Re: 2.6.32-rc3: low mem - only 378MB on x86_32 with 64GB. Why?

[Valdis . Kletnieks]

On Sat, 10 Oct 2009 00:28:42 +0800, Jeff Chua said:

> From all the reading I've read about how slow 64-bit was, after doing all
> the lean and mean compiling, 64-bit is definitely the way to go! Fast and
> worth every bit switching to 64-bit! Now I can go for 128GB ram.

When the MIPS, PowerPC, and Sparc architectures went from 32 to 64 bits,
they *did* take a bit of a performance hit because it basically doubled
the memory bandwidth usage.  However, they all had a reasonably large
number of registers in 32-bit mode.  When the x86 went 64-bit, the register
pressure relief from the additional registers usually more then outweighs
the additional memory bandwidth (basically, if you're spending twice as
much time on each load/store, but only doing it 40% as often, you come out
ahead...)

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