Tzvetan Mikov (tzvetanmi@yahoo.com) on 5/14/07 wrote:
---------------------------
>(Of course this is at the expense of complexity - for an ex-Windows kernel developer
>the Linux kernel seems like a revelation)
>
>Consider the case when the user is running many applications but none of them require
>more than a GB of _virtual space_, let alone RAM. 2 GB of RAM should serve this case nicely.
>
>Are we technically justified in requiring a 64-bit OS for that? Linus says yes
>- I tend to agree that the comparative simplicity of the Linux design is worth it.
>Still, it is interesting to discuss the alternatives. It is a pity that Dave Cuttler doesn't post here regularly :-)


I pretty sure that no one, Intel and MS included, thinks PAE is a better solution than a straight 64 bit OS.

And everyone understands that it has some pretty solid limitations. Linus and others (myself included) have discussed the kernel space issues at length (simply considering the size of the data structures needed to manage all those physical pages demonstrates the problem, and that's not the only issue), and it's hard to see how a 32 bit OS could actually make use of much more than about 64GB of memory. And even that's something of a stretch (for example, you can't use the /3GB user address space option in Windows with more than 16GB of RAM, IIRC).

I'm sure that's part of the reason why Intel never defined (until x86-64) support for more than an additional four address bits in PAE (which limits you to 64GB).

OTOH, in many situations, adding (some) PAE support to an OS is/was much simpler than doing a 64 bit port of the OS especially once you start counting all the device drivers and whatnot (and Linux is something of a special case where that port is actually fairly easy).

Is especially simpler if you can limit the generality of the extended pages somewhat (or a lot), and for certain applications (databases, paging, caching, for example), something like PAE is pretty easy to use.

And from a hardware context, PAE was a basically trivial change in the page table formats.

So PAE was a quick and easy way to provide some support for more than 4GB of physical address space. Is a straight 64-bit OS (supporting 32 bit applications) better? Yes. But getting there from here is the problem. And for most users, the limitations of a 32-bit OS don't start to bite until you get somewhere past 2GB of RAM, so what the motivation for accepting any degree in inconveniece associated with a 64-bit version of the OS?