IBM Brings DDR4 Memory To Bear On Power Systems
October 17, 2016 Timothy Prickett Morgan
Every new generation of DDR main memory brings advantages of lower latency, higher frequency, higher bandwidth, and lower energy consumption, and this is always a good thing for any server platform. The Power8 systems that launched in 2014 with the Power8 processors were based on DDR3 technology, which was mature at the time and, frankly, less costly for IBM to deploy in its systems then DDR4 DIMMs would have been.
Now that DDR4 pricing has come down, thanks in large part to the widespread adoption of this chip technology by Intel‘s “Haswell” and now “Broadwell” lines of Xeon E5 processors, the technology is suitable for addition in the special Centaur DIMM (CDIMM) memory chips that Big Blue created for its own Power Systems machines. These CDIMMs are very dense memory cards that include the “Centaur” buffer chips that were designed to offer very high bandwidth on the Power8 chips and that include L4 cache memory segments on that buffer chip that contribute greatly to that performance.
As part of the October 11 announcements, IBM is bringing a set of CDIMMs based on the DDR4 memory technology. Although DDR4 memory chips can spin up to 2.4 GHz, IBM is keeping the clock speed on the DDR4 memory it uses in the new CDIMMs at 1.6 GHz, which is the same speed that the DDR3 memory chips used in the prior generation of CDIMMs had, which means their performance is the same. Presumably, this lower-speed DDR4 has more energy savings compared to the 2.4 GHz variants, which obviously spin 50 percent faster and which no doubt throw off more heat. IBM says that the new DDR4 memory chips and the CDIMM cards that use them have the same reliability, availability, and serviceability as the prior DDR3-based CDIMMs, and warns that systems cannot mix and match DDR3 and DDR4 cards in the same systems, but the exception is that on multi-node E870, E870C, E880, and E880C systems, each node can be equipped with either DDR3 or DDR4 memory. The use of the DDR3 cards requires Power Systems firmware update 860 or higher to work, so you will have to patch a box if you upgrade.
DDR4 memory is not entirely new to the Power Systems line, you will remember. Back in February, IBM announced a 256 GB CDIMM card for its high-end NUMA systems that was based on DDR4 chips. At the time, IBM required customers to fully load up a Power E870 with 8 TB of memory or a Power E800 with 16 TB of memory to get the fatter cards, which were twice as capacious as the prior top-end 128 GB DDR3 cards.
Now, the scale-out Power Systems machines (S812L, S814, S822, S822L, S824, and S824L) all have DDR4 CDIMMs available in capacities of 16 GB, 32 GB, 64 GB, and 128 GB capacities, and the new Power E850C announced last week uses DDR4 CDIMM features in the same capacities, too. (The existing Power E850 will get DDR4 memory cards sometime in the second half of 2017, and it is not clear why this will take so long.) The Power E870, E870C, E880, and E880C systems all DDR4 CDIMM cards in the range of 16 GB through 128 GB plus the fatty that came out back in February.
In addition to the announcement of the memory, IBM is cutting the price of capacity on the DDR4-based cards. Take a look at the features and prices for the scale-out Power Systems:
On the scale-out Power Systems machines, you buy a card and it is fully activated when it plugs into the system. So the price reductions are a simple bit of math. On larger systems, you pay for a memory card loaded up with the full complement of memory for that card, and then you pay an additional fee for every 1 GB of memory you turn on using Capacity On Demand features in the system.
As you can see from the chart above, the new DDR4 CDIMMs for the scale-out Power Systems machines (the ones most commonly used by IBM i shops) saw a price cut of between 10 percent and 25 percent, depending on the make and model of the card. In the chart above, the four cards on the top are for the Power Systems S814, S824, and S824L machines and the three on the bottom are for the Power S812L, S822, and S822L machines. (The 4U boxes and the 2U boxes, as the chart says.)
On the larger Power Systems machines, customers have to buy CDIMMs in units of four, and as we said above, there is a base fee plus activation charges. The new DDR4 CDIMM prices for these machines match the price cuts IBM announced back in August and September in the Power Systems line. It works out to $117 per GB on a fully populated card set, except for those with the fattest cards that use the 256 GB CDIMMs, which cost $125 per GB on a fully activated card with 1 TB of memory, which would cost $128,824. So, a fully loaded Power E880 or E880C would need just under $2.1 million to load up on memory. Here is how the features and pricing stack up for the big NUMA machines:
Here’s the fun bit. Out there in the broader world of servers, DDR4 memory chips can use 30 percent less energy and therefore produces less heat than the same capacity of DDR3 memory. This lets companies either clock it up higher and get more performance in the same thermal envelope or keep the performance the same and save energy. IBM is keeping the clock speeds on its DDR3 and DDR4 CDIMMs the same, and even still there are times when they will burn less energy. In general, IBM says that fatter DDR4 cards will save more energy compared to DDR3 equivalents than skinny ones (which makes sense), and similarly, the busier the memory is, the bigger the delta will be, too. The other thing to consider is that the power dram on the memory controllers on the Power8 processors and the Centaur buffer chips will not change with the use of either DDR3 or DDR4 memory. The memory could burn maybe 20 percent less at the CDIMM level and maybe only 2 percent to 3 percent at the system level for most customers.
As always, it all depends.