Re: Best kernel config for exactly 1GB ram

[Mark Zealey <mark@zealos.org>]

On Sun, Nov 11, 2001 at 02:56:58PM -0600, Steve Bergman wrote:

> Hi,
> 
> 
> I have just upgraded my athlon 1200 system to 1GB ram.  I am unclear as 
> to how I should configure the kernel for this box.  The config.help says 
>    to say no to "high memory support" if the kernel will not run on a 
> machne with more than 1GB.  When I do this I notice that my available 
> memory as reported by top is ~ 120MB less than if I say I want 4GB 
> support.  I recall that linux reserves some of the address space for 
> itself (I thought it was just 64MB).
> 
> What are the trade offs involved here?  Am I better off sacrificing a 
> bit of the physical memory for reasons of efficiency elsewhere?  When I 
> request support for up to 4GB, what exactly changes with respect to the 
> visible virtual address space that apps see, etc?
> 
> This is a desktop machine, so it's not running Oracle or anything like 
> that.  I seem to recall Linus mentioning that big databases tend to like 
> the large (3GB) virtual address space.

Personally, I'd just leave it at the default no high-mem option.

The kernel will then be able to 'see' about 960MB of the memory, so you loose
about 64MB of it, but it's not worth the kernel using bounce-buffers etc just so
you can get 64MB more memory.

IIRC there was a 2GB patch that just redefined PAGE_OFFSET or something similar,
this means that you could see all the memory, but the max virtual memory a
process could see would be 2 gig (as opposed to 3 gig with the default).

-- 

Mark Zealey
mark@zealos.org
mark@itsolve.co.uk

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