Intel’s new Cascade Lake Xeon servers don’t just offer a dramatic increase in core count (up to 56 cores with the latest Cascade Lake-AP CPUs). They also offer a significant increase in total memory capacity, courtesy of Optane DC Persistent Memory.
Current conventional DDR4 DIMMs top out at 128GB, with 64GB a more common capacity. Optane turbo-charges this, with a minimum module size of 128GB and capacities of up to 512GB per DIMM. Up to 6TB of Optane DIMMs may be installed in a dual-socket system (3TB per CPU + 1.5TB of DRAM). At least one DIMM is required in any system that uses Optane for memory, but customers have the option to pair 1x 128GB DDR4 module with a 512GB Optane module for a total of 768GB of RAM in two DIMM slots. In some dense server configurations where DIMM slots are limited to allow for higher compute density, this kind of capacity configuration could prove extremely useful.
Questions about read/write cycles compared with DRAM have been raised, but Intel is guaranteeing Optane will last for at least three years, even if run at peak write rate for the entire time.
The overall state of Optane and its likelihood of adoption into the wider market are uncertain. As we discussed earlier today, there are questions about Optane’s performance advantage (Intel has stated they are targeting a 1.2x increase in performance per dollar relative to DRAM). The sharp decrease in RAM prices isn’t helping commercialization, either.
At the same time, however, we’ve heard that there are some specific tricks companies could use to improve database performance. Theoretically, multiple copies of a database or frequently-accessed files could be held in RAM simultaneously, allowing multiple simultaneous database reads in circumstances where CPUs would otherwise have to wait and access data sequentially. Keeping these copies of the database synchronized with each other would still be necessary, but that’s its own issue.
One interesting tidbit about Optane and its support for Cascade Lake: When Intel rolled out Optane for desktops, it chose to limit the support to its latest chips and products. In servers, Optane DC PM will be limited to the latest family of Xeon chips. In this case, however, the difference isn’t just a matter of product segmentation. There are physical differences in the DRAM controller on Cascade Lake compared with earlier Xeons to enable support for Optane DIMMs.
While Intel isn’t sharing much in the way of detail yet, the company has created its own modified version of the DDR4 memory interface, dubbed DDR-T. The changes needed to support DDR-T make Optane DC PM non-backward compatible with previous Xeon chips.
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