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Date: Fri, 13 Sep 2013 16:52:04 +0200
From: Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH 8/9] mm: implement split page table lock for PMD level
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To: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc: Alex Thorlton <athorlton@sgi.com>, Ingo Molnar <mingo@redhat.com>, Andrew Morton <akpm@linux-foundation.org>, Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>, "Eric W . Biederman" <ebiederm@xmission.com>, "Paul E . McKenney" <paulmck@linux.vnet.ibm.com>, Al Viro <viro@zeniv.linux.org.uk>, Andi Kleen <ak@linux.intel.com>, Andrea Arcangeli <aarcange@redhat.com>, Dave Hansen <dave.hansen@intel.com>, Dave Jones <davej@redhat.com>, David Howells <dhowells@redhat.com>, Frederic Weisbecker <fweisbec@gmail.com>, Johannes Weiner <hannes@cmpxchg.org>, Kees Cook <keescook@chromium.org>, Mel Gorman <mgorman@suse.de>, Michael Kerrisk <mtk.manpages@gmail.com>, Oleg Nesterov <oleg@redhat.com>, Rik van Riel <riel@redhat.com>, Robin Holt <robinmholt@gmail.com>, Sedat Dilek <sedat.dilek@gmail.com>, Srikar Dronamraju <srikar@linux.vnet.ibm.com>, Thomas Gleixner <tglx@linutronix.de>, linux-kernel@vger.kernel.org, linux-mm@kvack.org

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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