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Vista, Network Performance, and the MMCSS

October 14, 2007

Mark Russinovich wrote up an excellent article (along with some pretty interesting comments and dialog) regarding the current prioritization “glitch” between the Multimedia Class Scheduler Service and network throughput.  The gist is that when Windows Media Player begins playback, it’s being prioritized a bit too high—dragging network throughput almost down to nothing.

One commenter mentioned a partial fix—if you can live with it. 

By default, the Windows Audio service (rather important) depends on the MMCSS.  You can sever this relationship by modifying the dependencies in the registry.

If you go into the Windows registry, and look for the key:

HKLM > SYSTEM > CurrentControlSet > Services > AudioSrv key

There’s a string value called DependOnService.  By default, the value has three entries: AudioEndpointBuilder, RcpSs, and MMCSS.  Remove MMCSS.  You can close the registry editor now.

The final step is to go into the Services management console (Services.msc) and Disable the Multimedia Class Scheduler service.  It’ll squak and close the Windows Audio service down; that’s fine.

Reboot.

Upside:

When your computer comes back up, try moving some files around and see what the transfer speeds are.  On my Gb network, there is an improvement, but it’s not _real_ substantial.  The place I notice the most difference is while gaming.  S/R speeds in FFXI seem to be dramatically higher—Lower Jeuno actually loads, rather than just studders around.

Downside:

Audio sources that use Windows Media Player for playback do studder a bit if your computer is under heavy load; however, MP3/WMA formats seem pretty solid—even while gaming. 

The big issue seems to be streaming music.  Streaming music, tested from various sources, is almost unusable.  That, however, is only limited to those directly invoking the WMP application—it doesn’t seem to affect embeded browser instances or non-WMP players.

As many have pointed out, I hope these issues are fixed in Service Pack 1.

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