Will moving code into kernel space give more precise timing?

Question

Background information:

I presently have a hardware device that connects to the USB port. The hardware device is responsible sending out precise periodic messages onto various networks that it, in turn, connects too. Inside the hardware device I have a couple Microchip dsPICs. There are two modes of operation.

One scenario is where send simple "jobs" down to the dsPICs that, in turn, can send out the precise messages with .001ms accuracy. This architecture is not ideal for more complex messaging where we need to send a periodic packet that changes based on events going on within the PC application. So we have a second mode of operation where our PC application will send the periodic messages and the dsPICs simply convert and transmit in response. All this, by the way, is transparent to the end user of our software. Our hardware device is a test tool used in the automotive field.

Currently, we use a USB to serial chip from FTDI and the FTDI Windows drivers to interface the hardware to our PC software.

The problem is that in mode two where we send messages from the PC, the best we are able to achieve is around 1ms on average hardware range. We are subjected to Windows kernel pre-emption. I've tried a number of "tricks" to improve things such as:

1. Making sure our reader & writer threads live on seperate CPU affinities when possible.
2. Increasing the thread priority of the writer while reducing that of the reader.
3. Informing the user to turn off screen saver and other applications when using our software.
4. Replacing createthread calls with CreateTimerQueueTimer calls.

All our software is written in C/C++. I'm very familiar and comfortable with advanced Windows programming; such as IO Completions, Overlapped I/O, lockless thread queues (really a design strategy), sockets, threads, semaphores, etc...

However, I know nothing about Windows driver development. I've read through a few papers on KMDF vs. UDMF vs. WDM.

I'm hoping a seasoned Windows kernel mode driver developer will respond here...

The next rev. of our hardware has the option to replace the FTDI chip and use either the dsPIC's USB interface or, possibly, port the open source Linux FTDI stuff to Windows and continue to use the FTDI chip within our custom driver. I think by going to a kernel mode driver on the PC side, I can establish a kernel driver that can send out periodic messages at more precise intervals without preemption and/or possibly taking advantage of DMA.

We have a competitor in our business who I think does exactly something similar with their tools. As far as I know, user space applications can not schedule a thread any better than 1ms. We currently use timeGetTime in a thread. I've experiemented with timer queues (via CreateTimerQueueTimer) with no real improvement.

Is a WDM the correct approach to achieve more precise timing?

Our competitor some how is achieveing very precise timing from Windows driven signals to their hardware and they do load a kernel driver (.sys) and their device runs over USB2.0 as does ours.

If WDM is the way to go, can I get some advise on what kernel functions I should be studying for setting up the timings? Thanks for reading

Answer 1

In kernel mode, you have the luxury of getting a DPC triggered in multiples of 100-nanosecond intervals without dealing with interrupts. A DPC cannot be preempted (aka interrupted by thread scheduler) because thread scheduler is also a DPC. An interrupt can still preempt a DPC though. So an interval value of 10 should do the trick for you to have a callback with utmost precision.

However you don't have access to many features such as paged memory, or a specific thread's memory space at DPC level because they run in arbitrary context. It could be useful to defer processing to your own user mode process' context using an APC which has access to more features.

Kernel threads don't get any special treatment in terms of priority. They are the same as user threads from scheduler's perspective. There are couple more higher-priority levels kernel threads can get but usually no kernel thread uses any of them. I don't think your bottleneck is thread priority. It doesn't matter how big your priority number is, having just one above everyone else is enough for you to become the "god thread" which receives top priority. Having highest priority doesn't mean that you'll get continuous attention. OS will still pause your thread to run others so quantum starvation does not occur.

Another note on Windows preemption behavior: Balance Set Manager temporarily boosts a thread's priority when a thread is signaled by an asynchronous event (GUI click, timer trigger, I/O completion) to allow completion code to finish it's procesing with less preemption. Using an async timer handler should give enough boost to prevent preemption at least for a quantum. I wonder why your code does not fall into that window. However it seems like you are not the only one having problems with timer precision: http://www.virtualdub.org/blog/pivot/entry.php?id=272

I agree with Paul on complexity of driver development, but as long as you have a good justification it's not rocket science, just more effort.

Answer 2

This is one of the fundamental design aspects of the Windows kernel - that code running at passive level (=> all user-mode code) is subject to DPCs and interrupts taking up time, and if you want 1us accuracy, you're probably not going to get it with either a UMDF or user-mode driver.

However, writing a kernel driver is not a light or cheap undertaking, it is very difficult, both to even write, and to ensure that it works on your customers' machines (a lot of testing is required). Getting it right will cost you significant engineering resources.

As a stopgap, I'd look into MMCSS for >= Vista (http://msdn.microsoft.com/en-us/library/windows/desktop/ms684247(v=vs.85).aspx), it may give you enough priority that you can be satisfied.

If you really want to go down the rabbit hole, KMDF is what you should be using. KMDF is a framework on top of WDM that represents a lot of codified best-practices for drivers. Unless you're absolutely forced to, KMDF is always the best way to go for drivers. And to be honest, you're almost certainly going to want to either contract with OSR (http://www.osr.com) or hire someone (several people?) experienced in writing Windows drivers.

Answer 3

Your focus on drivers and kernel performance misses the forest for the trees. The elephant in the room is the fact that full-speed USB 2 bus frames happen with 1ms period. High speed USB 2 micro-frames happen every 1/8ms.

When you send data over full-speed USB (like for most FTDI chips), the best your application can hope for is that the data will get to the device sometime during the very next frame. With an unloaded USB bus, the transfer will happen very close to the start-of-frame. You'll observe it as 1ms granularity with small random deviation. This is precisely what you're seeing, and is not bad. For example, since all USB devices attached to the same host will see the frames at the same time, it's a simple way to synchronize multiple device clocks with better than microsecond precision. What your application can do is simply send a message that has not only the data, but some time in the near future when it should be sent out. Another issue with USB is that there are no guarantees as to when your requests for data transmission will be serviced. You're sharing a bus with other devices, after all.

I think you need to reengineer your system and not depend on any sort of timing from the PC end. The application that runs on the PC should be assumed to be, timing-wise, limited to the performance of the human that interacts with it. Anything that requires guaranteed real time performance must be on your dsPIC devices. Even the USB bus doesn't cut it as you have no guarantees at all as to how soon will your request be scheduled on the bus.

Basically, if you want guaranteed real-time performance on Windows, then there must be no user mode involved -- it must all run in kernel mode, and you must use communications channels that are for your exclusive use (or you make them act that way, e.g. by filtering right on top of the USB host).