FW: [PATCH] powerpc/mm: Export HPAGE_SHIFT
rdreier at cisco.com
Thu Feb 5 16:33:34 EST 2009
> Right, but then you need to set that in the VMA's, and thus gone is your
> nice fast g_u_p() that doesn't touch VMAs :-)
Registering memory is a slow path thing in the RDMA world. Speeding it
up is nice, so we make userspace do the madvise(VM_DONTCOPY) if it cares
but if it doesn't it can leave it out.
> > Yes, but unfortunately MPI says apps can allocate memory however they
> > damn well please... in any case these issues are all-too-well-known in
> > the RDMA world for quite a while.
> Yup. What do you think of the idea of pre-COWing pages with an elevated
> count at fork time ?
Super-duper sucks if the first thing the child does is exec() :)
Also if the parent has registered > half the memory in the system then
it's instant OOM. So not that useful for the RDMA case :)
The one thing that might make sense is to pre-COW any partial pages that
the parent has registered -- ie if half a page can be used by the child,
at least pre-COW that, but leave all the full pages with VM_DONTCOPY.
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