Re: The performance and behaviour of the anti-fragmentation relatedpatches
From: Martin J. Bligh
Date: Fri Mar 02 2007 - 19:00:19 EST
.. 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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