After weeks of troubleshooting DPC latency problems on my Lenovo Thinkpad T420 laptop which were causing audio dropouts and mouse freezes every few mins I have finally tracked down the source of the issue. It appears that the Intel 82579LM based Network Controller driver may not be correctly optimised for Windows 8. I'm running Windows 8 Pro 64-bit and am using the latest Intel driver for the controller (18.104.22.168). In troubleshooting the issue I tried everything I could think of such as:
Update BIOS and all drivers to latest versions
Disable CPU and PCI Power Management in BIOS
Disabled onboard Sound (I use a separate professional audio sound card), Wifi, Bluetooth
Change power management settings to High Performance and Disabled USB selective suspend
Turn on all offloading settings on Ethernet NIC
Ran bcdedit /set disabledynamictick yes
Disabling Network Location Awareness in Windows
Nothing made any noticable difference at all. I had also been noticing that my network performance had been pretty poor intermittently on this machine for a while, occasionally experiencing dropouts and low download speeds (yes I know that can be caused by many other factors but please hear me out). Every few mins on the PC I would experience a DPC latency spike of around 95000us to 100000us which would occur right when the audio dropouts, mouse freezes, and probably network pauses would occur. I use the PC for audio production amongst other things so this causes me big issues.
I was totally baffled by the problem until I ran an xperf trace in the Windows Performance Toolkit and saw that the spike was being generated from ndis.sys and tcpip.sys. That got me thinking down the lines of a network related problem so I disabled the onboard Intel(R) 82579LM based Network Controller in Device Manager in Windows 8 and what do you know the problem disappeared competely!! No more DPC latency spikes ever (tested for hours and many times). I can do pretty much whatever I like on the PC and the problem does not reoccur. Not one single audio dropout in my Audio DAW either. The onboard Wifi is still running (Intel Centrino Advanced-N 6205 - v22.214.171.124) providing me with network access and download speeds through the wireless nic are both much faster and more stable surprisingly (should be the other way around if anything). As soon as I re-enable the 82579LM NIC the problem is immediately back within a few mins.
I am desperate to get this fixed as I cannot use wireless all of the time. Intel can you please fix the 82579LM driver to make it fully compatible with Windows 8 Pro 64-bit. I'm sure many other people out there are having the same issue and just don't know the cause, or put up with it it as real time applications such as audio are less important to them. I have read about other people having these issues and it only appears to be with Windows 8. I think the drivers are fine for Windows 7, although I haven't confirmed this for sure.
Intel please consider patching the 82579LM NIC driver ASAP for full Windows 8 support. I can assist in providing you information if you require as I'm very keen to get this sorted.