Vmalloc Information Needed

[pramod gurav]

On Sun, Mar 23, 2014 at 9:54 AM, Anup Buchke <anup.estuff at gmail.com> wrote:

> For a user/kernel configuration of 3/1GB and (0-16M DMA , 16-896 - Low ,
> 896 - 1024 - High )
> *Q:* Is amount of memory allocated to Vmalloc limited to 128MB?
>
> The upper 128MB in virtual space(HIGHMEM) is not a limit for allocation
but for mapping of physical memory. So at a time you can allocate 128MB of
memory using vmalloc which is mapped in physical memory. If you try to
allocate more then the aged pages will be swapped out and new pages will
goven that space.

I read an article which said to increase the Vmalloc allocation size we
> need to either
> *1)* Change user/kernel distribution like 2.75/1.25 which proportionately
> increase the address space available to High memory
>

I don't think it is possible to have virtual address space split in
fractions like you mentioned. But it you can have U/K split like 3:1, 2:2
and 1:3 by configuring it in Kernel. But 3:1 U/K split is the recommended
one.

>
> *2)* For a fixed user/kernel distribution reduce the low memory like
> 16-800 thereby increasing the size available for high memory.
>
This is something that has to explored.

>
>
> In either of the case the memory for vmalloc is limited to user/kernel
> distribution . i.e. the amount of virtual address space available in high
> memory.
>

Again there is not limit on allocation of memory but it comes with a cost
of lot swap in, swap out of pages if you try to allocate more than 128MB.

>
>
I am a beginner, so please let me know if my understanding is correct..and
> rectify in-case of wrong.?
>
>
This all holds true for 32 bit architecture. For 64 bit machines (which I
am running on my host) the virtual address space is huge and it is equally
divided between U/K space. The HIGHMEM concept also does not come here as
you have a huge amount of physical memory mapped to virtual address space
in kernel.

Thanks,
> Anup Buchke.
>
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>


-- 
Thanks and Regards
Pramod
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