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Re: 64 Bit Linux shows 4GB... was Using all of 4GB RAM...



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 tmr com>
  "We have more to fear from the bungling of the incompetent than from
the machinations of the wicked."  - from Slashdot


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