Is a specific number of CPUs in the slot a mandate or a max?

[RBob]

I have 3 very conventional low-powered PCs running FAH in their spare time. Two have 4-core CPUs with the slot CPU count set to -1. But I have one 6-core machine with the CPU count set to 3 because going higher causes overheating.

Based on the wording of the config screen, I think the setting of -1 leaves the work assigner free to give tasks which require between 1-4 CPUs.

But, will the machine with a CPU count of 3 only be given tasks that need exactly 3 CPUs or also be given tasks that need only 1 or 2 (if such things exist)?

[Joe_H]

Welcome to the folding support forum.

The setting of -1 leaves the number of CPU threads (cores) up to the client software and the setting of the power slider. The client will reserve one CPU thread for a GPU if a usable one is detected. Then Full will use all remaining threads, Medium one less, and Light uses half the available threads.

Setting a specific number causes the client to always request WUs that can be folded on that many threads. If no WUs happen to be available for the number, a WU that can fold on fewer threads may be assigned. This will more often be seen by people with larger systems with more than 12 or so CPU threads supported.

[RBob]

Thanks. Maybe I have to look into why this machine runs so hot anyway.

The two machines running 4 cores full time (not much else for them to do right now) are stone cold; the antique i3 machine running Linux hits 151F and never higher.

The 6-core machine with only 3 cores active runs as high as 160F, although I can see the cores rotating the load constantly so nobody stays too hot for too long. When I first started, I got nervous when things hit 170+F and the fan(s) were roaring, so I then cut the number of active cores down until I got down to 3.

[Neil-B]

Check the tjmax for you cpu ... 170f is sub 80c iirc ... many cpus will readily cope with this even for sustained periods but fans will be ramped up ... some cpus have tjmax as 100c

[psaam0001]

Don't be afraid to power that overheating system down for a couple of hours, and open it up to see if there is dust build-up on the CPU heat sink/fan (or any of the other fans/vent slots you are able to clean--especially if you have pets). Once that is done, leave the side panel off as you initially power it up to see if the CPU fan is working (check fan power connections).

Note: Some of the newer cases come with filters you can rinse off and let air dry for a few hours before reinstalling them. Check the case user manual.

Paul

[MeeLee]

You haven't said what CPU the 6 core unit is, but most older (2014-era) CPUs, have a TJmax of ~80C, and more recent CPUs usually hit 90-95C without problems.
I would however do the following:
1- Clean any debris off of the heat sink.
2- Reapply the thermal paste.
3- If the CPU has turbo boost frequencies, you can disable turbo. Especially on older machines, the +100-200Mhz is really not going to make the speed difference, especially if it allows you to fold on all cores when disabling it.
4- Inspect your PC, to see if the CPU has sufficient airflow. If not, just buy a 12V case fan that fits (120mm or 140mm) and allow more case airflow to pass by the CPU..

170F isn't too high (~77C), but a bit on the high side.
You will preferably want to run your CPU below 75C (~166F), and most optimally around 65-70C under full load (150-160F).
That's only 10F lower.
Your PC is made to run at those temperatures just fine all day long!

[Hopfgeist]

Check the tjmax for you cpu ... 170f is sub 80c iirc ... many cpus will readily cope with this even for sustained periods but fans will be ramped up ...

I don't relate to Fahrenheit in my head, but the maximum allowed measured temperature depends on a number of factors. Typically, desktop machines allow the CPUs to run hotter before ramping up the fans, to keep them quiet: the i7 in my iMac runs up to 99°C (210°F) under full load at full fan speed, but when I limit it to 3 threads for FaH, it sits at around 93°C (199°F), and is almost silent.

Servers are set up differently to cope better with constant high load, and noise is much less of an issue: the big Xeon-machine (Dual 6-core X5675) runs at around 80°C (176°F) under full load, and the much smaller backup server (single 4-core X3430) runs really cool at 55°C (131°F) with all cores blazing.

some cpus have tjmax as 100c

Back in the day, the Motorola MC68040 (ceramic case) had a maximum junction temperature of 110°C (230°F), but that is rare with modern CPUs except for special applications.

Cheers,
HG

[RBob]

Didn't realize there were responses to my whining about the heat.

The CPU is an i5-9400, and the PC's a Dell XPS-8930, about 1.5 years old. So, I'm not ready yet to tear it down. I wouldn't be surprised if the box is inadequately cooled, or that I'm just a little too reluctant to push the temperature up beyond 160F.

I have a pair of antique Dell Optiplex 3010s, one with an i5 that runs at about 130F and the other with an i3 which runs at about 150F. Both are 4-core CPUs.

I'll check the temperature limits of the i5-9400 and see what the drop-dead value is.

[Neil-B]

Tjmax for the i5-9400 is 100C or 212°F... this is the temp at which MBs will start to throttle so running up to that should be fine (if noisy) but most people might by happier with running say less than 90C - it is really personal choice ... Dell cases whilst functional are pretty basic as far as cooling and most run stock cpu coolers which tbh are never great ... I got an 8C drop in temps on one of mine dells recently with a relatively mild upgrade switching out the stock for a small after market cooler that fit the case - you may want to consider that?

[RBob]

You've convinced me to push things a little harder.

Right now, it's running with 4 cores with no apparent heat or noise problems. When this task finished, I'll try 5. If that holds, I'll try 6, just to see what happens.

[gunnarre]

Note that even if you ask for 5, 6, or 7 threads, the work server will give you a work unit which uses the same or less number of threads. So you have to check the logs to see how many threads you're actually running on. I.e. you're most likely running 4 threads even if you ask for 5; if you ask for 12 you might get 12 or 8 for example.

[Neil-B]

And if you increase core count on a slot it wont come into play until the next wu is downloaded

[PantherX]

In addition to what is stated above, if you were to use CPU:5 then this is what you would see on your system:
FahCore_a7 WUs: They will print a message aboue avoiding large prime numbers and will use 4 CPUs even if 5 CPUs were configures.
FahCore_a8 WU: They will use 5 CPUs (or any number) even if it is a prime number.

The reason is that FahCore_a8 is new and in order to avoid domain decomposition error (when a WU is too small to be distributed across all the specified CPUs), it uses a workaround that ensures that any number would work fine. While it does have an impact on large CPU count, it would vary per Project. The idea being, FahCore_a8 will be able to smartly handle this domain decomposition error in future.