max memory limits ???

max memory limits ???

[BenHanokh Gabriel]

hi

can some1 explain the memory limits on the 2.4 kernel

- what is the maximum amount of physical memory it supports ?
- what is the limit for user-space apps ?
- what is the limit for kernel ?
- are there any limits on malloc, static-memory, stack-memory ?
- does using HIGHMEM results with performance penalty   ?
- anything else ?

please CC me for any answer

regards
Benhanokh Gabriel

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Re: max memory limits ???

[Matti Aarnio]

On Wed, Nov 22, 2000 at 03:37:09PM +0200, BenHanokh Gabriel wrote:
> hi
> 
> can some1 explain the memory limits on the 2.4 kernel
> 
> - what is the maximum amount of physical memory it supports ?

	There are some Alpha systems with 256 GB memory where Linux is
	sometimes tested.   Those processors support up to several
	terabytes of memory (nobody built such maximum machines, though.)

	For intel machines limits are different, of course.
	(PAE36 supports up to 16*4 GB = 64 GB physical address space,
         within which there must be some holes to support e.g. PCI
	 bus address spaces, and system boot rom, to name a few.)

	For intel ia64 -- that is a 64 bit machine, thus it propably
	follows Alpha and UltraSPARC model in this regard.

> - what is the limit for user-space apps ?

	At 32 bit systems:  3.5 GB with extreme tricks, 3 GB for more usual.
	At 64 bit systems -- 2^60 or some such semi-meaningless value.

> - what is the limit for kernel ?

	Whatever the hardware supports.

> - are there any limits on malloc, static-memory, stack-memory ?

	userspace rules.

> - does using HIGHMEM results with performance penalty   ?

	Of course, it causes extra mapping operations on machines
	needing it to support larger memory (intel PAE36 featured
	hardware).  User processes can access all what is mapped
	into them at the same time -- All programs, kernel included
	are limited to 32 bit addresses, but kernel can juggle maps
	to reach areas not mapped in its address space at some moment.

> - anything else ?
> 
> please CC me for any answer
> 
> regards
> Benhanokh Gabriel
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Re: max memory limits ???

[Stephen C. Tweedie]

Hi,

On Wed, Nov 22, 2000 at 04:11:04PM +0200, Matti Aarnio wrote:
> > - does using HIGHMEM results with performance penalty   ?
> 
> 	Of course, it causes extra mapping operations on machines
> 	needing it to support larger memory (intel PAE36 featured
> 	hardware).  User processes can access all what is mapped
> 	into them at the same time -- All programs, kernel included
> 	are limited to 32 bit addresses, but kernel can juggle maps
> 	to reach areas not mapped in its address space at some moment.

The other performance implications are:

HIGHMEM currently results in extra copies if you are on Intel and
using a 4G or 64G config, whenever you perform disk IO to memory above
1GB.  If you are using the 64G config (PAE36), then the page tables
double in size, occupying more of your cache.  Those are likely to be
the two biggest performance costs in enabling highmem in an i686
kernel.

Cheers,
 Stephen
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Re: max memory limits ???

[Raymond Nijssen]

>Matti Aarnio <matti.aarnio@zmailer.org> wrote:
>On Wed, Nov 22, 2000 at 03:37:09PM +0200, BenHanokh Gabriel wrote:

>> can some1 explain the memory limits on the 2.4 kernel

>> - what is the limit for user-space apps ?

>        At 32 bit systems:  3.5 GB with extreme tricks, 3 GB for more usual.


What are those tricks?   Do they involve changing TASK_SIZE ?

And why do programs get mapped at 0x08000000 instead of 0x00001000 ?
This is wasting another 5% of the addressing space.

Please CC me on your reply.

Thanks,
-Raymond
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