quoted

quoted

I think it will for most of the situations we run aground with now
(normally 5000 oracle tasks sharing a 2Gb shared segment, or some
such monster).

10 GB pagetable overhead, for 2 GB of data.  No customer I
know would accept that much OS overhead.

To reduce the overhead we could either reclaim the page
tables and reconstruct them when needed (lots of work) or
we could share the page tables (less runtime overhead).

Or you use 4MB pages.  That tends to work much better and has less 
complexity.  Shared page tables don't work well on x86 when you have 
a database trying to access an SGA larger than the virtual address 
space, as each process tends to map its own window into the buffer 
pool.  Highmem with 32 bit va just plain sucks.  The right answer is 
to change the architecture of the application to not run with 5000 
unique processes.

Bear in mind that large pages are neither swap backed or file backed
(vetoed by Linus), for starters. There are other large app problem scenarios 
apart from Oracle ;-)

M.

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