Request for reviews (XS): 6873799 and 6873800

Vladimir Kozlov Vladimir.Kozlov at Sun.COM
Thu Aug 20 11:49:08 PDT 2009

The main performance improvement of compressed oops came
after next changes:

6791178: Specialize for zero as the compressed oop vm heap base

If java heap (including perm gen) size is < 2Gb and OS allows
to reserve the heap below 4Gb boundary no encoding/decoding
needed since object pointers fit into 32-bit. So it is clear win.
(low 2Gb of virtual address space is used for java launcher and
malloc, also Win loads kernel libs there).

If java heap < 30Gb only shift instruction is used for encoding/decoding.

Look on the wiki page
for details on CompressedOops and other technologies Hotspot VM uses.


Christian Thalinger wrote:
> Andrew Haley wrote:
>> Vladimir Kozlov wrote:
>>> I think EA and COOP are stable enough to enable them
>>> by default in HS17. I will do separate pushes to have
>>> separate build bundles.
>>> Fixed 6873799: enable escape analysis by default
>>> Fixed 6873800: enable compressed oops by default
>> That's interesting.  So, even with the (small) extra effort needed to
>> unpack a compressed oop, the memory saving means it's always a win?
> Well, "always" is probably not true.  But nowadays most applications
> rely heavily on memory throughput and it's definitely a win there.  But
> sure Vladimir has some numbers when he wakes up.
> -- Christian

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