linux-kernel - Re: The performance and behaviour of the anti-fragmentation related patches

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Date:	Fri, 02 Mar 2007 15:58:41 -0800
From:	"Martin J. Bligh" <mbligh@...gle.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Mark Gross <mgross@...ux.intel.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Balbir Singh <balbir@...ibm.com>, Mel Gorman <mel@...net.ie>,
	npiggin@...e.de, clameter@...r.sgi.com, mingo@...e.hu,
	jschopp@...tin.ibm.com, arjan@...radead.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: The performance and behaviour of the anti-fragmentation related
 patches

> .. and think about a realistic future.
> 
> EVERYBODY will do on-die memory controllers. Yes, Intel doesn't do it 
> today, but in the one- to two-year timeframe even Intel will.
> 
> What does that mean? It means that in bigger systems, you will no longer 
> even *have* 8 or 16 banks where turning off a few banks makes sense. 
> You'll quite often have just a few DIMM's per die, because that's what you 
> want for latency. Then you'll have CSI or HT or another interconnect.
> 
> And with a few DIMM's per die, you're back where even just 2-way 
> interleaving basically means that in order to turn off your DIMM, you 
> probably need to remove HALF the memory for that CPU.
> 
> In other words: TURNING OFF DIMM's IS A BEDTIME STORY FOR DIMWITTED 
> CHILDREN.

Even with only 4 banks per CPU, and 2-way interleaving, we could still
power off half the DIMMs in the system. That's a huge impact on the
power budget for a large cluster.

No, it's not ideal, but what was that quote again ... "perfect is the
enemy of good"? Something like that ;-)

> There are maybe a couple machines IN EXISTENCE TODAY that can do it. But 
> nobody actually does it in practice, and nobody even knows if it's going 
> to be viable (yes, DRAM takes energy, but trying to keep memory free will 
> likely waste power *too*, and I doubt anybody has any real idea of how 
> much any of this would actually help in practice).

Batch jobs across clusters have spikes at different times of the day,
etc that are fairly predictable in many cases.

M.

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