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What's stopping you from shutting down cores with the traditional big-only design? I'm looking at per-core power consumption of my AMD 5800H right now, and I ca

by amq 5y ago

What's stopping you from shutting down cores with the traditional big-only design? I'm looking at per-core power consumption of my AMD 5800H right now, and I can see that it is able to fully shut down up to 7 cores out of 8, bringing down the entire package power consumption as low as 3.8 W.

masklinn 5y ago
Nothing, but if you need to run tasks then you need to wake these cores up. If you have low-priority or very short tasks to run, then you can wake just a few small cores, and that's way less expensive than to wake big cores. An other advantage is that it can help progress background tasks more efficiently: if you run background tasks on small cores then you don't need to suspend tasks which need the big cores or migrate them between threads to run background tasks.
hajile 5y ago
Rush to idle is a compromise. There are two basic kinds of loads. User responsive and background tasks. User tasks want to use the rush to idle because it improves responsiveness. Most background tasks don’t really care. High performance cores trade power efficient for execution speed by using lots of transistors to analyze the code and optimize on the fly (literally a semi-programmable core in itself to run your core faster). In addition, power consumption is linear with clocks in theory, but not at all in practice. Let’s consider AMD specifically. In order or semi in order cores are much narrower. All the speculation bits go away. The extra load/store complexity (very significant) also disappears because the core simply can’t use it. SMT will also go away. As it adds a ton of duplication, that also reduces size (10% if I remember Intel’s claims from a few years ago). Simple cores get much better performance per watt and per transistor, but at the expense of time. Makes a lot of sense if time isn’t critical. On desktop, a zen 3 core at 4.5GHz will use around 12w. Dropping frequency just 200MHz will lower power usage per core by almost 2w. (from Anandtech). In the space of that one core you mention, you could fit 4 complete in order cores with lots of room to spare. They could all run at a couple GHz and still use less than half the 3.8w you mention.

by amq 5y ago

masklinn 5y ago

Nothing, but if you need to run tasks then you need to wake these cores up. If you have low-priority or very short tasks to run, then you can wake just a few small cores, and that's way less expensive than to wake big cores. An other advantage is that it can help progress background tasks more efficiently: if you run background tasks on small cores then you don't need to suspend tasks which need the big cores or migrate them between threads to run background tasks.

hajile 5y ago

Rush to idle is a compromise. There are two basic kinds of loads. User responsive and background tasks. User tasks want to use the rush to idle because it improves responsiveness. Most background tasks don’t really care. High performance cores trade power efficient for execution speed by using lots of transistors to analyze the code and optimize on the fly (literally a semi-programmable core in itself to run your core faster). In addition, power consumption is linear with clocks in theory, but not at all in practice. Let’s consider AMD specifically. In order or semi in order cores are much narrower. All the speculation bits go away. The extra load/store complexity (very significant) also disappears because the core simply can’t use it. SMT will also go away. As it adds a ton of duplication, that also reduces size (10% if I remember Intel’s claims from a few years ago). Simple cores get much better performance per watt and per transistor, but at the expense of time. Makes a lot of sense if time isn’t critical. On desktop, a zen 3 core at 4.5GHz will use around 12w. Dropping frequency just 200MHz will lower power usage per core by almost 2w. (from Anandtech). In the space of that one core you mention, you could fit 4 complete in order cores with lots of room to spare. They could all run at a couple GHz and still use less than half the 3.8w you mention.