Re: Bootmem allocator

["Joseph A. Knapka" <jknapka@earthlink.net>]

Joseph A. Knapka wrote:

> Disclaimer: I have ignored the bootmem allocator
> thus far, but I presume it works very much like
> the old 2.2 bootmem allocation code, so I'll
> try to answer your questions on that basis... but
> maybe I'll be wrong :-)

I've looked at it and started writing some docs, but I
think I should correct my mistakes here first :-)

 
> A. Supreeth Reddy wrote:
> 
>> Hi All,
>> 
>> I am trying to understand how the bootmem allocator is designed. I 
>> have the
>> following questions in the bootmem allocator.
>> 
>> 1. Does it give memory in a sequential fashion in terms of pages or 
>> does it
>> allow any allocation of powers of 2 ?
> 
> 
> Sequentially; you ask for a particular number of pages
> and you get the lowest-address available free pages.

I was wrong. You ask for a particular number of bytes with
a particular alignment (power of 2 bytes), and you get the
lowest-address available chunk that fulfills your
alignment requirement. The bootmem allocator attempts to
merge sub-page-sized requests if possible, within the
constraints of the requested alignment.
That's what all the masking and shifting is about in
__alloc_bootmem_core() around line 190 of bootmem.c,
right after the "found:" label. (I'm looking at 2.4.0 source,
incidentally).

> 
>> 2. Is only 1/8 th of the whole memory after "_end" given to bootmem
>> allocator, since in init_bootmem_core () there is something like
>>         mapsize = (( end - start) + 7)/8;
> 
> The bootmem map is a bitmap, so 1 bit/page, not
> 1 byte/page.

Got that one right...

>> 3. Does the function __alloc_bootmem_alloc () try to concatenate 2
>> subsequent allocations to avoid internal fragmentation ?
> 
> 
> I expect so.
> 
> 
>> 4. And what is the purpose of last_offset and last_pos fields in the
>> bootmem_data_t structure, I find that they are used only in
>> __alloc_bootmem_alloc () function.
>> 
> 
> This may be something involving NUMA.

Bzzzt, wrong answer. last_pos is (I think) the last
page (highest address) from which an allocation was done,
and last_offset was the offset within the page of that
allocation. The offset data is used to do merges of sub-page
requests.

Regards,

-- Joe
-- Joseph Knapka
"If I ever get reincarnated... let me make certain I don't come back
  as a paperclip." -- protagonist, H Murakami's "Hard-boiled Wonderland"
// Linux MM Documentation in progress:
// http://home.earthlink.net/~jknapka/linux-mm/vmoutline.html
* Evolution is an "unproven theory" in the same sense that gravity is. *

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