Monday, March 19, 2007

Misleading Symptoms

Computer systems have become so complicated that it is likely to misdiagnose the problem if you look only at the symptoms and lack understanding of how it works.

A friend's old Windows XP computer was getting the error "Windows---Display Driver Stopped Responding: The i81xdnt5 display driver has stopped working normally," and he asked me to look at it. Whenever it happens, the display switches back to 640x480 resolution at 16 colors and messes up desktop icon arrangement. He could restart the computer, but it gets annoying.

A quick search turns up a Microsoft Knowledge Base article explaining that the device driver might be faulty and suggested that we upgrade the display driver. We tried that but it didn't help.

He said he has a way to reproduce it---the error happens when he runs a lot of applications. I suspect it has something to do with virtual memory issues, so I checked:
  1. If the disk is running low on available space. But it had plenty, with 9GB free.
  2. How much memory is free, using Task Manager. I discovered that Windows only uses up to 384MB of page file, and it is nearly full right after the system was rebooted.
  3. The performance setting, and found out that someone had limited page file size to 384MB.
I changed the page file size to "system managed" and solved the problem.

The computer originally had only 128MB of ram, but I helped my friend upgrade to 256MB at some point. I think whoever bought the computer for my friend before had followed the rule that page file should be three times the physical memory size, but the memory upgrade requires the size limit to be updated. That's why he's running into this problem only after memory upgrade.

I can see there are many possible misdiagnoses:
  1. As the Microsoft Knowledge Base states, it could be due to a faulty display driver. Cost of misdiagnosis is the time to find display driver and install it; not bad so far.
  2. It could be due to a faulty hard drive for failing to provide a reliable page file. We had a hard drive problem on that computer before. It was fixed after a chkdsk, but that could indicate that the drive is dying. Cost of misdiagnosis is ~$80 for a new hard drive.
  3. Since the problem only happened after a memory upgrade, it could be due to the new memory modules. Cost of misdiagnosis is ~$100 for a new memory module.
  4. It could be due to a faulty display card for "not responding" to a driver. However, the computer uses integrated graphics chip on the motherboard, so that could imply the motherboard needs replacement. Cost of misdiagnosis is either ~$50 for a new display card, or ~$150 for a new motherboard; trying both and finding out that neither help cost ~$200.
As you can see, it could easily cost us $300+ sans labor if we had tried everything we can imagine under the sun.

Think about the implication of a medical misdiagnosis. Human bodies are much more complicated than computers.
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