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%CPU utilization is a lie

brendanlong.com

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Re: %CPU utilization is a lie

#91

Post by zekrioca »

kqr wrote: post #76

It might be a lie, but it surely is a practical one. In my brief foray into site reliability engineering I used CPU utilisation (of CPU-bofund tasks) with queueing theory to choose how to scale servers before big events. The %CPU suggestions ran contrary to (and were much more conservative than) the "old wisdom" that would otherwise have been used. It worked out great at much lower cost than otherwise. What I'm tryin…

I noticed exactly the same thing. The author is saying something that has been repeatedly written in queueing theory books for decades, still they are noticing this only now.

Re: %CPU utilization is a lie

#93

Post by Aissen »

Funny that it talks about matrixprod, which I think is not that relevant as benchmark — unless you care about x87 performance specifically. I recently sent a pull request to try to address that in a generic manner: https://github.com/ColinIanKing/stress-ng/pull/561

Yet I'm still surprised by this benchmark. On both Zen2 and Zen4 in my tests (5900X from the article is Zen3), matrixprod still benefits from hyperthreading and scales a bit after all the physical cores are filled, unlike what the article results show.

All of this is tangential of course, as I'd tend to agree that CPU utilization% is just an imprecise metric and should only be used as a measure of "is something running".

Re: %CPU utilization is a lie

#94

Post by bob1029 »

I think looking at power consumption is potentially a more interesting canary when using very high core count parts.

I've ran some ML experiments on my 5950x and I can tell that the CPU utilization figure is entirely decoupled from physical reality by observing the amount of flicker induced in my office lighting by the PWM noise in the machine. There are some code paths that show 10% utilization across all cores but make the cicadas outside my office window stop buzzing because the semiconductors get so loud. Other code paths show all cores 100% maxed flatline and it's like the machine isn't even on.

Re: %CPU utilization is a lie

#96

Post by therealdrag0 »

kqr wrote: post #76

It might be a lie, but it surely is a practical one. In my brief foray into site reliability engineering I used CPU utilisation (of CPU-bofund tasks) with queueing theory to choose how to scale servers before big events. The %CPU suggestions ran contrary to (and were much more conservative than) the "old wisdom" that would otherwise have been used. It worked out great at much lower cost than otherwise. What I'm tryin…

> semi-crappy indicator … good enough.

Agree. Another example of this is for metrics as percentiles per host that you have to average, vs histograms per host that get percentile calculated at aggregation time among hosts. Sure an avg/max of a percentile is technically not a percentile, but in practice switching between one or the other hasn’t affected my operations at all. Yet I know some people are adamant about mathematical correctness as if that translates to operations.

Re: %CPU utilization is a lie

#97

Post by fuzzfactor »

Windows users try this:

Ctrl-Alt-Del then launch TaskManager.

In TaskManager, click the "Performance" tab and see the simple stats.

While on the Performance tab, then click the ellipsis (. . .) menu, so you can then open ResourceMonitor.

Then close TaskManager.

In ResourceMonitor, under the Overview tab, for the CPU click the column header for "Average CPU" so that the processes using the most CPU are shown top-down from most usage to least.

In Overview, for Disk click the Write (B/sec) column header, for Network click Send (B/sec), and for Memory click Commit (KB).

Then under the individual CPU, Memory, Disk, and Network tabs click on the similar column headers. Under any tab now you should be able to see the most prominent resource usages.

Notice how your CPU settles down after a while of idling.

Then click on the Disk tab to focus your attention on that one exclusively.

Let it sit for 5 or 10 minutes then check your CPU usage. See if it's been climbing gradually higher while you weren't looking.

Re: %CPU utilization is a lie

#99

Post by 4gotunameagain »

mustache_kimono wrote: post #84

Reminds me of Brendan Gregg's "CPU Utilization is Wrong" but this blog fails to discuss that blog's key point that CPU utilization is a measure of whether or not the CPU is busy, including whether the CPU is waiting [0]. That blog also explains that the IPC (instructions per cycle) metric actually measures useful work hidden within that busy state. [0]: https://www.brendangregg.com/blog/2017-05-09/cpu-utilization...

What's up with Brendans and CPU utilisation concerns, any Brendan to shine some light ?

Re: %CPU utilization is a lie

#100

Post by shim__ »

c2h5oh wrote: post #44
PaulKeeble wrote: post #20

This is bang on, you can't count the hyperthreads as double the performance, typically they are actually in practice only going to bring 15-30% if the job works well with it and their use will double the latency. Failing to account for loss in clockspeed as the core utilisation climbs is another way its not linear and in modern software for the desktop its really something to pay careful attention to. It should be po…

To complicate things more HT performance varies wildly between CPU architectures and workloads. e.g. AMD implementation, especially in later Zen cores, is closer to a performance of a full thread than you'd see in Intel CPUs. Provided you are not memory bandwidth starved.

Whats the difference between Intels and AMDs approach?
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