> Except that the flipper isn't enabled yet and that the backpressure > mechanism is busted somewhow. At least that is what the recent > experiment with cranking up the buffer cache limit showed us.
> People screamed and we backed the change out again. And there were > problems on amd64 and sparc64 alike. Which means the generic backpressure mechanism is busted. As a result, we currently rely on the 4GB dma limit as a forwardpressure subsystem, and tuneables which keep the buffer cache small. The buffer cache flipper was going to give us very large buffer cache compared to other systems. Until it is finished, we are still doing fine. > What we probably need is help fixing the buffer cache. Then we can > enable the flipper. And then we see if 64-bit DMA is still a > requirement. I was simply pointing out that massive (well above 4GB) buffer cache on a 64-bit DMA-reachable machine worked poorly. Likely due to data structures managing the memory with rather large O... Chasing DMA-reachability on a theory that it helps some subsystem... some substantiation is required. In my experience (and I think yours), there are other hurdles.
