What Outranks Thread Priority? | Hacker News Reader

What Outranks Thread Priority?

randomascii.wordpress.com

101 pointsbenaadams19 comments

Excellent writeup of a useful technique. I'm going to have to learn how to do this myself one day. Of course, if you're not inside Microsoft it may be very hard to fix any of it.

It really does show how the CPU gains of the past decades have been immediately wasted by badly organised processes doing work inefficiently which is then immediately discarded, while simultaneously blocking more important work.

The Windows kernel scheduler causes much trouble for our simulation software which runs 7 out of 8 cores at full pace, and then mysteriously gets suspended by the kernel when left to run over night. No Microsoft, its not OK to suspend a user task, and it’s not OK to reboot my box for an update while I’m running simulation software. If I end up having to reboot to a non Microsoft OS to run the simulations, I may End up never rebooting back to Win10 (once a solution to our external 3rd party tools is found).

This is triggering some bad memories. I havent used windows for anything serious in almost a decade, but it's pretty mind blowing they still havent fixed their basic reliability and availability issues. It's especially farcical because it flies completely in the face of the marketing claims used to push commercial software over FOSS. I work at a facility where running experiments effectively costs $1000/hour, and we have learned the hard way to not let windows anywhere near anything remotely critical. We dont even use windows to host guis for talking to the back end. About 10 years ago we made a long series of attempts to remedy these problems of windows machines rebooting themselves etc when told not to and losing irreplacable overnight data etc. The only solution that can be trusted long term is total excorcism and switching to an OS that is written to help users compute instead of to make money for MS shareholders. If you absolutely cant use WINE, see if you can run it in a VM to at least mitigate some of the nonsense. Plus you have the option of taking snapshots to hard restore the machine. /rant

What does the suspension of your tasks look like? I've never seen this happen. Was it a UWP app?

I've left calculations running overnight many times on my Windows machine and never had a problem. Was your machine going to sleep based on the power settings? I'll bet that was it.

for the *nix among us, the equivalent is the scheduling class (SCHED_FIFO, SCHED_RR, SCHED_OTHER). A given (kernel) thread's priority only applies within its scheduling class; if there are ready-to-run threads in a higher "ranked" scheduling class, threads in the lower "ranked" classes will not run.

This is widely used when using Linux (for example) as a soft-real time system. The same basic concepts apply to macOS (since they really come from POSIX and even Mach inherited them).

Great Post!

You would think that unlocking the screen would be highest priority task for a general purpose OS after it has woken up from sleep, but surprising to see that other processes manage to outrank it.

Whats interesting to me is that this LockApp.exe was waiting to get the CPU for a full 19 seconds after it was ready. 19 seconds should be an eternity, I guess.

Since the screen was locked and LogonUI.exe had not got the chance to run yet, what other processes were holding on to CPU at that time. Any ideas on that?

>Any ideas on that?

As he explains, processes with a GUI trying really hard to render despite it not being possible, as the screen hasn't been unlocked yet. Presumably another Windows bug.

Also hardware drivers initializing stuff. Including spinning for an eternity – his mouse and keyboard driver apparently spinned for 700ms.

There is a whole series of very interesting (even exciting) analyses by this author (randomascii, Bruce Dawson). Most of them are using his own tool UIforETW (https://github.com/google/UIforETW/).

I wonder, what alternative tool would you use on Mac? I guess Xcode Instruments in some variant? And this is based on DTrace, right? Would that give you all the same possibilities and traces as UIforETW?

And what alternative tool would you use on Linux? perf? eBPF? That would give you the same information as UIforETW? And what GUI would be similar to UIforETW or Xcode instruments?

It's important to note that UIforETW is just my tool for recording ETW traces. The underlying ETW tracing and WPA (the trace viewer) are all Microsoft's.

That said, UIforETW definitely fixes some significant issues with recording and managing ETW traces.

As someone who spends most of the time in *nix... Windows just sounds nightmarish to debug. The few times I've had to debug something in Linux kernel space I was super glad I had the full source code.

All that effort, and then... "the problem magically went away and we'll probably never know the true root cause"

Microsoft apparently makes symbols for Windows available, which is quite nice. On macOS you're often stuck with stripped binaries…

I have the opposite problem, spent most of my early career in win32 and x360. Having been on Linux derivatives for the last ~10 years I've yet to see debugging tools as good as Pix or Visual Studio.

The tools described in the post are really good, as are a bunch of other MS debugging tools, but it is a world unto itself and unfortunately the best place to learn is in Redmond.

In the Unix world I feel similarly about things like dtrace. Amazing tool with a learning curve.

Windows has really nice tracing and WPA is pretty powerful. I've not yet seen a tool that is similarly easy to use on Linux. You can get most of the raw data from perf, but the tooling for analyzing the data isn't as good. I'd love to hear about a tool that for Linux allows doing Dawson's wait analysis easily if anyone knows of one.

I agree about the lack of source - it's really nice to be able to see exactly what the kernel is doing on Linux.

>I agree about the lack of source - it's really nice to be able to see exactly what the kernel is doing on Linux.

Debug symbols do help a ton on Windows.

That's definitely true, and the debug symbols story is better on windows (in my experience) than other platforms - AFAIK linux has no real equivalent to symbol servers.

Debian debug symbol packages are pretty nice [0].

But if you want a repository for your own debug symbols, multitude of symbol versions and ease of use then you're out of luck.

[0]: https://wiki.debian.org/DebugPackage

Fascinating analysis! Thank you. I have noticed this effect is especially prominent on corporate laptops configured with monitoring features. I notice a similar effect on entering standby. Wonder if it’s related to another Rank 2 demotion of a critical process.

My laptop's startup time reduced by few seconds, since I tried the mitigation steps.

I hope Bruce looks into other issues too

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