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FreeBSD Scheduling on Hybrid CPUs

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Re: FreeBSD Scheduling on Hybrid CPUs

#11

Post by mrheosuper »

nine_k wrote: post #10
dehrmann wrote: post #7

The high-level pitch of P cores and E cores seems so elegant, but when it actually comes to scheduling, it gets messy fast. Even in a laptop running off a battery, you can't simply switch to E cores because some short-lived work might be latency-sensitive. You also can't assume long-running work should be on an E core because maybe you're anxious to get that video encoded. Even for lots of small work, different core…

Funny enough, Unix already has user-settable priorities, aka "nice level". ACPI gives us an idea how plentiful the power is. So, when powered by AC power, schedule everything on P cores when possible, schedule processes that eat a lot of CPU on P cores, same for any process with a negative nice value. When powered by a battery, schedule anything with non-negative nice value on E cores, keep one P core up for real-tim…

>when powered by AC power, schedule everything on P cores when possible

Sometime I feel like that is undesirable. It may make system consume more power, thus more heat output and louder.

Re: FreeBSD Scheduling on Hybrid CPUs

#12

Post by wtallis »

nine_k wrote: post #10
dehrmann wrote: post #7

The high-level pitch of P cores and E cores seems so elegant, but when it actually comes to scheduling, it gets messy fast. Even in a laptop running off a battery, you can't simply switch to E cores because some short-lived work might be latency-sensitive. You also can't assume long-running work should be on an E core because maybe you're anxious to get that video encoded. Even for lots of small work, different core…

Funny enough, Unix already has user-settable priorities, aka "nice level". ACPI gives us an idea how plentiful the power is. So, when powered by AC power, schedule everything on P cores when possible, schedule processes that eat a lot of CPU on P cores, same for any process with a negative nice value. When powered by a battery, schedule anything with non-negative nice value on E cores, keep one P core up for real-tim…

> So, when powered by AC power, schedule everything on P cores when possible, schedule processes that eat a lot of CPU on P cores, same for any process with a negative nice value.

Even when plugged in, you may have thermal limitations. P cores will chew through your power budget more aggressively than E cores. For latency-sensitive workloads you do want to emphasize the P cores, but when throughput is the goal you'll usually be better off not ignoring the E cores, and not trying to run the P cores at high frequency where they're much less efficient. Intel started adding E cores to consumer chips in large part so they could score better on throughput-oriented multithreaded benchmarks like Cinebench; they're decent at compiling code, too, but you'll still want the P core for the linker.

Re: FreeBSD Scheduling on Hybrid CPUs

#13

Post by wtallis »

themafia wrote: post #2

> Apart from some models of Alder Lake, it is now impossible to buy an Intel chip that does not have at least P (Performance) and E (Efficiency) cores. Really? I just bought one: https://www.intel.com/content/www/us/en/products/sku/236786/...

> Apart from some models of Alder Lake

That bit actually still applies. Intel may have branded the 14100F as Raptor Lake, but it is almost certainly Alder Lake silicon, just a higher speed bin of the 12100F.

See https://www.intel.com/content/www/us/en/products/compare.htm... and note how none of them get the higher DRAM frequency support or larger L2 caches characteristic of Raptor Lake silicon.

Re: FreeBSD Scheduling on Hybrid CPUs

#14

Post by 01HNNWZ0MV43FF »

jcelerier wrote: post #8
dehrmann wrote: post #7

The high-level pitch of P cores and E cores seems so elegant, but when it actually comes to scheduling, it gets messy fast. Even in a laptop running off a battery, you can't simply switch to E cores because some short-lived work might be latency-sensitive. You also can't assume long-running work should be on an E core because maybe you're anxious to get that video encoded. Even for lots of small work, different core…

As a user with a laptop, the last thing I want is the OS to decide for me. I want to tell it myself "this is sensitive, put all your energy into it because I'm five minutes away from pushing that important work and I have seven minutes of battery left" or "this won't work at all if run at less than 2 GHz" vs "I must drag what I'm doing along for as long as I can, save every bit of battery possible. The computer can't…

I feel like cases 1, 2, and 3 broadly fit into "Battery Saver", "Performance", and "Battery Saver" modes?

