Adrian Chadd <adr...@creative.net.au> writes:
> On Wed, Oct 28, 2009, Alan Tyree wrote:
>
>> That's interesting, Daniel. So what are my tradeoffs. Run the normal kernel:
>> faster but only 2.5gb; Run the bigmem kernel and suffer the performance but
>> have more memory.
>
> Hm, does Linux actually -copy- data around when doing PAE?
Sometimes, the same way as the FreeBSD stuff you mention, using "bounce
buffers" to help incapable hardware out.
The big performance hit is that 32-bit needs to keep page tables in low
memory, which is a scarce resource, and that it needs to use the kmap / kunmap
infrastructure to provide access to pages in high memory, outside the easily
accessed address space.
That later is where most of the cost comes from.
> The whole point behind PAE is to give you a larger amount of address space
> usable by concurrent processes. So each process will have a differently
> mapped set of "PAE" RAM pages as needed, along with the shared kernel stuff.
Yeah. The real cost is the HIGHMEM stuff, not PAE directly, although that
does increase pressure in low memory because the page tables are a bit bigger.
Daniel
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