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Asymetrical performance between PCIe peer-to-peer devices

S_G_2
Beginner
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Hi,

@admins: if this is a wrong place, feel free to move it to the relevant topic.

I have developed a driver for enabling direct communication between two PCIe devices. So, the default path is PCI A device -> Host Memory -> PCI B device. My framework allows the direct path PCI A -> PCI B. The problem is that while, the speedup in one direction (A->B) yields significant speedup, in the other direction (B->A) there is an order of magnitude drop down.

I can't figure the reason of the above behavior (both device firmware are closed-source). The chipset of the setup is the 5520 (IOH-36D). I have checked out the manual (http://www.intel.com/content/dam/doc/datasheet/5520-5500-chipset-ioh-datasheet.pdf), but still no clue. So, I was wondering if there is an obvious reason for that. If you can sched some light for my case I would be glad.

Thanks in advance!

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