Re: [Xen-devel] Memory fragmentation and PCI passthrough

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 06.09.11 at 19:54, Marek Marczykowski <marmarek@xxxxxxxxxxxxxxxxxxxxxx> 
>>> wrote:
> In tmem documentation is also described some workaround for this:
> reserve some memory region for allocations with 0<order<=9. But SWIOTLB
> tries to allocate 64MB, which much bigger than 2MB... Is it really
> needed to allocate such big region of contiguous memory in one piece?

For one, you can tweak the size (you shouldn't normally need 64Mb in
a DomU, and even in Dom0 that's mostly needed for certain SCSI
devices iirc).

Second, the allocation isn't done in a single 64M contiguous chunk (from
hypervisor perspective) - xen_create_contiguous_region() is being
called on much smaller pieces (IO_TLB_SEGSIZE << IO_TLB_SHIFT,
i.e. 128 << 11, yielding 256k chunks).

Jan


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