Huge memory on PAE kernel

[mtz]

Hi,

I'm developing a kernel module that requires more than 1GB RAM.
Most of this memory is taken from the slab, using kmem_cache_create() and kmem_cache_alloc().
Also using vmalloc() and kmalloc().

It works fine on RHEL4, kernel 2.6.9-42.ELhugemem, having 3.1 GB Low-Memory (LowTotal in /proc/meminfo).

Porting to RHEL5 kernel 2.6.18-128.el5PAE, there's only 715 MB Low-Memory.
My questions:
1. How can a kernel module allocate high memory?
2. Should using high memory degrade performance?

Note:
RHEL4 kernel is built with CONFIG_HIGHMEM4G=y
RHEL5 kernel is built with CONFIG_HIGHMEM64G=y

Thanks,
Mtz

[mtz]

I will try to answer myself...
1. How can a kernel module allocate high memory?
Simply, by calling vmalloc().
2. Should using high memory degrade performance?
It should, but I'm not sure how much.
I'm also not sure if allocating more than 128MB VM will exhaust the 896-1024MB zone, and further degrade performance.

Comments, anyone?
Thanks,
Mtz

[Rubberman]

Any reason why you cannot use an x86_64 kernel? If you can, then you don't have to worry about himem/lowmem issues. It will still run your 32bit applications without problem.

[mtz]

Any reason why you cannot use an x86_64 kernel? If you can, then you don't have to worry about himem/lowmem issues. It will still run your 32bit applications without problem.

Hi Rubberman,

Porting to x86_64 kernel is indeed in our roadmap; however, suspecting this porting will exceed our schedule, it was decided to currently stick to x86_32.
So for now, we'll probably have to manage with the current limitations:
LowTotal: 714 MB ; for kmalloc(), kmem_cache_alloc(), …
VmallocTotal: 114 MB ; for vmalloc()

I've tried to increase the vmalloc space as described in the link :
lkml.org/lkml/2006/6/28/155
but kept getting the described kernel panic… Any tips on this?

Thanks,
Mtz

[Rubberman]

Hi Rubberman,

Porting to x86_64 kernel is indeed in our roadmap; however, suspecting this porting will exceed our schedule, it was decided to currently stick to x86_32.
So for now, we'll probably have to manage with the current limitations:
LowTotal: 714 MB ; for kmalloc(), kmem_cache_alloc(), …
VmallocTotal: 114 MB ; for vmalloc()

I've tried to increase the vmalloc space as described in the link :
lkml.org/lkml/2006/6/28/155
but kept getting the described kernel panic… Any tips on this?

Thanks,
Mtz

Actually, it might be faster to just update to x86_64. Sorry that I don't know what the best means to tune low memory and vmalloc memory. I would think that this might be possible with boot-time parameters. Have you posted this query to the kernel.org forums?