linux-kernel - Re: [PATCH 8/9] mm: implement split page table lock for PMD level

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Message-ID: <20130913145204.GG21832@twins.programming.kicks-ass.net>
Date:	Fri, 13 Sep 2013 16:52:04 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc:	Alex Thorlton <athorlton@....com>, Ingo Molnar <mingo@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	"Eric W . Biederman" <ebiederm@...ssion.com>,
	"Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	Andi Kleen <ak@...ux.intel.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Dave Hansen <dave.hansen@...el.com>,
	Dave Jones <davej@...hat.com>,
	David Howells <dhowells@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Kees Cook <keescook@...omium.org>,
	Mel Gorman <mgorman@...e.de>,
	Michael Kerrisk <mtk.manpages@...il.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Rik van Riel <riel@...hat.com>,
	Robin Holt <robinmholt@...il.com>,
	Sedat Dilek <sedat.dilek@...il.com>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH 8/9] mm: implement split page table lock for PMD level

On Fri, Sep 13, 2013 at 05:25:13PM +0300, Kirill A. Shutemov wrote:
> Peter Zijlstra wrote:
> > On Fri, Sep 13, 2013 at 04:06:15PM +0300, Kirill A. Shutemov wrote:
> > > The basic idea is the same as with PTE level: the lock is embedded into
> > > struct page of table's page.
> > > 
> > > Split pmd page table lock only makes sense on big machines.
> > > Let's say >= 32 CPUs for now.
> > 
> > Why is this? Couldn't I generate the same amount of contention on PMD
> > level as I can on PTE level in the THP case?
> 
> Hm. You are right. You just need more memory for that.
> Do you want it to be "4" too?

Well, I would drop your patch-1 and use the same config var.
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