4 ms·

Yes, I see your point, which is what the primary part of my comment was directly addressing. seconds/clock is a linear measurement of time, graphs better, and f

Yes, I see your point, which is what the primary part of my comment was directly addressing. seconds/clock is a linear measurement of time, graphs better, and for people who actually deal with timing of performance sensitive code, time/op is a much more natural measurement that is generally preferred when measuring and profiling code.

deathanatos 9y ago
I have the same question, but I'm not grasping whatever it is you're trying to say. What does it mean to be a "linear" measurement of time? (or even "graphs better"? That sounds subjective. I thought "up and to the right" was better /s) > for people who actually deal with timing of performance sensitive code While it is — at least in my experience — true that code segments are usually profiled in time / operation, aren't CPUs typically measured in IPS (instructions/sec), since clock speed is now somewhat decoupled from performance given optimizations present in modern CPUs, such as instruction pipelining?
- Arelius 9y ago
  It's X vs 1/X. If it's 1/X of time, you get properties such as something that is twice as fast is a 0.5 times the value, but something 4 times as fast is only .25 times the value. thus making the result non-linear. Also I think something that makes it a bit more clear the advantages of being time/ops consider optimizing a video game, and we have the task of trying to get from 60 FPS to 90 FPS and we have say 10 sub tasks that each take say 10% of the frame time, it's hard to say how much optimization I'll have to do off the top of your head. But if instead I say that you have to get from a 16.6 ms frame to an 11.1 ms frame, and you have 10 tasks that take 10% of the frame time, which is obviously about 1.66 ms each, and I also know I need to save 5.5 ms of frame, the scales of which I'm thinking in are all nice and easy to reason about.
fermienrico 9y ago
Thanks, I think that makes sense since 1/x is a non-linear function. I have a follow-up question - what's then the advantage of measuring speed as ops/unit-time (Freq)? How come we don't see 0.25 ns/op as advertised speed on a Processor?
- Arelius 9y ago
  Exactly. Honestly I don't know. I suspect it's mostly historical and marketing driven. I think CPU clocks are driven by some sort of oscillator, which is likely thought of as a frequency. And ops/unit-time may be a bit ambiguous, since if we try to choose say instructions, not all instructions take the same amount of clock cycles. Additionally from a marketing perspective, I suspect you get a bit of an hz, more is better, sort of increase of numbers. Honestly though I really don't know, this is all just speculation. Similarly video games are often "marketed" in Frames/Second, which is exactly the same problem.
  - fermienrico 9y ago
    You're kind of blowing my mind. The more I think, the more stupid I feel. If Freq is non-linear function of period, which it is... then Period is also a non-linear function of Freq. They're an inverse of each other. So, if we're talking about Frequency, then the same argument could be made that Period is non-linear and it cannot be used to compare speed, since Period is 1/f - a non-linear function. wtf is going on here... my brain hurts.
    - Arelius 9y ago
      Yes, that's very true. I think it can be sometimes hard to ground yourself in what the actual thing is you care about. I think here is where experience trying to use the values is helpful, hence my video game example in a sibling thread.
- eesmith 9y ago
  "How come we don't see 0.25 ns/op as advertised speed on a Processor?" Because different operations take different numbers of cycles and have different latencies. See http://www.agner.org/optimize/instruction_tables.pdf http://www.agner.org/optimize/instruction_tables.pdf for a table of such.

deathanatos 9y ago

I have the same question, but I'm not grasping whatever it is you're trying to say. What does it mean to be a "linear" measurement of time? (or even "graphs better"? That sounds subjective. I thought "up and to the right" was better /s) > for people who actually deal with timing of performance sensitive code While it is — at least in my experience — true that code segments are usually profiled in time / operation, aren't CPUs typically measured in IPS (instructions/sec), since clock speed is now somewhat decoupled from performance given optimizations present in modern CPUs, such as instruction pipelining?

Arelius 9y ago
It's X vs 1/X. If it's 1/X of time, you get properties such as something that is twice as fast is a 0.5 times the value, but something 4 times as fast is only .25 times the value. thus making the result non-linear. Also I think something that makes it a bit more clear the advantages of being time/ops consider optimizing a video game, and we have the task of trying to get from 60 FPS to 90 FPS and we have say 10 sub tasks that each take say 10% of the frame time, it's hard to say how much optimization I'll have to do off the top of your head. But if instead I say that you have to get from a 16.6 ms frame to an 11.1 ms frame, and you have 10 tasks that take 10% of the frame time, which is obviously about 1.66 ms each, and I also know I need to save 5.5 ms of frame, the scales of which I'm thinking in are all nice and easy to reason about.

fermienrico 9y ago

Thanks, I think that makes sense since 1/x is a non-linear function. I have a follow-up question - what's then the advantage of measuring speed as ops/unit-time (Freq)? How come we don't see 0.25 ns/op as advertised speed on a Processor?

Arelius 9y ago
Exactly. Honestly I don't know. I suspect it's mostly historical and marketing driven. I think CPU clocks are driven by some sort of oscillator, which is likely thought of as a frequency. And ops/unit-time may be a bit ambiguous, since if we try to choose say instructions, not all instructions take the same amount of clock cycles. Additionally from a marketing perspective, I suspect you get a bit of an hz, more is better, sort of increase of numbers. Honestly though I really don't know, this is all just speculation. Similarly video games are often "marketed" in Frames/Second, which is exactly the same problem.
- fermienrico 9y ago
  You're kind of blowing my mind. The more I think, the more stupid I feel. If Freq is non-linear function of period, which it is... then Period is also a non-linear function of Freq. They're an inverse of each other. So, if we're talking about Frequency, then the same argument could be made that Period is non-linear and it cannot be used to compare speed, since Period is 1/f - a non-linear function. wtf is going on here... my brain hurts.
  - Arelius 9y ago
    Yes, that's very true. I think it can be sometimes hard to ground yourself in what the actual thing is you care about. I think here is where experience trying to use the values is helpful, hence my video game example in a sibling thread.
eesmith 9y ago
"How come we don't see 0.25 ns/op as advertised speed on a Processor?" Because different operations take different numbers of cycles and have different latencies. See http://www.agner.org/optimize/instruction_tables.pdf http://www.agner.org/optimize/instruction_tables.pdf for a table of such.