how to get a DMA channel near a process?

From: Brice Goglin
Date: Tue Aug 25 2009 - 09:04:56 EST


I am playing with DMA engine on a Nehalem box with two X58 chipsets
(Supermicro X8DAH). My understanding is that there are 8 DMA channels on
each chipset, so 8 channels near each processor. Unfortunately, my BIOS
and 2.6.31-rc still wrongly reports the physical location of my devices
(it claims all PCI devices are near the first processor) but I worked
around the problem manually.

The offloaded copy performance changes a lot depending on whether the
process memory is allocated near the DMA device. So first I would like
to know if DMA channels are allocated near the requesting
processor/process. Then I guess it's possible to read the cpu mask near
a given chan by following chan->dev.device up to the pci device, right?
But is there any way to request a DMA channel near a specific socket or
NUMA node or cpu mask?


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