Re: [Xen-devel] [PATCH][2/17] USB virt 2.6 split driver---xenidc buffer resource provider

[Muli Ben-Yehuda <mulix@xxxxxxxxx>]

On Tue, Nov 22, 2005 at 11:12:29AM +0000, Keir Fraser wrote:

> If the buffer is > PAGE_SIZE, you should use vmalloc if at all 
> possible. Order != 0 allocations can easily fail on a churned box with 
> lots of memory fragmentation.

Quoth Linus[1[:

"vmalloc() is NOT SOMETHING YOU SHOULD EVER USE! It's only valid for
when you _need_ a big array, and you don't have any choice. It's slow,
and it's a very restricted resource: it's a global resource that is
literally restricted to a few tens of megabytes. It should be _very_
carefully used."

The main reason not to use vmalloc has a run-time overhead due to the
increased tlb footprint and is also a limited resource (128MB on x86,
IIRC). I've heard of systems running out of vmalloc space and
dying with plenty of memory otherwise available. There are other ways
to deal with high order allocations failing, e.g. pre-allocating, not
using GFP_ATOMIC where not absolutely necessary, etc.

[1] http://www.ussg.iu.edu/hypermail/linux/kernel/0311.1/0576.html

Cheers,
Muli
-- 
Muli Ben-Yehuda
http://www.mulix.org | http://mulix.livejournal.com/


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