Best kernel config for exactly 1GB ram

Best kernel config for exactly 1GB ram

[Steve Bergman]

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.

Any enlightenment would be greatly appreciated.



Thanks,
Steve Bergman
steve@rueb.com

Re: Best kernel config for exactly 1GB ram

[Xavier Bestel]

le dim 11-11-2001 à 21:56, Steve Bergman a écrit :
> 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 

Actually it's a bit less than 1GB, so you'd better go for 4GB support.

	Xav

Re: Best kernel config for exactly 1GB ram

[Mark Zealey]

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

UL++++>$ G!>(GCM/GCS/GS/GM) dpu? s:-@ a16! C++++>$ P++++>+++++$ L+++>+++++$
!E---? W+++>$ N- !o? !w--- O? !M? !V? !PS !PE--@ PGP+? r++ !t---?@ !X---?
!R- b+ !tv b+ DI+ D+? G+++ e>+++++ !h++* r!-- y--

(www.geekcode.com)

Best kernel config for exactly 1GB ram

[Zwane Mwaikambo]

you actually lose about 128Mb ram (128Mb HIGHMEM) if you don't specify the
4gig option, and you might find that you'd rather have the extra memory
than the "noticeable?" slowdown. I set all my 1G boxes to 4G mostly because
i put that amount of ram because i need every bit.

Regards,
	Zwane Mwaikambo

Re: Best kernel config for exactly 1GB ram

[David Lang]

I leave mine at the 1G settings, it gives me 960MB instead of 1024M but
avoids all the potential problems that show up with himem.

there are still a lot of potential problems with himem (see linus' post
last week about the pitfalls of locking memory on himem machines in the
wrong zone for one example)

those potential problems are worth dealing with if you really do need a
lot of ram, but the difference between 960M and 1024M doesn't seem like
enough benifit to be worth it. If you really need every bit of your ram
you should not be running X and should shut down a LOT of the default
services, if you haven't gone through and optimized your system you can
probably free up more then 64M of ram that way.

David Lang


On Mon, 12 Nov 2001, Zwane Mwaikambo wrote:

> Date: Mon, 12 Nov 2001 08:38:29 +0200 (SAST)
> From: Zwane Mwaikambo <zwane@linux.realnet.co.sz>
> To: linux-kernel@vger.kernel.org
> Cc: steve@uhura.rueb.com
> Subject: Best kernel config for exactly 1GB ram
>
> you actually lose about 128Mb ram (128Mb HIGHMEM) if you don't specify the
> 4gig option, and you might find that you'd rather have the extra memory
> than the "noticeable?" slowdown. I set all my 1G boxes to 4G mostly because
> i put that amount of ram because i need every bit.
>
> Regards,
> 	Zwane Mwaikambo
>
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