[Haskell-cafe] compressed pointers?
jmaessen at alum.mit.edu
Wed Apr 16 08:17:10 EDT 2008
On Apr 16, 2008, at 4:45 AM, Ketil Malde wrote:
> I notice BEA uses something called "compressed pointers" to get the
> 64-bit (more registers, etc) benefits without paying the
> (cache-thrashing) cost.
But only if you're not *actually* using a 64-bit address space. From
their own documentation:
The heap size will be limited to less than 4 GB; therefore, you can
only use this option for applications that demand less than 4 GB of
live data. The heap will be reduced to meet this size limitation if
you specify a larger initial (-Xms) or maximum (-Xmx) heap size.
So this amounts to saying "we can use the 64-bit ISA but still use 32-
bit pointers with all the restrictions that accompany them". You
might be able to keep non-heap data around in excess of 4GB (eg it
might be possible to mmap a file *and* have 4GB of heap data, and
maybe even keep thread stacks off-heap as well).
You can take advantage of pointer alignment to get address spaces of
8-32GB (by shifting 32-bit pointers before dereferencing them), but
that requires taking back the pointer bits that GHC just stole for
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