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Computing News | Microsoft Responds to Re-discovery of Vista Network Slowdowns
Posted by: Tweaker
Date added: 23:27 Monday, 27th August 2007 GMT
Source: Beta News
A curious network performance reduction noticed by many Windows Vista users of the 2CPU forum that became the talk of Slashdot last week has been identified as having been caused not by DRM, as Slashdot users expected, but by a curious prioritization "feature" of Vista that's intentionally biased toward Media Player at the expense of network and system resources.
The effects of this feature were first revealed last June, as BetaNews reported, by Microsoft security engineer Mark Russinovich.
In a blog post to SysInternals this afternoon, Russinovich once again provides a clear view of what some prominent bloggers have called a bug, but which Microsoft maintains is an intentional choice: favoring multimedia playback when the CPU is stressed, even if it means slowing down file and network transfers and even the mouse pointer.
The culprit, Russinovich writes, is the Multimedia Class Scheduler Service (MMCSS), one of the many services run within Vista's service host. When the CPU is under stress, Windows Media Player 11 places a call to MMCSS, which in turn boosts the player's own priority relative to other processes. Many Vista users found out about this through the use of Vista's Performance Monitor.
"When a multimedia application begins playback, the multimedia APIs it uses call the MMCSS service to boost the priority of the playback thread into the realtime range, which covers priorities 16-31, for up to 8 ms of every 10 ms interval of the time, depending on how much CPU the playback thread requires. Because other threads run at priorities in the dynamic priority range below 15, even very CPU intensive applications won't interfere with the playback."
At least, that's the intention. Internet traffic shouldn't be affected by this performance throttling, he continues, due to multiple connections that fragment Internet traffic even in the best conditions anyway. Conceivably, the lesser the bandwidth, the fewer the fragments.
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