On May 18, 2007, at 2:24 AM, Peter Young wrote:
Analysing the PCIe bus behaviour, we noticed that we were experiencing multiple retries followed by a 'disconnect with data' when trying to read the data from the Mac memory.
This does transfer data but at a very slow rate (see above).
Our interface card uses a PLX9056 chip connected via a PCIexpress bridge to perform the DMA.
Are there any known issues with DMA'ing data from Mac memory to memory onboard a PCIe card?
Haven't heard of any.
Any ideas as to why I am getting such terrible data rates?
Not without a lot more information. I do have a couple suggestions for tests which may shed light on whether hardware has anything to do with the problem.
1. Download Boot Camp, install Windows on your Mac Pro (I'm assuming you're testing on a Mac Pro), and test under Windows.
2. If performance problems persist under #1, and if you have the hardware handy to do so, test under Windows on an Intel 5000 series chipset PC from another vendor. Any workstation PC which uses 5100 or 5300 series Xeons should do. (This would eliminate the possibility that something in Apple's firmware or somewhat different (*) motherboard design is responsible.)
(*) Re: different, I'm not aware of any other 5000 series chipset motherboard which has four PCIe slots with software-configurable assignment of lanes to slots. I presume the reconfiguration feature requires some extra silicon on the motherboard which is not likely to be present on other 5000 chipset boards, and very likely adds some latency to PCIe traffic.
