Re: sparsemem & sparsemem extreme question

[Heiko Carstens <heiko.carstens@de.ibm.com>]

> > I'm just wondering why there is all this indirection stuff here and why not
> > have one contiguous aray of struct pages (residing in the vmalloc area) that
> > deals with whatever size of memory an architecture wants to support.
> This is exactly what ia64 does today.  Programatically, it does remove a
> layer of indirection.  However, there are some data structures that have
> to be traversed during a lookup: the page tables.  Granted, the TLB will
> provide some caching, but a lookup on ia64 can potentially be much more
> expensive than the two cacheline misses that sparsemem extreme might
> have.

Sure, just that on s390 we have a 1:1 mapping anyway. So these lookups would
be more or less for free for us (compared to what we have now).

> In the end no one has ever produced any compelling performance reason to
> use a vmem_map (as ia64 calls it).  In addition, sparsemem doesn't cause
> any known performance regressions, either.

As far as I understand the memory hotplug patches they won't work without
SPARSEMEM support. So the ia64 approach with a vmem_map will not work here,
right?

Actually my concern is that whenever the address space that is covered with
SPARSEMEM_EXTREME is not sufficient just another layer of indirection needs
to be added.

Heiko

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