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Tracking too many interrupts with my tablet PC

  • Artigo

  • I have been very happy with my tablet once I scanned my fingerprint to "calm down" the fingerprint reader software. I happened to notice the fan on the tablet was still running more often than what I expected after making that change. I figured I had something still causing the CPU to be used too much (thereby generating the heat which was causing the fan to run) and started troubleshooting again.

  • My first stop was Process Explorer. It gave me a big clue: something was using about 7% of the CPU time making "DPCs." A quick check on MSDN told me DPCs were Deferred Procedure Calls (aka "interrupts") and, essentially, they could cause a CPU to be used excessively. This seemed to jibe with what I was thinking, so I fired up Windbg to attach to the System process to see what was slamming the CPU with DPCs. It failed, of course, since the System process is protected and I can't attach to it.

  • Looking around, I found this article about using xperf to track DPCs. Talk about timing - it had only been written a few days before I needed that information. I read through the article and since I already had Windows symbols installed, I was set to go. I don't have a lot to add to that article since I was able to follow it pretty much step by step.  I was able to drill down pretty quickly to see 2 possible drivers eating the CPU.

  • The top line here shows ndis.sys consuming 7.3% of the CPU over the time span I chose (a few seconds). The article stated that I could expand the view to show the calls being made, but for some reason, symbols were not resolving for me, and I just got blank lines for each call. Still, this gave me a good starting point, and I would also keep an eye on the USBPORT.SYS driver.

  • NDIS is used by networking, so I went to the control panel \ system \ device manager to see what was installed as far as network drivers go. I hazily remember something about a default NDIS driver in Windows 95 from my tech support days which was used if not 32 bit drivers were available, and I hoped to be able to use the device manager to get a version number of what my tablet was using.

  • Now, if I had one complaint about the tablet, it is the sheer number of drivers installed. I'm not joking when I say there are over 100 drivers total, and there were 4 network drivers. 2 were for 802.11 a/b/g (wireless and LAN), one was for Bluetooth, and there was an Nvidia nForce driver. Since I had to guess which was the culprit, I went for the Nvidia first, for no particular reason. I disabled the driver and waited a few seconds. The CPU fell to 0%, and the number of DPCs dropped to near zero. After a few seconds, the fan went quiet. I think I found the culprit.

  • I tried googling up Nvidia Nforce problems to see if there were other people reporting similar performance hits. I neglected to think about my wireless network not working since I had disabled this driver, but lo and behold, it still worked. I rebooted, verified the driver was not loaded, and still had network connectivity.  I looked around and found a few scattered reports of problems with NVidia network drivers, but nothing definitive.

  • I'm tempted to leave it alone at this point, and keep my eye on Nvidia to see if they update their drivers. But I haven't fixed the problem - I've only eliminated a symptom, and I haven’t had time to find what functionality I may have lost. For instance, I did not have time to verify Bluetooth still working, so it may be completely broken at this point. I also have a wireless printer connected to which I need to print. And I still do not know why the USB driver is using more than 1% of the CPU, or why the storport.sys calls are taking so much time relative to all other calls.

  • Questions, comments, concerns and criticisms always welcome,

  • John

Comments

  • Anonymous
    April 21, 2008
    PingBack from http://microsoftnews.askpcdoc.com/?p=3469

  • Anonymous
    April 21, 2008
    The comment has been removed

  • Anonymous
    May 02, 2008
    Excellent. I have a motion tablet and using the process explorer by sysinternal (now MS), I too noticed DPC eating up as much as 70% of my cpu! Ouch. I searched on how to track down what was causing this but I had no luck -- until your article! Thanks so much!