Messages in this thread | | | Subject | Re: bigphysarea in 2.2 | Date | Sat, 11 Apr 1998 11:08:11 -0600 | From | Stephen Williams <> |
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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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