Re: HIGHMEM64G Kernel (2.6.23.1) makes system crawl

[Boaz Harrosh <bharrosh@panasas.com>]

On Wed, Oct 24 2007 at 18:35 +0200, lsorense@csclub.uwaterloo.ca (Lennart Sorensen) wrote:
> On Wed, Oct 24, 2007 at 04:41:16PM +0200, Boaz Harrosh wrote:
>> I thought that for 4GB of memory the CONFIG_HIGHMEM4G=y CONFIG_HIGHMEM64G is not set
>> would be enough.
>>
>> From looking in source code the CONFIG_HIGHMEM64G is broken in many respects,
>> in my opinion. It's really a 64-bit with 32-bit quirks enabled. Does it even
>> run on None 64-bit machines? Not many! 
>>
>> You have a magnificent 64-bit Machine, why?
> 
> CONFIG_HIGHMEM4G means 4GB address space.  Since PCI and BIOS use some
> of the first 4GB address space, a machine with 4GB of actual RAM must
> map some of that RAM above the 4GB address space, so you access it all
> you need to enable PAE (address space extensions intel invented to give
> 36bit addressing on 32bit machines), which lets the OS map memory above
> 4GB (up to 64GB), to let applications use it.  Applications are still
> limited to a 32bit address space.  64bit machines of course just use all
> memory directly without any silly extensions.
> 
> The common intel BIOS bug relating to configuring the MTRR and such for
> caching of memory makes the top portion of memory uncached and hence
> very slow, which means any code placed in the top part of memory gets
> slow.  The kernel likes to place itself at the top of memory which means
> the kernel gets slow and hence the whole system gets slow.  Holefully
> intel will learn to make proper BIOSes real soon so that people with
> intel boards can stop having such slow machines when they have lots of
> ram.
> 
> --
> Len Sorensen

So one thing I don't understand is the difference between CONFIG_HIGHMEM4G
and regular 32-bit. I always thought that 32 bit means 4GB address space
and that the HIGHMEM4G is using your above trick but with 32-bit dma_addr_t
since there is only actual 4GB of real memory and the rest is mapping tricks.
But I guess I was wrong.

The other point I was making is why use 36 bit trickery when you have a machine
that is capable of the full 64-bit, and runs much faster at that? Just a waste of
a good machine, wont you say? Perhaps we learn to use x86_64 ARCHs before Intel
fixes their BIOS, and they where right all along?

Boaz

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