Revision as of 22:55, 14 September 2019

z15 is the successor to the z14, a 14 nm z/Architecture mainframe microarchitecture designed by IBM and introduced in 2019.

Process Technology

IBM fabricates its z15 microprocessors and system controllers on GlobalFoundries's 14 nm (14HP) FinFET Silicon-On-Insulator (SOI) process featuring highly-dense deep trench structures used for high-density eDRAM.

Release Dates

The z15 was launched by IBM on September 12, 2019. General availability of the z15 mainframe started September 23.

Architecture

Key changes from z14

• Higher scalability
  • Up to 190-way multiprocessing (from 170-way)
• Central Processor (CP)
  • 2 more cores (12, up from 10)
  • Core
    • 10-13% higher IPC (IBM claim)
    • Front-end
      • Improved branch predictor
        • New TAGE predictor
        • BTB pre-buffer (BTBp) replaced by a simpler write buffer
          • single double-bandwidth port (two independentread ports)
        • 2x larger L1 BTB (8 sets of 2K rows, up from 4 sets of 2K rows)
    • Back-end
      • Larger GCT (60 groups, up from 48 groups)
        • Wider retire (12 instructions/cycle, up from 10)
      • Larger Issue Queues (2 x 36-entry, up from 2 x 30-entry)
      • 2x larger mapper (128-entry, up from 64-entry)
      • Larger integer physical register files (???, up from 120 entries)
      • Larger vector physical register files (???, up from 127 entries)
    • Execution engine
      • New Modulo Arithmetic (MA) unit
    • Memory subsystem
      • 2x larger L2 instruction cache (4 MiB, up from 2 MiB)
      • 2x larger 2 GiB pages STLB (256-entry, up from 64 entries)
  • Shared L3
    • 2x larger L3 (256 MiB, up from 128 MiB)
  • I/O
    • GX Bus removed
    • X Bus removed (2 interface, down from 3)
    • New PCIe Gen interface (3 interfaces, up from 2)
  • Memory
    • Larger memory support (40 TiB, up from 32 TiB)
  • New integration
    • Nest Acceleration Unit (NXU)

• System Controller (SC)
  • 1.4x Larger L4 cache (960 MiB, up from 672 MiB)
    • Non-exclusive (from inclusive)

Block Diagram

CP Chip

Individual core

Memory Hierarchy

The z15 features a memory structure very similar to the z14.

• Cache:
  • L1 instruction cache
    • 128 KiB, 8-way set associative
    • 64 sets, 256 B line size
  • L1 data cache
    • 128 KiB, 8-way set associative
    • 64 sets, 256 B line size
  • L2 instruction cache
    • 4 MiB, 8-way set associative
    • 2K sets, 256 B line size
  • L2 data cache
    • 4 MiB, 8-way set associative
    • 2K sets, 256 B line size
  • L3 cache
    • 256 MiB, 32-way set associative
    • 32K sets, 256 B line size
  • L4 cache (off-chip, on the SC chip)
    • 960 MiB, 40-way set associative
    • 64K sets, 256 B line size
    • Non-exclusive
  • System Memory
    • Up to 40 TiB
    • Up 5 DIMMs (one DIMM per channel)
      • 5th DIMM for Redundant Array of Independent Memory (RAIM) (Note: No non-RAIM option)
      • 32, 64, 128, 256 and 512 GiB DIMMs

• TLBs:
  • ITLB
    • 4 sets x 128 entries
  • DTLB
    • 4 KiB pages
      • 4 sets x 256 entries
    • 1 MiB pages
      • 4 sets x 128 entries
    • 2 GiB pages
      • 2 sets x 32 entries
  • STLB
    • 4 KiB pages
      • 6K entries
    • 2 GiB pages
      • 256 entries
    • Segment Table
      • 8 sets x 256 entries
    • Page Table
      • 8 sets x 256 entries

Overview

This article is a work in progress!

Mainframe

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System

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Drawer

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Central Processor

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Core

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Die

Central Processor (CP) Chip

• 14HP FinFET on SOI
  • 17 metal layers
• 9,200,000,000 transistors
• 5.2 GHz
• 12 cores
• Die size
  • 25.3 mm x 27.5 mm
  • 695.75 mm²

Core

System Controller (SC) Chip

• 14HP FinFET on SOI
  • 17 metal layers
• 9,700,000,000 billion transistors (note that this number, from the technical document, is likely incorrect as it's the same number as the z14)
• 960 MiB shared eDRAM L4 cache.
• Die size
  • 25.3 mm x 27.5 mm
  • 695.75 mm²