Re: how to get a DMA channel near a process?

From: Dan Williams
Date: Wed Aug 26 2009 - 02:16:34 EST

On Tue, Aug 25, 2009 at 6:05 AM, Brice Goglin<Brice.Goglin@xxxxxxxx> wrote:
> Hello,
> 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?

If we can determine the topology of which chipset is closer to which
cpu package then we can modify the dmaengine channel_table to return
"local" channels via dma_find_channel(). The current allocation
policy is blind per-cpu round robin such that cpu-0 gets channel-0,
cpu-1 gets channel-1... As you pointed out the challenge will be
finding a reliable source for the topology information.

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