The L4-cache and Memory Subsystem

Each POWER8 memory controller has access to four "Custom DIMMs" or CDIMMs. Each CDIMMs is in fact a "Centaur" chip and 40 to 80 DRAM chips. The Centaur chip contains the DDR3 interfaces, the memory management logic and a 16 MB L4-cache.

The 16 MB L4-cache is eDRAM technology like the on-die L3-cache. Let us see how the CDIMMs look in reality.

Considering that 4Gb DRAM chips were available in mid 2013, the 1600 MHz 2Gb DRAM chips used here look a bit outdated. Otherwise the (much) more expensive 64GB CDIMMs use the current 4Gb DRAM chips. The S822L has 16 slots and can thus use up to 1TB (64GB x 16) in DIMMs.

Considering that many Xeon E5 servers are limited to 768 GB, 1 TB is more than competitive. Some Xeon E5 servers can reach 1.5 TB with 64 GB LR-DIMMs but not every server supports this rather expensive memory technology. It is very easy to service the CDIMMs: a gentle push on the two sides will allow you to slide them out. The black pieces of plastic between the CDIMMS are just place-holders that protect the underlying memory slots. For our testing we had CDIMMs installed in 8 of our system's 16 slots.

The Centaur chip acts as a 16MB L4-cache to save memory accesses and thus energy, but it needs quite a bit of power (10-20 W) itself and as a result is covered by heatsink. CDIMMs have ECC enabled (8+1 for ECC) and have also an extra spare DRAM chip. As result, a CDIMM has 10 DRAM chips while offering capacity of 8 chips.

That makes the DRAM subsystem of the S822L much more similar to the E7 memory subsystem with the "Scalable memory interconnect" and "Jordan Creek" memory buffer technology than to the typical Xeon E5 servers.

Inside the S822L: Hardware Components Benchmark Configuration and Methodology
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  • extide - Friday, November 6, 2015 - link

    No he meant that in a lot of the european countries they use the dot as a comma, so it would be 50.000 to mean 50 thousand. Reply
  • Murloc - Sunday, November 8, 2015 - link

    the international system dictates that , and . are the same thing, and as a separator you should use a space.
    In many countries in Europe, ' is also used. That's fine too as there is no ambiguity.
    Using . and , for anything that is not the decimal separator in international websites just creates confusion imho.
    I guess AT doesn't have a style book though.
    Reply
  • duploxxx - Friday, November 6, 2015 - link

    nice review.
    but Xeon is not 95% of the market. AMD is still just a bit above 5% on its own. so it deserves a bit salt :) not to mention the fact that competition is good for all of us. if reviewers continue like this all narrowed readers will think there is no competition.
    Reply
  • silverblue - Friday, November 6, 2015 - link

    I'm left wondering what a Steamroller-based 16+ core CPU would do here, considering multithreading is better than with previous models. Yes, the Xeons have a large single-threading lead, but more cores = good in the server world, not to mention that such a CPU would severely undercut the price of the competition.

    Shame it isn't ever going to happen!
    Reply
  • lmcd - Friday, November 6, 2015 - link

    Or even an Excavator! It's a shame AMD didn't just keep Bulldozer developing internally until at least Piledriver, and iterate on Thuban. Reply
  • Kevin G - Saturday, November 7, 2015 - link

    AMD killed off both Streamroller and Excavator chips early on as the Bulldozer and Piledriver chips weren't as competitive. More importantly, OEMs simply were not interested even if those parts were upgrades based upon existing designs. Thus the great AMD server drought began as they effectively have left that market and are hoping for a return with Zen.

    Also I should point out that Seattle, AMD's first ARM based Opteron has yet to arrive. This was supposed to be out a year ago and keep AMD's server business going throughout 2015 during the wait for Zen and K12 in 2016. Well K12 has already been delayed into 2017 and Seattle is no where to be found in commercial systems (there are a handle of Seattle developer boards).
    Reply
  • JoeMonco - Saturday, November 7, 2015 - link

    When you account for only 5% of the market while the other side commands 95%, you aren't really much of a credible competitor. Reply
  • xype - Sunday, November 8, 2015 - link

    That’s not always correct, though. You can have 5% of the market and 20% of the profits, for example, which would put you in a way better position than your competitors (because only a small increase in market share would pay big time). Reply
  • Murloc - Sunday, November 8, 2015 - link

    that applies more to consumer products, e.g. apple. Reply
  • dgingeri - Friday, November 6, 2015 - link

    I've been dealing with IBM Power based machines for 5 years now. Such experience has only given me a major disdain for AIX.

    I do NOT advise it for anyone. It sucks to work on. There is a certain consistent, spartan logic to it, but it is difficult to learn, and learning materials are EXTREMELY expensive. I never liked the idea of paying $12,000 for a one week class that taught me barely a tenth of what I needed to know to run an AIX network. (My company paid for the class, but I could not get them to pay for the rest of them, for some reason.) This makes people who can support AIX extremely expensive to employ. Figure on paying twice the rate of a Windows admin in order to employ an AIX admin. Then there is the massive expense of maintenance agreements. Even the software only maintenance agreement, just to get patches for AIX, is $4000 per year per system. They may be competitive in cost up front, but they drain money like vampires to maintain.

    Even the most modern IBM Power based machine takes 20-30 minutes to reboot or power up due to POST diagnostics. That alone is annoying enough to make me avoid AIX as much as I can.
    Reply

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