RE: LIBCFS_ALLOC

Julia Lawall
2015-06-30T15:01:03Z

On Tue, 30 Jun 2015, Simmons, James A. wrote:

> >Yeah. You're right. Doing a vmalloc() when kmalloc() doesn't have even
> >a tiny sliver of RAM isn't going to work. It's easier to use
> >libcfs_kvzalloc() everywhere, but it's probably the wrong thing.
>
> The original reason we have the vmalloc water mark wasn't so much the
> issue of memory exhaustion but to handle the case of memory fragmentation.
> Some sites had after a extended period of time started to see failures of
> allocating even 32K using kmalloc. In our latest development branch we moved
> away from using a water mark to always try kmalloc first and if it fails then we
> try vmalloc. At ORNL we ran into severe performance issues when we entered
> vmalloc territory. It has been discussed before on what might replace vmalloc
> handling in the case of kmalloc fails but no solution has been worked out.

OK, but if a structure contains only 4 words, would it be better to just
use kzalloc? Or does it not matter? It would only save trying vmalloc in
a case that it is guaranteed to fail, but if a structure with 4 words
can't be allocatted, the system has other problems. Another argument is
that kzalloc is a well known function that people and bug-finding tools
understand, so it is better to use it whenever possible.

Some of the other structures contain a lot more fields, as well as small
arrays. They are probably acceptable for kzalloc too, but I wouldn't know
the exact dividing line.

julia