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What should the CPU usage be of a fully-loaded CPU that has been throttled?

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Re: What should the CPU usage be of a fully-loaded CPU that has been throttled?

#21

Post by wtallis »

awesomeusername wrote: post #18
magila wrote: post #11

A key problem with this proposal is that modern CPUs do not have a single definitive maximum frequency. You have the base frequency which is rarely relevant outside of synthetic workloads then you have a variety of turbo frequencies which interact in complex ways. AMD's latest CPUs don't even have clear upper bounds on their frequency scaling logic. It's a big black box and the results can vary depending on the workl…

The OS can keep a record over time of the maximum frequency each CPU core has ever hit. This will take into account machine to machine variance, and even environmental factors effecting maximum speed.

That strategy guarantees that a process that runs for multiple minutes while consuming all available CPU cycles will be reported as using 100% CPU at most during the first few seconds, after which it will usually be reported as using somewhere less than 90%, and realistically could be reported as low as 65%. How is this helpful?

Re: What should the CPU usage be of a fully-loaded CPU that has been throttled?

#22

Post by jeffbee »

I don't like to speak about "throttling" because all modern CPUs are in a closed-loop control system where the capacity of one core-second varies. There is no question about whether your CPU is throttled. It is, always. That leads to all the uncertainty about the denominator. We know how many cycles passed while a certain thread had the CPU, but we don't have very good ways to estimate the number of cycles that were available. If you take the analysis one layer deeper, does a program that waits on main memory while chasing pointers randomly use 100% of the CPU, or does it waste 99% of it, since it's not using most of the execution resources? Such a program could be said to be using 100% CPU time, but it won't respond to higher CPU clock speeds. When waiting for loads it makes no difference if time passes at 4GHz or 400MHz.

So anyway, it is complicated.

Re: What should the CPU usage be of a fully-loaded CPU that has been throttled?

#23

Post by wtallis »

undfg wrote: post #14
spullara wrote: post #7

Related, 50% CPU usage on a hyper-threaded CPU isn't 50%. It is usually closer to 80-90% depending on the workload. Something to watch out for when monitoring.

Interesting. Does this mean that if you are not going to use all HT threads it's better to turn off HT?

Depends on the machine. The earliest implementations of HT worked by statically partitioning various caches and other resources in the processor core in half, which meant that a single-threaded process really could slow down by having HT enabled but not actively used. Newer desktop-class processors tend to have no significant downsides to leaving HT enabled, but there might still be some SMT implementations on niche products that don't handle this well.

Re: What should the CPU usage be of a fully-loaded CPU that has been throttled?

#24

Post by jeffbee »

awesomeusername wrote: post #18
magila wrote: post #11

A key problem with this proposal is that modern CPUs do not have a single definitive maximum frequency. You have the base frequency which is rarely relevant outside of synthetic workloads then you have a variety of turbo frequencies which interact in complex ways. AMD's latest CPUs don't even have clear upper bounds on their frequency scaling logic. It's a big black box and the results can vary depending on the workl…

The OS can keep a record over time of the maximum frequency each CPU core has ever hit. This will take into account machine to machine variance, and even environmental factors effecting maximum speed.

Estimating x86 core frequency is a lot trickier than you've implied.

Re: What should the CPU usage be of a fully-loaded CPU that has been throttled?

#26

Post by SavantIdiot »

This is precisely why all of the large volume cloud server farms I worked with turn off throttling: they need 100% predictable CPU utilization. I worked on power control strategies at Intel for quite some time, and we would often joke in server (Xeon) parts that it was pointless because all of our work was disabled.

Early throttles were 50% duty cycles, then L1 bubble injections, then V/F frequency scaling. The author only addresses the early mechanisms, but it gets even more complex with the PCUs in the later Xeons.

It is not an easy question to answer, but I think the question can be modified. A single number doesn't solve the problem, you need to know utilization in the context of throttling (and magnitude). Then decide what you are trying to solve: scheduling or app-level throttling? Personally, the Hz denominator should change if it used in utilization , since that covers the majority of cases. Any other case should read both metrics.

EDIT: Removed generalization and worded as anecdote.

Re: What should the CPU usage be of a fully-loaded CPU that has been throttled?

#27

Post by buran77 »

annoyingnoob wrote: post #16

I think we can have more nuance than a choice of 50% or 100%. I fall in the 'Show 100%' camp for sure, showing 50% but not indicating why isn't particularly helpful without knowing the background here. I think a separate indicator for an overall throttling condition would be helpful. Show a 100% usage and a throttle indicator together.

Determining how much is in use out of what is practically available is relatively easy. But determining what is 100% is still difficult to decide on, CPUs have too many frequency options to decide what the "real" 100% point is. Even picking the base frequency as the reference would be a pain to properly monitor and predict, and to take decisions based on that.

Re: What should the CPU usage be of a fully-loaded CPU that has been throttled?

#28

Post by annoyingnoob »

buran77 wrote: post #27
annoyingnoob wrote: post #16

I think we can have more nuance than a choice of 50% or 100%. I fall in the 'Show 100%' camp for sure, showing 50% but not indicating why isn't particularly helpful without knowing the background here. I think a separate indicator for an overall throttling condition would be helpful. Show a 100% usage and a throttle indicator together.

Determining how much is in use out of what is practically available is relatively easy. But determining what is 100% is still difficult to decide on, CPUs have too many frequency options to decide what the "real" 100% point is. Even picking the base frequency as the reference would be a pain to properly monitor and predict, and to take decisions based on that.

Its more about determining what throttling means, 100% for conditions. In any case, the tools we are used to using have traditionally showed a percentage scale of usage - seems like a reasonable measure even if its hard to measure.

Re: What should the CPU usage be of a fully-loaded CPU that has been throttled?

#29

Post by toast0 »

awesomeusername wrote: post #18
magila wrote: post #11

A key problem with this proposal is that modern CPUs do not have a single definitive maximum frequency. You have the base frequency which is rarely relevant outside of synthetic workloads then you have a variety of turbo frequencies which interact in complex ways. AMD's latest CPUs don't even have clear upper bounds on their frequency scaling logic. It's a big black box and the results can vary depending on the workl…

The OS can keep a record over time of the maximum frequency each CPU core has ever hit. This will take into account machine to machine variance, and even environmental factors effecting maximum speed.

Oh, but it gets more fun. The same operation can take more clocks for various reasons... If I really undervolt my Zen 2 apu, power usage and benchmarks go way down, but clock frequency stays high; the CPU is clock stretching and it gets a lot less work done.

Anyway, current processors run a separate clock per core, and maximum clocks are only available when a small number of cores are active; if all cores are busy, that should really be 100%, even if each core is only doing 80% of max for a single core.

Mostly, I want to see % of time cpu is busy, and separately, stats on how throttled the cpu is, because it's hard to combine both into a coherent number. Maybe also some idea of how much of the core is being exercised, if it can be easily measured... I'd love to know when a program is keeping the cpu busy, but not making good use of it.

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