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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?

#61

Post by gruez »

dataflow wrote: post #34

> half-speed for whatever reason IMHO the reason actually does matter. Utilization should be relative to the maximum frequency the CPU could be running the same instructions at. If the CPU is throttling to save power, then it could be running the same instructions at a higher frequency, so utilization should be relative to the higher one. If it's throttling to lower the temperature, then it can't be running the same…

>Utilization should be relative to the maximum frequency the CPU could be running the same instructions at

How do you define "maximum frequency"? For desktops it's relatively easy to define since they usually have good cooling and power delivery that they can always hit the top frequency, but for laptops this often doesn't apply. It might be only able to sustain top frequency for less than a minute before thermal throttling kicks in.

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

#62

Post by sundvor »

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.

Agree 100% (he!). The utilisation really needs to show busy as in % of presently available resources, and displaying what those are is a completely different concern.

For Windows, if anything I'd love to see an additional metric, where the app resource usage is presented in % of available cores.

Eg if I have a single thread app (i.e. a game) on let's say a 10 core CPU with SMP disabled, it might report 10% usage. But looking at the core graphs we see one at 100% with the other 9 idling. So the app is really maxed out at its architectural limit - ie 100%. It would be nice to have an additional column representing this - and maybe an idea of the foreground app's available % usage in other reporting (am I cpu limited, or GPU? people have bought Intel over AMD CPUs for this metric even though multi thread workload capacity has been far greater in AMDs).

Taking a step back again, especially for Ryzen CPUs unless manually locked you'll have different per core frequencies - so the article's suggestion becomes an almost impossible mission: A Mandelbrot processor can be split across all threads rather equally for an unthrottled 10 core run say at 4ghz, but a core limited config may be able to run it at 4.5ghz prior to thermal throttling - what is the real max to report in a 10 core run following that then? And on a very cold day, with fans at a higher RPM, the core limited run might hit a higher 4.8ghz - now what? You just can't; reporting on current CPU capability is a separate concern altogether.

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

#63

Post by gruez »

Tempest1981 wrote: post #52
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.

Yep, and Windows never goes above 100%, iirc. My linux box can hit 800% if I can keep all the cores fed. More confusion.

That seems like something entirely different. Windows is displaying cpu usage relative to all the cores (total load divided by total number of cores), whereas linux is displaying usage relative to one core (total load divided by one core).

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

#64

Post by taneq »

wtallis wrote: post #10

When the OS has asked the CPU to slow down to more closely match the performance currently required by the software, then it is somewhat misleading to report that an application is using 90+% of the CPU time, even if the CPU is actually spending 90+% of its time running that application. However, when the CPU's speed has been reduced because it's too hot or the system is otherwise unable to allow the processor to sus…

> However, when the CPU's speed has been reduced because it's too hot …

Maybe just measure overall system load as current CPU temperature as a percentage of maximum?

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

#65

Post by dataflow »

gruez wrote: post #61
dataflow wrote: post #34

> half-speed for whatever reason IMHO the reason actually does matter. Utilization should be relative to the maximum frequency the CPU could be running the same instructions at. If the CPU is throttling to save power, then it could be running the same instructions at a higher frequency, so utilization should be relative to the higher one. If it's throttling to lower the temperature, then it can't be running the same…

>Utilization should be relative to the maximum frequency the CPU could be running the same instructions at How do you define "maximum frequency"? For desktops it's relatively easy to define since they usually have good cooling and power delivery that they can always hit the top frequency, but for laptops this often doesn't apply. It might be only able to sustain top frequency for less than a minute before thermal thr…

Whatever frequency it could execute the next cycle at, assuming there was no desire to minimize power usage. It's a transient measure, not a steady-state measure.

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

#66

Post by namibj »

toast0 wrote: post #55
jrockway wrote: post #35

Earlier quoted context omitted.

SMT is usually a throughput win, and usually a latency loss. I was playing with my Threadripper a while back and for C++ builds of large projects, HT results in about a 10% improvement in compilation speed. 10% is a big deal and you should take it. The downside is that games had noticeably lower framerates even with the rest of the CPU idle (at least the games I play are bounded by single-thread performance across ma…

> 64GB wasn't enough for 64 concurrent clang runs. You need a little bit more (but of course 128GB is the next installable increment). That's not really true, you can mix and match memory, and you might not get ideal bandwidth, but for a lot of uses it's just fine.

Notably, triple-rank configurations work just fine. It's just not trivial to find both a dual-rank module and a single-rank module with the same components used on both (so all the sub-timings match and performance will be nice and proper).

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

#68

Post by foobar33333 »

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…

CPU usage is an incredibly complex metric that doesn't really explain what is actually going on. I noticed while running a benchmark I can get my CPU to 50c and 100% usage and it stays steady like that. But then I tried prime95 and my cpu very quickly hit 99c also at 100%. Likely the different benchmarks were both running as fast as they can but the prime95 one ran on a faster part of the cpu which could generate more heat and does not get stuck waiting on memory or other slow ops.

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

#69

Post by foobar33333 »

wmf wrote: post #19

Power saving and "throttling" (usually it's un-turboing) are different cases that shouldn't be conflated; in one case the processor could run faster but in the other it can't. Ultimately we may want different metrics depending on what they're going to be used for. If you calculate relative to base frequency you will get utilization over 100% which is going to confuse some people. Linux has done some work in this area…

MacOS has a weird solution to throttling. They put a fake process in the process list which looks like it consuming x% of the resources but really it is just blocking some usage to allow the CPU to cool.

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

#70

Post by naikrovek »

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…

not only that, but modern CPUs aren't even deterministic in how they run an instruction. the result of any particular instruction is deterministic, but the location of the silicon and the amount of silicon used to complete the instruction is not deterministic anymore.

CPU Usage measured in percentage doesn't make sense on any CPU with modern performance features like speculative execution and branch prediction.

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