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

[Bill Davidsen]

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