Re: [PATCH] gfs2: use __vmalloc GFP_NOFS for fs-related allocations.

From: Steven Whitehouse
Date: Wed Feb 04 2015 - 04:50:16 EST

Hi,

On 04/02/15 07:13, Oleg Drokin wrote:
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

I was thinking back to this thread:
https://lkml.org/lkml/2010/4/12/207

More recent than 2008, and although it resulted in a patch that apparently fixed the problem, I don't think it was ever applied on the basis that it was too risky and kmalloc was the proper solution anyway.... I've not tested recently, so it may have been fixed in the mean time,

Steve.

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