8-way multithreading in Windows 7

Windows 7 Beta is out today. It’s free to participate in, so if you’re interested, please check it out and give us feedback.

To whet the appetites of my compressionist friends, one much-requested new feature in the Format SDK .dll in Windows 7: It now can go up to 8-way threading, compared to the 4-way threading introduced with WMP 11 and Vista! This should nearly double encoding speed on 8-core machines doing HD encoding.

The number of threads the encode gets is determined by the height of the video frame.

• 1-119 lines 1 thread
• 120-239 lines: 2 threads
• 240-479 lines: 4 threads
• 480+ lines:  8 threads

This is because each thread gets a slice of the video frame, so more threads mean that vertical motion gets a little less efficient to encode. And as always, the number of threads used can be overrridden with Alex Zambelli’s WMV9 PowerToy. Its looks like we’ll get one last update out of him after all to add “8” to the drop-down list.

This new mode will work with any program that uses the Format SDK .dll, including Windows Media Encoder, the new Windows Live Movie Maker beta, and Virtual Earth. It won’t apply to VC-1 Encoder SDK based apps like Expression Encoder 2, but obviously we’ll want to add this to the VC-1 Encoder SDK as well.

So, if you’ve got an 8-core processor, or a 4-core with hyperthreading, give the Win 7 beta shot. I’d love to hear from anyone who gets some comparisons for encoding time between XP/Vista and Win 7.