linux-kernel - Re: [PATCH] gfs2: use __vmalloc GFP

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Message-Id: <BD2045CE-45AD-4D79-8C8D-C854D112DCC5@linuxhacker.ru>
Date:	Wed, 4 Feb 2015 02:13:29 -0500
From:	Oleg Drokin <green@...uxhacker.ru>
To:	Dave Chinner <david@...morbit.com>
Cc:	Steven Whitehouse <swhiteho@...hat.com>, cluster-devel@...hat.com,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-mm@...ck.org
Subject: Re: [PATCH] gfs2: use __vmalloc GFP_NOFS for fs-related allocations.

Hello!

On Feb 3, 2015, at 5:33 PM, Dave Chinner wrote:
>> I also wonder if vmalloc is still very slow? That was the case some
>> time ago when I noticed a problem in directory access times in gfs2,
>> which made us change to use kmalloc with a vmalloc fallback in the
>> first place,
> Another of the "myths" about vmalloc. The speed and scalability of
> vmap/vmalloc is a long solved problem - Nick Piggin fixed the worst
> of those problems 5-6 years ago - see the rewrite from 2008 that
> started with commit db64fe0 ("mm: rewrite vmap layer")....

This actually might be less true than one would hope. At least somewhat
recent studies by LLNL (https://jira.hpdd.intel.com/browse/LU-4008)
show that there's huge contention on vmlist_lock, so if you have vmalloc
intense workloads, you get penalized heavily. Granted, this is rhel6 kernel,
but that is still (albeit heavily modified) 2.6.32, which was released at
the end of 2009, way after 2008.
I see that vmlist_lock is gone now, but e.g. vmap_area_lock that is heavily
used is still in place.

So of course with that in place there's every incentive to not use vmalloc
if at all possible. But if used, one would still hopes it would be at least
safe to do even if somewhat slow.

Bye,
    Oleg
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