What Does an Idle CPU Do?

http://duartes.org/gustavo/blog/post/what-does-an-idle-cpu-do/

by brunorsini • 12 years ago

563 55 12 years ago

AceJohnny2• 12 years ago

Gustavo Duarte's (not to be confused with Androider Matias Duarte) blog posts are awesome. I still highly recommend his 2009 in-depth review of how the Linux kernel manages memory: http://duartes.org/gustavo/blog/post/how-the-kernel-manages-...

pwelch• 12 years ago

Wish I could upvote you twice. His posts are amazing.

mindslight• 12 years ago

Spread spectrum modulate its emissions to backhaul cached crypto keys?

(/me ducks)

edit: oops, the downvoters are correct; modern cpus have dedicated hardware for that.

amluto3• 12 years ago

Tiny little correction: On x86 multi-processor machines, the CPU can be woken from idle by an interrupt or by a write to a designated address in main memory from another CPU. Linux uses* the latter technique to very efficiently wake up a process on one CPU when it receives data from another CPU. This is a big speedup on some workloads.

* This didn't work very well until Linux 3.16.

DogeDogeDoge• 12 years ago

nop nop nop nop

leonatan• 12 years ago

When I was young, I never understood why there was an idle process in Windows, eating my CPU... The days without Internet.

typedweb• 12 years ago

I seem to recall the idle 'task' on Windows zero's empty memory pages, so there might be less of what's happening in the article on those machines.

DonGateley8• 12 years ago

In the days of OS/360 and OS/370 (IBM's mainframe OS in the 60's and '70s) no instructions at all executed in the wait state. There was no OS level idle process. Instead, the internal CPU microcode itself ran a little loop without dispatching any machine instructions until the tentacles that reached out from that loop were tickled by an external interrupting event. That loop had to be very short for latency reasons and very prickly to test for all the conditions that could take the processor out of it to resume dispatching instructions or do critical internal things.

I know this because I wrote that 6 (initially 8) micro-instruction loop for the IBM System/370 Model 155-157 in the late '60s. I was told back then that since that model became the most widely sold machine ever I had written the code most executed in the history of computing. :-)

The status latch in the machine that indicated that loop was running was fed to a light on the console and, yep, it was nearly always lit other than when booting the OS.

beachstartup• 12 years ago

8 to 6... a 25% optimization. what'd you take out?

DonGateley8• 12 years ago

I got the hardware guys to merge some of the conditions that caused a loop exit into condition groups that were tested by other micro-instructions until a couple went away. Getting out quick was important, sorting out why was less so.

wglb3• 12 years ago

Awesome.

Were some of the smaller machines (135 or smaller) non-microcode? E.g. RTL or TTL hardwired?

DonGateley8• 12 years ago

Everything in that generation of machines was micro-coded except for the Model 195 which was then IBM's super scientific computer.

beat3• 12 years ago

They kept that trick up for a long time. As of AIX 3.2.5 back in the 1990s, an "idle" RS/6000 ran 70% cpu utilization. They were super-fast and efficient in their day, too.

Dunno how true that is today, but I wouldn't be surprised if AIX still uses the same kind of tight event loop internally.

socceroos• 12 years ago

RS/6000. Heh - I'm the only person left in my state who still knows how one of these works.

Diagnosing startup problems with all those super quick flashing codes was a nightmare.

Angostura• 12 years ago

Presumably these were the pre-PowerPC machines?

esmi2• 12 years ago

This can be true on modern CPUs as well. In fact, if the software chooses, it can even remove power from the core dynamically so one doesn't even have to pay the leakage power.

See for example Intel's C6 state. http://www.hardwaresecrets.com/article/Everything-You-Need-t...

s_kanev• 12 years ago

Yeah, cpu idleness is pretty fascinating and, as it turns out, quite important. Shameless plug, I just published a paper [1], showing that if you mess up how deeply you go to sleep in cpuidle, you can loose up to ~15% latency in datacenter services like websearch.

[1] http://www.skanev.org/papers/iiswc14ep.pdf

amluto3• 12 years ago

This reminds me: if you're stuck on a Linux kernel before 3.9 and you're using Sandy Bridge hardware or newer, turn off C1E in BIOS. The "C1E" setting controls a misfeature in which the CPU will look at the OS's request for a light sleep and say "Ha! You must have asked for light sleep by accident! I'm going to go all the way to sleep instead!" This causes surprisingly severe performance problems (we've seen milliseconds of unexplained random hiccups).

Linux 3.9 and higher is immune: it just forcibly disables this feature.

makomk• 12 years ago

Hah. I guess that's a major benchmark win - decreases power usage in common cases, and few CPU benchmarks test wake-from-sleep latency either directly or indirectly.

amelius• 12 years ago

But what does a CPU running an idle virtual machine do?

zheng• 12 years ago

Not sure why this is being downvoted, it's actually a good question. If all of your VM's are idle, you would want your host CPU to idle as well, although my guess is this doesn't happen in things like Xen, VMWare, etc.

vegardx• 12 years ago

There has actually been problems with that, there was a bug with Citrix XenServer [0] where the CPU would, if left on idle for a long time, step down and turn off more or less all cores, which resulted in a locked up and unresponsive host. You can imagine what that did to the virtual machine supposedly running on that host.

[0] http://support.citrix.com/article/CTX127395

amluto3• 12 years ago

Off the top of my head: the VM executes either HLT or MWAIT, and those instructions are generally programmed to cause a VM exit, so the hypervisor can take action. The hypervisor will deschedule the VM and possibly go idle itself.

TBH, I have no idea how MONITOR works in a VM. It might be quite messy.

Edit: Here's a really nice explanation: http://www.contrib.andrew.cmu.edu/~somlo/OSXKVM/mwait.html

hderms• 12 years ago

Does it reinitiate VM execution by waiting for the hypervisor to be scheduled again which schedules out to its constituent vms?

ciupicri• 12 years ago

By the way, FreeDOS and MS-DOS use 100% of the CPU when running on a VirtualBox virtual machine [1].

[1] http://www.freedos.org/wiki/index.php/VirtualBox_-_Chapter_7

voltagex_• 12 years ago

Which is why someone wrote DOSIDLE.

I'm only just starting to learn asm so I'm not entirely sure how it works, but the source is available at http://maribu.home.xs4all.nl/zeurkous/download/mirror/dosidl....

userbinator• 12 years ago

DOS-style operating systems were written at a time when halting the CPU to reduce power consumption was basically not a concern, so the "idle loop" is actually an infinite loop (TSRs often hooked into this loop to do "background processing", a rudimentary form of task switching.)

I seriously doubt that it is healthy for any computer to run for a longer time with 100% CPU usage.

Computers in the DOS and Win9x days would be at 100% CPU usage for basically as long as they were switched on; and a modern one should be absolutely fine running at 100% for an indefinite amount of time provided that things like the cooling system are in good working condition. The biggest difference between CPUs now and CPUs then is the difference in power consumption between idle/sleep and full power - e.g. a 66MHz 486 consumes <5W at full power (and not much difference when halted) vs. ~5W at idle and close to 100W for a recent Haswell i7.

If I have a DOS guest running for some time in VirtualBox, with full CPU usage, OS X eventually will pop up a nice half transparent window. It tells me that my computer needs now to be shut down and that I should push the power button long enough to do so. It's the Mac version of the BSOD.

That suggests insufficient cooling. If it's a system that's been in use for a while, it's probably time to clean out the heatsinks and fans. If a new system is overheating from 100% CPU usage, then I would consider it defective.