chapter1-AppendixA-m4-ziavras

204 mm2 ratio sqrt 2x some material is adapted from d

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Unformatted text preview: ns: 68 vs. 423 pins (6X) » DRAM Pins: 16 vs. 66 pins (4X) Smaller, faster transistors but communicate faster over (relatively) longer lines: limits latency » Feature size: 1.5 to 3 vs. 0.18 micron (8X,17X) » DRAM Die Size: DRAM Si 47 vs. 217 mm2 (ratio sqrt ⇒ 2X) 2X) » MPU Die Size: 35 vs. 204 mm2 (ratio sqrt ⇒ 2X) Some material is adapted from D. Culler & D. Patterson (UCB) S. Ziavras 6 Reasons Latency Lags Bandwidth (cont’d) 2. Distance limits latency • 3. • • • • • • • • Size of DRAM block ⇒ long bit and word lines ⇒ most of DRAM access time Speed of light and computers on network 1. & 2. explains linear latency vs. square BW? Bandwidth easier to sell (“bigger=better”) E.g., 10 Gbits/s Ethernet (“10 Gig”) vs. 10 µsec latency Ethernet 4400 MB/s DIMM (“PC4400”) vs. 50 ns latency Even if just marketing, customers now trained customers Since bandwidth sells, more resources thrown at bandwidth, which further tips the balance HOWEVER (Recent Trends) SERVER & BUSINESS APPS: require bandwidth MULTICORES: require bandwidth Some material is adapted from D. Culler & D. Patterson (UCB) S. Ziavras 6 Reasons Latency Lags Bandwidth (cont’d) 4. Latency helps BW, but not vice versa • • • Spinning disk faster improves both bandwidth and rotational latency » 3600 RPM ⇒ 15000 RPM = 4.2X » Average rotational latency: 8.3 ms ⇒ 2.0 ms » Things being equal, also helps BW by 4.2X Lower DRAM latency ⇒ More accesses/second (higher bandwidth) Higher linear density helps disk BW (and capacity), but not disk Latency » 9,550 BPI ⇒ 533,000 BPI ⇒ 60X in BW Some material is adapted from D. Culler & D. Patterson (UCB) S. Ziavras 6 Reasons Latency Lags Bandwidth (cont’d) 5. Bandwidth hurts latency • • Queues help Bandwidth, hurt Latency (Queuing Theory) Adding chips to widen a memory module increases Bandwidth but higher fan-out on address lines may increase Latency 6. Operating System overhead hurts Latency more t...
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