linux-kernel - RE: LIBCFS

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Message-ID: <alpine.DEB.2.10.1506301657470.2330@hadrien>
Date:	Tue, 30 Jun 2015 17:01:03 +0200 (CEST)
From:	Julia Lawall <julia.lawall@...6.fr>
To:	"Simmons, James A." <simmonsja@...l.gov>
cc:	'Dan Carpenter' <dan.carpenter@...cle.com>,
	Julia Lawall <julia.lawall@...6.fr>,
	"devel@...verdev.osuosl.org" <devel@...verdev.osuosl.org>,
	"Dilger, Andreas" <andreas.dilger@...el.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	"kernel-janitors@...r.kernel.org" <kernel-janitors@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Drokin, Oleg" <oleg.drokin@...el.com>,
	"lustre-devel@...ts.lustre.org" <lustre-devel@...ts.lustre.org>
Subject: RE: LIBCFS_ALLOC



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
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