[CentOS] [OT] Memory Models and Multi/Virtual-Cores -- WAS: 4.0-> 4.1 update failing

[Feizhou]

>>> where does the performance hit for 4G/4G on Intel (whether
>>> ia32e or not) come from?
>>
>>
>>
>> The performance hit is for _all_ IA-32 compatible architectures running
>> Linux/x86, because there is definitely a hit.
>>
>> There's a hit for the 4G+4G HIGHMEM model.
>> And there is another, bigger one if you go 64G model (more than 4GiB
>> user).
>>
>> As far as _both_ Intel IA-32 on Linux/x86 _and_ Intel IA-32e (EM64T) on
>> Linux/x86-64, you _always_ have "bounce buffers" (c/o the Soft I/O MMU,
>> Soft IOTLB in Linux/x86-64 on EM64T) if you are doing a transfer between
>> two memory areas -- e.g., user memory and memory mapped I/O -- when
>> _one_ area is above 4GiB.  No way around that, and a major problem with
>> Intel right now.
> 
> 
> Right, so if I have 2G of RAM, I want 2G/2G (kernel/user) split instead 
> of 1G/3G so that I don't have to turn on HIGHMEM and thus avoid the 
> penalty of using HIGHMEM.

ugh...RHEL4 kernels do not provide 2G/2G split...only a 4G/4G option. 
Documentation say between 0% -> 30% performance hit...and to treat it as 
20%...

Sorry about the 'no need for HIGHMEM' part...that is still needed to see 
more than 1G, my mistake.