Write Combine...

[Korporativ]

I know what write combining does, but my understanding is that PCI-e doesn't really use it, and it causes more problems than anything. I know in my old system that had an AGP 6800GT OC I had to disable it because it caused some bizzare pauses in games. My question is... does PCIe even use it currently, or is it safe to turn it off? I notice some difference in things like 3dmark, but honestly... synthetic benchmarks don't mean a whole lot.

 

[Peter]

Write combining is a CPU feature, collecting write data into bursts right there inside the CPU before the data even go out over the front side bus (Intel) or the HyperTransport link (AMD).

Of course, every kind of modern bus becomes more effective when it transports stuff in burst - including PCIE.

The answer is yes, everybody "still" uses it, particularly for the graphics card's primary frame buffer.

Why so little difference in benchmarks? Because it only matters when the CPU writes into the graphics card RAM - and your benchmarks tests mostly stuff that the GPU renders locally.

Try CPU rendered 2D video, like DivX or something, and you'll see the effects of WC much more prominently. The speed increase on the CPU->PCIE path is from below 100 MB/s without WC to nearly 4,000 MB/s with WC enabled on a 16x PCIE link.

 

[Korporativ]

Because Intel is stubborn and still uses shared bus architecture for the FSB... hmm. Still, trying to figure out why turning it off fixes a lot of jitters, even with synchronized refresh in certain programs.