Deterministic Threading

Question

We are working on a project where we need our threads to run at accurate intervals. We tried doing this in user mode on windows, but we are not able to get the accuracy we need. The thread does not always run at exact time we expect.

In our application, we need to send and receive data from the NIC at specific intervals. We are also using the Npcap library, but the main issue is still the timing of the thread. Since windows is not a real time operating system, we understand that user mode timing can vary. That is why we would like to know what options windows provides to help improve timing accuracy.

We want to know if there are any recommended method? And if accurate timing is not possible in user mode, we would like to know whether moving this part into kernel mode is the right approach.

We would appreciate your guidance or any documentation that can help us understand the best way to achieve more deterministic thread behavior on windows.

Answer 1

Hi @Vaibhav Gaikwad ,

Thanks for sharing your details!

From what I understand, Windows is designed as a general-purpose OS and not a real-time one. As to why user mode on Windows didn't get the accuracy you need is probably your thread competing with other processes, background tasks, and hardware interrupts. You can check here for more info.

So getting microsecond-level precision in user mode is very hard even if you set a timer, the OS scheduler can delay your thread because something else is running.

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Windows can’t guarantee perfect timing, but here are suggestions that I think could make this more predictable:

High-Resolution Timers:

Think of this as a super-accurate stopwatch. QueryPerformanceCounter lets you measure time down to microseconds so you can schedule work closer to the exact moment you want.

• Reference: Acquiring high-resolution time stamps

Waitable Timers:

Instead of looping and checking the clock (which wastes CPU), you can set a timer that “wakes up” your thread at the right time. This is more efficient and helps keep timing consistent.

• Reference: Waitable Timer Objects

Thread Priority & Timer Resolution:

If your thread is competing with others, it might get delayed. By raising its priority (SetThreadPriority), you tell Windows, “Run this first.”

• Reference: SetThreadPriority

And timeBeginPeriod makes the system clock tick faster (e.g., every 1 ms instead of 15 ms), so your timing calls are more accurate.

• Reference: timeBeginPeriod

Multimedia Timers:

These were designed for audio/video apps that need smooth timing. They give you better precision for periodic tasks than normal timers.

• Reference: Multimedia Timers

I know these methods won't make Windows behave like a real-time OS, but they should reduce jitter and make timing much more predictable.

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Is Kernel Mode the Right Approach?

Moving timing-sensitive logic into kernel mode can help reduce latency and improve predictability because it avoids user-mode scheduling delays. For example, implementing an NDIS driver for NIC operations gives you more direct control over timing.

But here’s the catch: even in kernel mode, Windows isn’t a hard real-time system. It can still pause your thread for interrupts or other high-priority tasks. So, kernel mode might reduce jitter and improve consistency, but it won’t guarantee microsecond-perfect timing under all conditions.

If your application truly needs hard real-time guarantees, you’ll need to go beyond kernel mode:

• Use NIC hardware features like timestamping or packet scheduling so the timing happens in hardware, not software.
  • Reference: Overview of NDIS Packet Timestamping
• Or add real-time extensions to Windows (e.g., IntervalZero RTX or INtime) that turn it into a system capable of deterministic scheduling.
  • Reference:Soft Real-Time on Windows IoT

Bottom line: Kernel mode is the right approach if you need tighter control and lower jitter, but for absolute determinism, hardware offload or real-time extensions are recommended.

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I hope my suggestions are helpful and help you move forward! If you have any questions, please comment below. I'll be happy to assist!