linux-kernel - Re: [PATCH] mm/hugetlb: per-vma instantiation mutexes

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Date:	Mon, 15 Jul 2013 17:12:31 -0700
From:	Davidlohr Bueso <davidlohr.bueso@...com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	David Gibson <david@...son.dropbear.id.au>,
	Hugh Dickins <hughd@...gle.com>,
	Rik van Riel <riel@...hat.com>,
	Michel Lespinasse <walken@...gle.com>,
	Mel Gorman <mgorman@...e.de>,
	Konstantin Khlebnikov <khlebnikov@...nvz.org>,
	Michal Hocko <mhocko@...e.cz>,
	"AneeshKumarK.V" <aneesh.kumar@...ux.vnet.ibm.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Hillf Danton <dhillf@...il.com>, linux-mm@...ck.org,
	LKML <linux-kernel@...r.kernel.org>,
	Anton Blanchard <anton@...ba.org>
Subject: Re: [PATCH] mm/hugetlb: per-vma instantiation mutexes

On Mon, 2013-07-15 at 16:08 -0700, Andrew Morton wrote:
> On Mon, 15 Jul 2013 17:24:32 +1000 David Gibson <david@...son.dropbear.id.au> wrote:
> 
> > I have previously proposed a correct method of improving scalability,
> > although it doesn't eliminate the lock.  That's to use a set of hashed
> > mutexes.
> 
> Yep - hashing the mutexes is an obvious and nicely localized way of
> improving this.  It's a tweak, not a design change.
> 
> The changelog should describe the choice of the hash key with great
> precision, please.  It's important and is the first thing which
> reviewers and readers will zoom in on.
> 
> Should the individual mutexes be cacheline aligned?  Depends on the
> acquisition frequency, I guess.  Please let's work through that.

In my test cases, involving different RDBMS, I'm getting around 114k
acquisitions.

> 
> Let's not damage uniprocesor kernels too much.  AFACIT the main offender
> here is fault_mutex_hash(), which is the world's most obfuscated "return
> 0;".

I guess we could add an ifndef CONFIG_SMP check to the function and
return 0 right away. That would eliminate any overhead in
fault_mutex_hash().

> 
> >  It wasn't merged before, but I don't recall the reasons
> > why. 

So I've forward ported the patch (will send once everyone agrees that
the matter is settled), including the changes Anton Blanchard added a
exactly two years ago:

https://lkml.org/lkml/2011/7/15/31

My tests show that the number of lock contentions drops from ~11k to
around 500. So this approach alleviates a lot of the bottleneck. I've
also ran it against libhugetlbfs without any regressions.

Thanks,
Davidlohr

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