Alan Cox wrote:
You probably understand this better than I do, but I have never been able to
find enough performance difference in PAE vs. default kernels to worry me, at
PAE costs you a few percent on tlb loads. The big hit with > 1GB RAM is
the cost of the remapping of user pages and the TLB flushes it causes,
and on the 'hugemem' > 4GB referencing kernel that grows a lot more as
well as getting a lot of problems with 32bit capable I/O devices and
Intel processors with non IOMMU.
It depends a lot on workload and CPU variant.
I'm writing this in a VM running FC9 under a native "2.6.22.14-72.fc6PAE" kernel
which is heavily used in native mode using all 4GB memory. I did measure this
against both the non-PAE 32 bit kernel and x86_64 kernel for desktop, gimp, and
kernel builds.
If you are using a VM you've already totally shot your page table
performance to bits so it won't make any difference.
As noted, I did the testing on PAE, default, and x86_64 on the bare iron, and
didn't really see any significant performance changes. The differences were all
down just at the limits of noise in repeated runs, in the 2-3% range. For many
cases the performance benefit from using more memory is vastly greater than the
small loss in memory management overhead.
I have the feeling that running 32 bit apps under a 64 bit kernel is actually
slower than PAE, but again it's down in the noise. Thanks for your input.
--
Bill Davidsen <davidsen@xxxxxxx>
"We have more to fear from the bungling of the incompetent than from
the machinations of the wicked." - from Slashdot
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