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From: Theo <theom+news@chiark.greenend.org.uk>
Newsgroups: comp.sys.raspberry-pi
Subject: Re: What do I need to go with a Pi 4
Date: 16 Apr 2024 10:59:00 +0100 (BST)
Organization: University of Cambridge, England
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The Natural Philosopher <tnp@invalid.invalid> wrote:
> > The Pi 4B will definitely throttle with only a ventilated case if it is 
> > anything other than sitting idle all the time.
> > 
> I am not interested in proof by assertion
> I had mine up to 130% on 'top' and it never made more than 76°C

You do know that 'top' won't show throttling?  Throttling means the CPU is
clocked lower than the maximum frequency to reduce heat generation - top
will still show '100%' of CPU (for one core) but that will be 100% of a
lower clock speed.

cat /sys/devices/system/cpu/cpu0/cpufreq/scaling_cur_freq

shows you the current clock of CPU core 0 and:

sudo vcgencmd get_throttled 

will tell you the throttling status:


#### get_throttled

Returns the throttled state of the system. This is a bit pattern.

| Bit | Meaning |
|:---:|---------|
| 0 | Under-voltage detected |
| 1 | Arm frequency capped |
| 2 | Currently throttled |
| 3 | Soft temperature limit active |
| 16 | Under-voltage has occurred |
| 17 | Arm frequency capped has occurred |
| 18 | Throttling has occurred |
| 19 | Soft temperature limit has occurred


For example if I run 'stress -c 4' then get_throttled gives me:
throttled=0xe0008

so the temperature limit is in operation and throttling has occurred in the
past.  (this Pi4 has cooling, I can't remember but I think there's a
heatsink and fan in there)

$ sudo vcgencmd measure_temp
temp=84.7'C

so it's up near its thermal limit.

> > I don't see the point of letting it throttling when an inexpensive fan 
> > will keep it at full speed under any load.
> > 
> I question that it will in fact throttle.
> 
> Like so much 'everybody knows'  when you look at it it is in fact 
> 'everyone believes because people selling fans told them so.

'Everybody knows' because they have evidence, not assertions.

> The whole point of ARM is its lower power and lack of need for forced 
> cooling

Everyone's been thermally limited for maybe 15 years, it's just that Arm
cores have traditionally targeted a lower thermal envelope in devices where
forced air cooling isn't an option.  The way this works is that CPUs work
until they hit their thermal envelope and then throttle.  No popular
application processor for maybe a couple of decades has been able to power
all the silicon at once to max performance and stay within the thermal
budget.

Theo