linux-ext4 - Re: Use of kmalloc vs vmalloc in ext4?

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Date:	Mon, 06 Apr 2009 03:33:10 -0600
From:	Andreas Dilger <adilger@....com>
To:	Michael Rubin <mrubin@...gle.com>
Cc:	Curt Wohlgemuth <curtw@...gle.com>, linux-ext4@...r.kernel.org
Subject: Re: Use of kmalloc vs vmalloc in ext4?

On Apr 05, 2009  23:45 -0700, Michael Rubin wrote:
> Anyone have any comments? Or historical reasons? We operate with some
> constrained memory situations, and were wondering if a patch to move
> from kmalloc to vmalloc would be well received.

On 32-bit machines vmalloc space is tiny, and in all cases vmalloc
performance sucks, so traditionally very little kernel allocation
is done with vmalloc.  For one-off allocations like per-fs it is
probably OK to change them to vmalloc.

> On Wed, Mar 11, 2009 at 7:32 AM, Curt Wohlgemuth <curtw@...gle.com> wrote:
> > I've been running various tests of ext4 partitions lately, and have
> > found that with very low memory situations, I'm getting intermittent
> > mount failures due to ENOMEM from ext4_mb_init() and
> > ext4_fill_flex_info() .  Here's a typical dmesg from the latter:
> >
> >          EXT4-fs: not enough memory for 8198 flex groups
> >          EXT4-fs: unable to initialize flex_bg meta info!
> >
> > This is from a kzalloc() call of size ~64k .  I think the
> > ext4_mb_init() calls to kmalloc() and alloc_percpu() are even smaller.
> >
> > I was wondering why all the code in ext4 (and ext[23], for that
> > matter) uses kmalloc() and friends instead of vmalloc(), at least
> > where it's safe; is it just for performance reasons?
> >
> > I've seen the above errors when I do a mount -a, causing several
> > partitions to be mounted; I can usually mount the failed ones by hand
> > right afterwards, but this is a big difference for us, in our
> > environment, compared to, say, ext2 partitions.
> >
> > Thanks,
> > Curt
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Cheers, Andreas
--
Andreas Dilger
Sr. Staff Engineer, Lustre Group
Sun Microsystems of Canada, Inc.

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