Re: [Xen-devel] Load increase after memory upgrade (part2)

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 17.01.12 at 22:02, Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx> wrote:
> The issue as I understand is that the DVB drivers allocate their buffers
> from 0->4GB most (all the time?) so they never have to do bounce-buffering.
> 
> While the pv-ops one ends up quite frequently doing the bounce-buffering, 
> which
> implies that the DVB drivers end up allocating their buffers above the 4GB.
> This means we end up spending some CPU time (in the guest) copying the 
> memory
> from >4GB to 0-4GB region (And vice-versa).

This reminds me of something (not sure what XenoLinux you use for
comparison) - how are they allocating that memory? Not vmalloc_32()
by chance (I remember having seen numerous uses under - iirc -
drivers/media/)?

Obviously, vmalloc_32() and any GFP_DMA32 allocations do *not* do
what their (driver) callers might expect in a PV guest (including the
contiguity assumption for the latter, recalling that you earlier said
you were able to see the problem after several guest starts), and I
had put into our kernels an adjustment to make vmalloc_32() actually
behave as expected.

Jan


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