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Latency Lags Bandwidth (last ~20 years)

Performance Milestones Disk : 3600, 5400, 7200, 10000, 15000 RPM. Latency Lags Bandwidth (last ~20 years). Performance Milestones Memory Module : 16bit plain DRAM, 16b Page Mode DRAM, 32b, 64b, 64b SDRAM, 64b DDR SDRAM Disk : 3600, 5400, 7200, 10000, 15000 RPM.

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Latency Lags Bandwidth (last ~20 years)

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  1. Performance Milestones Disk: 3600, 5400, 7200, 10000, 15000 RPM Latency Lags Bandwidth (last ~20 years)

  2. Performance Milestones Memory Module: 16bit plain DRAM, 16b Page Mode DRAM, 32b, 64b, 64b SDRAM, 64b DDR SDRAM Disk: 3600, 5400, 7200, 10000, 15000 RPM Latency Lags Bandwidth (last ~20 years)

  3. Performance Milestones Ethernet: 10Mb, 100Mb, 1000Mb, 10000 Mbit/sec Memory Module: 16bit plain DRAM, 16b Page Mode DRAM, 32b, 64b, 64b SDRAM, 64b DDR SDRAM Disk: 3600, 5400, 7200, 10000, 15000 RPM Latency Lags Bandwidth (last ~20 years)

  4. Performance Milestones Processor: ‘286, ‘386, ‘486, Pentium, Pentium Pro, Pentium 4 Ethernet: 10Mb, 100Mb, 1000Mb, 10000 Mbit/sec Memory Module: 16bit plain DRAM, 16b Page Mode DRAM, 32b, 64b, 64b SDRAM, 64b DDR SDRAM Disk: 3600, 5400, 7200, 10000, 15000 RPM Latency Lags Bandwidth (last ~20 years)

  5. Rule of Thumb for Latency Lagging BW • In the time that bandwidth doubles, latency improves only by factors of 1.2 to 1.4 (and capacity approximately triples in the time that bandwidth doubles)

  6. CPU Memory LAN Disk Latency / Year 1.17 1.07 1.10 1.09 Capacity / Year -- 1.52 -- 1.28 Bandwidth / Year 1.50 1.27 1.39 1.18 Years for 2X Bandwidth 1.7 2.9 2.1 4.2 Capacity Improvement -- 3.4 -- 2.8 Latency Improvement 1.3 1.2 1.2 1.4 Improvement in Time for BW to Double

  7. CPU Memory LAN Disk Latency / Year 1.22 1.06 1.13 1.09 Capacity / Year -- 1.49 -- 1.37 Bandwidth / Year 1.55 1.30 1.78 1.24 Years for 2X Bandwidth 1.6 2.7 1.2 3.2 Capacity Improvement -- 2.9 -- 2.8 Latency Improvement 1.4 1.2 1.2 1.3 Recent Improvement in Time for BW to Double (Last 3 Performance Milestones)

  8. 6 Reasons LatencyLags Bandwidth • Latency helps BW, but not vice versa • Spinning disk faster improves both bandwidth and rotational latency • Higher linear density helps BW, not Latency • Bandwidth hurts latency • Longer queues help BW, hurt Latency • Distance limits latency • Speed of light and computers on network • Size of DRAM block => long bit and word lines => most of DRAM access time

  9. 6 Reasons LatencyLags Bandwidth 4. Moore’s Law helps BW more than latency • Smaller, faster transistors communicate over relatively longer lines: limits latency • Faster transistors, more transistors, more pins all help BW 5. Software overhead hurts latency • Long messages amortize overhead; overhead more for short messages • Bandwidth is easier to sell • E.g., 10 Gbit/s vs. 10 msec latency Ethernet

  10. 4 Ways to Cope with LatencyLags Bandwidth • Caching (Leveraging Capacity) • Processor caches, file cache, disk cache • Replication (Leveraging Capacity) • Read from nearest head in RAID, Distributed caches in SMP, Distributed ISPs • Parallelism and Out-of-Order Processing (Leveraging Bandwidth) • Processors, Disk controllers, Messages • Prediction (Leveraging Bandwidth) • Branches, Prefetching: disk, caches

  11. Conclusion • If everything improves at the same rate, then nothing really changes • When rates vary, we see real dislocation that in turn requires real innovation • Hardware and software developers should plan that performance advances will emphasize BW over latency

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