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SubjectRe: bigphysarea in 2.2
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I deal with some high-performance image processing hardware. I work in
gray a lot, and a 200DPI high-speed color scanner is soon to come to
reality. That said...

David.Mentre@irisa.fr said:
> With the new kernel memory manager, and if the defragmenting code
> which is under development works, wouldn't it be more useful to use
> standard kernel memory allocation. Static allocation like in
> bigphysarea is more a work-around that a real solution.

It is common for me to need many megabytes per board. In most cases
they are perfectly capable of scatter-gather, but the huge volume, even
broken into 4K pages, is usually hard to get, especially after the system
has been running for a while. (Think loadable modules.)

I do occasionally run into boards that do *not* do scatter-gather, and
they require lots of contiguous memory. These are typically not high
quantity, and so not worth the effort of fancy DMA controllers.

It actually strikes me as a bad idea to try to make a memory manager
that handles these extreme cases at the expense of the normal case. The
bigphysarea patch offers a different memory allocator intended for doing
only a few allocations in a big space. This way it doesn't get in the
way or needlessly complicate the normal allocator. It can also be
practically turned off without recompiling the kernel.

I think it is reasonable to expect my customers of such extreme hardware
to set aside memory specifically for the board so that such allocation
doesn't over-tax the normal allocation mechanism. Putting bigphysarea=foo
in /etc/lilo.conf is not too much to ask, patching the kernel source or
restricting them to a specific kernel version, well...

I believe that the bigphysarea patch is worth making a part of the kernel
as a whole because the Linux kernel is very well suited to these extreme
tasks that occasionally have special memory requirements. The bigphysarea
support is simple, yes, and it helps *a lot*.

(The patch can also be modified to put to good use that non-cached memory
that some computers seem to have.)


--
Steve Williams "The woods are lovely, dark and deep.
steve@icarus.com But I have promises to keep,
steve@picturel.com and lines to code before I sleep,
http://www.picturel.com And lines to code before I sleep."



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