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Performance difference 32bit/64bit userland

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Hi there,
while preparing other tests I created two installations on two hosts
with identical hardware (LPARs on IBM POWER):
- powerpc (64 bit kernel, 32 bit userland)
- ppc64 (64 bit kernel, 64 bit userland)
Both are up-to-date sid with systemd held to 232-10 (#852811).
Now the surprise: Using the 32 bit userland, CPU bound operations like
gzip or xz are significantly faster (5 to 10 percent). Comparing to x86
where i386 is 10 to 15 percent slower than amd64. Also running
debootstrap showed a similar pattern. All tests were repeated a few
times to rule out any caching or similar effects.
This is not quite satisfying. Anyone an explanation for this?
    Christoph

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