Processor scheduling and quanta in Windows (and a bit about Unix/Linux)

One of the more exotic and exciting IT subjects is the one of processor scheduling (if you’re not excited, read on, practical stuff to be seen later in the text). Multi-tasking OSes just give the illusion that they’re doing things in parallel – in reality, the CPUs rapidly skip from task to task using various algorithms and heuristics, making one think the processes truly are running simultaneously. The choice of scheduling algorithm can be immensely important.

Wikipedia has a nice article on schedulers in general:, good primer.

To cut a long story short: the processors are allowed to spend finite chunks of time (quanta) per process. Note that the quantum has nothing to do with task priority, it’s simply the amount of time the CPU will spend on the task. Every time the CPU switches to a new process, there’s what’s called a context switch (, which is computationally expensive. Obviously, we need to avoid excessive context switching but still maintain the illusion of concurrency.

In Windows Server (that uses a multi-level feedback queue algorithm, FYI), the default quantum is a fixed 120ms, close to many UNIX variants (100ms) and generally accepted as a reasonably short length of time that can fool humans into believing concurrency. Compare this to the workstation-level products (Windows Vista/XP/2000 Pro) that have a variable quantum that’s much shorter and also provide a quantum (not priority) boost to the foreground process (the process in the currently active window). In the workstation products, the quantum ranges from 20-60ms typically, with the background processes always relegated to the smallest possible quantum, ensuring that the application one is currently using “feels” responsive and that no background task hampers perceived performance too much. Typically, in a box that’s used as a busy terminal server this will be the better setting to use since it will ensure that the numerous “in-focus” user processes will all get a quantum sooner rather than later.

The longer, fixed quantum of Windows Server means that fewer system resources are wasted on context switching, and that all processes have the same quantum. More total system throughput can be realized with such a scheme, and it’s a more of a fair scheduler. It also explains the higher benchmark numbers when running the scheduler in “background services” mode. It’s obviously best for systems that are running a few intensive processes that can benefit from the longer quantum (and, believe it or not, games and pro audio apps run better like this).

Note that I/O-bound threads (processes waiting on disk, mouse, screen and keyboard I/O) are given priority over CPU-bound threads anyway, which explains why the longer quantum doesn’t harm interactivity much. Try it – have 4 winzip/winrar/7zip sessions running concurrently. You CAN still move your mouse 🙂 Here’s a great primer on internal windows architecture: Another, deeper dive:

Of course, there are ways to tune the timeslice in a more fine-grained fashion. In the registry, check out HKLM\SYSTEM\CurrentControlSet\Control\PriorityControl\Win32PrioritySeparation . Here are some explanations about how it works: and are great.

For instance – what if you don’t care to increase the quantum on the foreground window but, instead, just want short, fixed quanta (effectively around 60ms) for all processes to improve response time on a system with a lot of processes? Setting Win32PrioritySeparation to 0x28 will take care of that.

Here’s a useful Win32PrioritySeparation chart from

2A Hex = Short, Fixed , High foreground boost.
29 Hex = Short, Fixed , Medium foreground boost.
28 Hex = Short, Fixed , No foreground boost.

26 Hex = Short, Variable , High foreground boost.
25 Hex = Short, Variable , Medium foreground boost.
24 Hex = Short, Variable , No foreground boost.

1A Hex = Long, Fixed, High foreground boost.
19 Hex = Long, Fixed, Medium foreground boost.
18 Hex = Long, Fixed, No foreground boost.

16 Hex = Long, Variable, High foreground boost.
15 Hex = Long, Variable, Medium foreground boost.
14 Hex = Long, Variable, No foreground boost.

Here are some other pages where others have figured out the effective quanta (and remember the numbers are not in ms): (for embedded Windows, I have doubts about the accuracy of his calculations regarding the effective quantum but still interesting), (for Windows 2000, probably still valid).

Here’s a really nice article on the effects of schedulers and I/O-bound processes on virtualization:

Linux, on the other hand, has not one but several totally different CPU schedulers and I/O elevators available. Just see this page, comparing 2.6.22 with Vista’s kernel, and note how many non-standard features are available as patches: . You can get schedulers with cool names such as genetic, anticipatory, etc. Linux used to suffer on the desktop, but with recent patches interactivity has improved tremendously, and is now far more viable as a desktop OS. Here’s some cool info on anticipatory schedulers: Anticipatory schedulers can help systems with slower I/O (laptops and desktops, especially) feel more interactive, and was the default I/O elevator for a while (CFQ is the current default for I/O, though can have issues with desktop users, see A list of all the I/O elevators in the kernel: Whitepapers:,, .

Recently, Linux moved to the Completely Fair Scheduler model (, sparking a lot of controversy ( since it’s not quite done yet ( More info on CFS:

Interesting benchmarks showing the effects of scheduling on Linux performance:,

For anyone wishing to test the various Linux schedulers’ impact on interactivity, Con Kolivas has something: Con’s Staircase/Deadline (SD) scheduler ( didn’t make it to the mainline kernel, unfortunately, and a miffed Con announced he’s dropping out of kernel development. Pity, since I think he single-handedly contributed more to the advancement of Linux interactivity on the desktop than anyone else. It’s great to have the choice of schedulers depending on how you’re planning to use your system – it’s already done with the I/O elevator, let it be done with the CPU scheduler. Instead, Linus invoked his Papal-like powers and made what I consider to be an unsound decision.

The real issue with Linux though is the userland. Here’s a great paper showing issues with the userland and how it robs us of speed: . A lot of the CPU and I/O scheduler design is workarounds for those issues. Unless one deliberately chooses a stripped-down Linux distribution, the amount of bloat in the current code is incredible.

Finally, Solaris 10 also comes with a bunch of different schedulers, which you can assign globally or on a per-process/project basis. Tons more info:,,,

Heady reading, no?


3 Replies to “Processor scheduling and quanta in Windows (and a bit about Unix/Linux)”

  1. FYI – Bill Gates (through the agent of sold-out ex-deccie Dave Cutler), stole large portions of that scheduler design from dec OSes like Rsx-11 and VMS. Lest anyone think bill gates &Co. has ever done any original work, even his business practices were stolen from Attila the Hun, and Vlad Tepes.

  2. VMS (now called OpenVMS) is way better in scheduling processes. On a 2 CPU machine you can have 4-5 CPU bound processes and still work, perhaps a bit more slowly, but not much. On Windows, you’re unable to do any work in the same situation. Almost a case for a reboot if you want to regain control of the machine.
    Also I don’t understant the fact that an I/O bound process on Windows will take so much resources, that you’re constantly waiting for the system to react even though the CPUs is only 5% used. I experienced this many times.
    Maybe Windows can be tweaked a bit, but I doubt it can ever be as efficient.

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