linux-kernel - Re: [RFC] Transparent on-demand memory setup initialization embedded in the (GFP) buddy allocator

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Date:	Wed, 26 Jun 2013 10:02:16 -0500
From:	Nathan Zimmer <nzimmer@....com>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Mike Travis <travis@....com>, "H. Peter Anvin" <hpa@...or.com>,
	Nathan Zimmer <nzimmer@....com>, holt@....com, rob@...dley.net,
	tglx@...utronix.de, mingo@...hat.com, yinghai@...nel.org,
	gregkh@...uxfoundation.org, x86@...nel.org,
	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [RFC] Transparent on-demand memory setup initialization
	embedded in the (GFP) buddy allocator

On Wed, Jun 26, 2013 at 03:37:15PM +0200, Ingo Molnar wrote:
> 
> * Andrew Morton <akpm@...ux-foundation.org> wrote:
> 
> > On Wed, 26 Jun 2013 11:22:48 +0200 Ingo Molnar <mingo@...nel.org> wrote:
> > 
> > > except that on 32 TB 
> > > systems we don't spend ~2 hours initializing 8,589,934,592 page heads.
> > 
> > That's about a million a second which is crazy slow - even my 
> > prehistoric desktop is 100x faster than that.
> > 
> > Where's all this time actually being spent?
> 
> See the earlier part of the thread - apparently it's spent initializing 
> the page heads - remote NUMA node misses from a single boot CPU, going 
> across a zillion cross-connects? I guess there's some other low hanging 
> fruits as well - so making this easier to profile would be nice. The 
> profile posted was not really usable.
> 
That is correct, from what I am seeing, using crude cycle counters, there is
far more time spent on the later nodes, i.e. memory near the boot node is 
initialized a lot faster then remote memory.

I think the other low hanging fruits are currently being drowned out by the
lack of locality.

Nate

> Btw., NUMA locality would be another advantage of on-demand 
> initialization: actual users of RAM tend to allocate node-local 
> (especially on large clusters), so any overhead will be naturally lower.
> 
> Thanks,
> 
> 	Ingo
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