW I've always considered IPC as an exact term related to a specific code path. Current usage is quite analogous to a nub thinking it means "instructions per core".
 
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