On Tue, 2012-03-20 at 12:18 +0200, Avi Kivity wrote:

On 03/19/2012 10:03 PM, Peter Zijlstra wrote:

quoted

On Mon, 2012-03-19 at 14:16 +0200, Avi Kivity wrote:

quoted

quoted

Afaik we do not use dma engines for memory migration. 

We don't, but I think we should. 

ISTR we (the community) had this discussion once. I also seem to
remember the general consensus being that DMA engines would mostly
likely not be worth the effort, although I can't really recall the
specifics.

Esp. for 4k pages the setup of the offload will likely be more expensive
than actually doing the memcpy.

If you're copying a page, yes.  If you're copying a large vma, the
per-page setup cost is likely to be very low.

Especially if you're copying across nodes.

But wouldn't you then have to wait for the entire copy to complete
before accessing any of the memory? That sounds like a way worse latency
hit than the per-page lazy-migrate.

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