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Date: Mon, 2 Dec 2013 13:35:59 +0100
From: Ingo Molnar <mingo@kernel.org>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>, Davidlohr Bueso <davidlohr@hp.com>,
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        Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [RFC patch 0/5] futex: Allow lockless empty check of hashbucket
 plist in futex_wake()
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* Linus Torvalds <torvalds@linux-foundation.org> wrote:

> On Sun, Dec 1, 2013 at 8:55 AM, Ingo Molnar <mingo@kernel.org> wrote:
> >
> > Keeping the page invariant would bring significant performance 
> > advantages to hashing.
> 
> Or not. Rather, it would make things much worse. The virtual address 
> is much simpler and better to avoid needing any page table lookup 
> etc crap. The key is just the mm and the virtual address, and no 
> silly page table walks etc necessary.

In theory the CPU could give us the phyisical page address, as the TLB 
is already there: for most futex ops when the kernel runs we just 
attempted access of the virtual address on the user-space side, so the 
hardware already did the hard work of looking up everything in the 
page tables and has it cached most likely.

But yeah, you are right :-/

Thanks,

	ngo