Mainframe CPU: IBM's z15 gets 12 cores and 256 MB of L3 cache


The z15 from summer 2017 is followed by the z15: The processor is intended for IBM's z15 mainframe systems of the same name, which are used, among other things, in the financial industry and for flight bookings. Even the z14 was a gigantic chip, the z15 is even more monstrous than its predecessor – even though IBM uses the same manufacturing process.

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  1. Fraunhofer Institute for Software and Systems Engineering ISST, Dortmund-Hombruch
  2. Leica Geosystems GmbH, Stuttgart

Again, 14HP is used (14 nm node with Silicon on Insulator), with which IBM builds the two processors that make up a z15. It consists of CPs (Central Processor) as actual CPUs with cores including memory controller and SCs (System Controller) as interconnect and buffer level. The new CP now has 9.2 billion instead of 6.7 billion transistors and the SC has 9.7 billion transistors as before. According to the Technical Guide (PDF) both chips should continue to have 696 mm².

That would be unusual, because the CP has twelve instead of ten CPU cores again with 5.2 GHz. In addition, IBM has increased the L2 cache to 4 MB and the L3 buffer to 256 MB, which explains the increased transistor count, but not the chip area. Possibly the manufacturer has already activated existing functions for the z15, which could also apply to the L4 cache of the SC, which has grown from 674 MB to 960 MB. Elsewhere, however, IBM speaks of a doubled EDRAM macro density.

Improvements have been made to the actual processors, such as a day-based jump prediction. Their speculative executions are removed on erroneous jumps, which prevents side channel attacks. Further enhancements affect internal buffers and latencies, and the zEDC (zEnterprise Data Compression) has been integrated into the CPU so that it reaches a multiple of the throughput. For the z15, IBM has also optimized 2-way Simultaneous Multithreading (SMT) for an average of + 25 percent more performance.

Complete systems consisting of five so-called drawers (6x CP + 1x SC) now have up to 190 cores instead of 170 cores and can address up to 40 TB of DDR4 memory instead of up to 32 TB of RAM (see comparison). As usual, IBM also sells the z15 mainframes only on request, the price of the more powerful machines loosely at several million US dollars.

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