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

[Alan Cox <alan lxorguk ukuu org uk>]

> 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.

Alan