Re: FreeBSD Scheduling on Hybrid CPUs

#15

Post by mlyle »

swills wrote: post #4
pixl97 wrote: post #3

Earlier quoted context omitted.

How about change that to "Anything with more than 6 cores". Anything with 4 cores only has one speed of core. At 6 cores it more of a mixed bag, some have all the same cores, some have a split of performance and efficient cores. Anything over an i5 will have E cores.

Hmm, I think Granite Rapids is all P-Cores and goes up to 86 cores (172 threads): https://en.wikipedia.org/wiki/Granite_Rapids

Here there's weirdness, still, though because there's such a frequency difference you'll get between "low priority" and "high priority" cores.

Re: FreeBSD Scheduling on Hybrid CPUs

#16

Post by nine_k »

mrheosuper wrote: post #11
nine_k wrote: post #10

Earlier quoted context omitted.

Funny enough, Unix already has user-settable priorities, aka "nice level". ACPI gives us an idea how plentiful the power is. So, when powered by AC power, schedule everything on P cores when possible, schedule processes that eat a lot of CPU on P cores, same for any process with a negative nice value. When powered by a battery, schedule anything with non-negative nice value on E cores, keep one P core up for real-tim…

>when powered by AC power, schedule everything on P cores when possible Sometime I feel like that is undesirable. It may make system consume more power, thus more heat output and louder.

A laptop and a desktop certainly would balance P and E differently!

Re: FreeBSD Scheduling on Hybrid CPUs

#17

Post by Melatonic »

wtallis wrote: post #12
nine_k wrote: post #10

Earlier quoted context omitted.

Funny enough, Unix already has user-settable priorities, aka "nice level". ACPI gives us an idea how plentiful the power is. So, when powered by AC power, schedule everything on P cores when possible, schedule processes that eat a lot of CPU on P cores, same for any process with a negative nice value. When powered by a battery, schedule anything with non-negative nice value on E cores, keep one P core up for real-tim…

> So, when powered by AC power, schedule everything on P cores when possible, schedule processes that eat a lot of CPU on P cores, same for any process with a negative nice value. Even when plugged in, you may have thermal limitations. P cores will chew through your power budget more aggressively than E cores. For latency-sensitive workloads you do want to emphasize the P cores, but when throughput is the goal you'll…

Always personally disable turbo boost. Especially on laptops

Re: FreeBSD Scheduling on Hybrid CPUs

#18

Post by ethan_smith »

themafia wrote: post #2

> Apart from some models of Alder Lake, it is now impossible to buy an Intel chip that does not have at least P (Performance) and E (Efficiency) cores. Really? I just bought one: https://www.intel.com/content/www/us/en/products/sku/236786/...

The i3-14100F is just one example - Intel still sells numerous non-hybrid models across their lineup including most i3s, Pentiums, Celerons, and many server/workstation Xeons. The documentation's claim about availability is overstated.

Re: FreeBSD Scheduling on Hybrid CPUs

#19

Post by dijit »

dehrmann wrote: post #6
themafia wrote: post #2

> Apart from some models of Alder Lake, it is now impossible to buy an Intel chip that does not have at least P (Performance) and E (Efficiency) cores. Really? I just bought one: https://www.intel.com/content/www/us/en/products/sku/236786/...

There are also Xeons, but it limits an OS to use in data centers.

There are workstation Xeons. Though it seems that mobile Xeons are defunct now.

Re: FreeBSD Scheduling on Hybrid CPUs

#20

Post by [deleted] »

dehrmann wrote: post #7

The high-level pitch of P cores and E cores seems so elegant, but when it actually comes to scheduling, it gets messy fast. Even in a laptop running off a battery, you can't simply switch to E cores because some short-lived work might be latency-sensitive. You also can't assume long-running work should be on an E core because maybe you're anxious to get that video encoded. Even for lots of small work, different core…

[deleted]
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