1 00:00:00,000 --> 00:00:06,560 If I tell you I'm multitasking, don't believe me, the best that I can do as a weak pitiful human 2 00:00:06,560 --> 00:00:12,640 is to quickly switch my attention from one single thing to another. But is that how your CPU works? 3 00:00:13,440 --> 00:00:19,040 Actually, no. Your CPU relies on a strategy called scheduling, where the operating system 4 00:00:19,040 --> 00:00:24,480 will decide what sets of instructions or threads will get put through the CPU first, 5 00:00:24,560 --> 00:00:30,000 second, third, and so on. Understanding scheduling is crucial to understanding 6 00:00:30,000 --> 00:00:35,360 exactly how modern multi-core CPUs work. So we got a little bit of help for this video, 7 00:00:35,360 --> 00:00:39,200 and we'd like to thank our friends at Intel for their assistance as we run through this thought 8 00:00:39,200 --> 00:00:44,400 experiment. Each thread that your computer is working on gets assigned a value called priority, 9 00:00:44,400 --> 00:00:49,120 so a thread with a higher priority value will be further ahead in line to get processed by your 10 00:00:49,120 --> 00:00:55,120 CPU. Developers can assign priority values when they write a program, and Windows can change a 11 00:00:55,120 --> 00:01:00,080 thread's priority based on how many system resources it needs and what the exact nature 12 00:01:00,080 --> 00:01:05,600 of the instructions are. For example, if a thread is tied to what you're inputting with a keyboard 13 00:01:05,600 --> 00:01:10,720 and mouse, such as clicking buttons in your web browser or aiming your weapon in a game, 14 00:01:10,720 --> 00:01:16,080 it's likely going to get a higher priority. By contrast, background activities that aren't 15 00:01:16,080 --> 00:01:21,920 time sensitive, those will get a lower priority. Similarly, developers can assign another value 16 00:01:21,920 --> 00:01:29,200 called quality of service, or QOS, to each thread. QOS is a way for devs to signal to Windows 17 00:01:29,200 --> 00:01:34,080 how much processor power a thread might need, and while Windows doesn't treat it as important 18 00:01:34,080 --> 00:01:39,440 as priority, QOS is still a factor that the operating system will take into account when it 19 00:01:39,440 --> 00:01:45,440 comes to scheduling. QOS is especially relevant when it comes to power management, as the OS can 20 00:01:45,440 --> 00:01:51,440 use QOS to request a certain frequency from the CPU, and the CPU's hardware can decide 21 00:01:51,440 --> 00:01:56,640 if it can ramp up to that frequency while staying within power limits. That's especially useful 22 00:01:56,640 --> 00:02:04,000 on laptops. Which is all very cool, but in terms of our guy doing two things at once analogy, 23 00:02:04,000 --> 00:02:10,640 priority and QOS pertain more to what I'm choosing to do, like texting while watching a movie, 24 00:02:10,640 --> 00:02:17,440 rather than how I'm juggling those two tasks. So let's drill down into that. Traditionally, 25 00:02:17,440 --> 00:02:24,000 CPU threads would go through one at a time, like my brain, but this isn't how modern multicore 26 00:02:24,000 --> 00:02:29,760 systems operate, especially considering some newer chips even have different kinds of cores 27 00:02:29,760 --> 00:02:36,080 all on one package. One goal in scheduling on multicore is not to context switch more than is 28 00:02:36,080 --> 00:02:42,560 necessary. And context switching is just another fancy term for moving a thread from one core 29 00:02:42,560 --> 00:02:48,800 onto a different one. In general, you want each thread to start and finish on one single core, 30 00:02:48,800 --> 00:02:53,440 and you want to load balance across cores so that none of them are getting overwhelmed. 31 00:02:53,440 --> 00:02:57,680 And there's been a lot of tweaking over the years to figure out how to do this easily 32 00:02:57,680 --> 00:03:03,200 and efficiently, such as with libraries, which are software resources that sit between the 33 00:03:03,200 --> 00:03:08,320 program and the operating system and allow multi-threading without making a developer do 34 00:03:08,320 --> 00:03:14,720 a ton of extra coding. On newer chips, how exactly threading works out across cores becomes even more 35 00:03:14,720 --> 00:03:20,960 important because of hybrid chips like Intel's Alder Lake lineup. These chips have higher performance, 36 00:03:20,960 --> 00:03:28,800 or P cores, and lower performance, but more power saving, E cores. It might be confusing as to why 37 00:03:28,800 --> 00:03:33,440 they put E cores on a desktop CPU where you aren't worried about your battery running out, 38 00:03:33,440 --> 00:03:38,880 but because E cores are physically smaller, you can actually pack more of them onto a chip, 39 00:03:38,880 --> 00:03:44,880 meaning you can in some cases get better performance if the scheduler can spread a program out 40 00:03:44,880 --> 00:03:50,720 across many E cores compared to a smaller number of P cores. Now, even though the Windows scheduler 41 00:03:50,720 --> 00:03:57,280 will still use P cores first by default if they're not busy, it can then use an E core for a second 42 00:03:57,280 --> 00:04:03,520 thread before using the slower hyperthreading feature of a P core. We're also getting to the 43 00:04:03,520 --> 00:04:08,080 point where, although Windows is still making final decisions on scheduling, there are now 44 00:04:08,080 --> 00:04:15,040 hardware assist features on newer CPUs that will actually give advice to Windows, so a CPU can look 45 00:04:15,040 --> 00:04:20,960 at the incoming workload and then advise Windows on the best current core for a certain thread, 46 00:04:20,960 --> 00:04:25,040 depending on whether Windows wants more performance or better power efficiency. 47 00:04:25,600 --> 00:04:31,040 So there's a lot that goes into deciding who gets to go first when it comes to your CPU, 48 00:04:31,040 --> 00:04:37,680 and a multi-core model can legitimately multitask. Also, my wife can do it somehow. 49 00:04:37,680 --> 00:04:44,160 If you guys enjoyed this video, check out our other videos, leave a comment if you have a suggestion for a future fastest Techquickie, 50 00:04:44,160 --> 00:04:45,520 and don't forget to subscribe.