Computer Networking Latency
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Questions and Answers

What is the latency of a main memory reference for a double-word?

  • 5
  • 0.5
  • 100 (correct)
  • 7

    • What is the primary motivation behind building warehouse-scale computers using commodity components?

  • To fulfill the demand of an increasing number of users (correct)
  • To improve the reliability of computing
  • To reduce the cost of computing
  • To reduce energy consumption

    • What is the latency of sending 2 KiB of data over a 1 Gbps network?

  • 200
  • 20,000 (correct)
  • 2,000
  • 20

    • Who predicted that computing would become a public utility?

    <p>John McCarthy</p> Signup and view all the answers

    What is the latency of reading 1 MiB sequentially from disk?

    <p>20,000,000</p> Signup and view all the answers

    What is the latency of a round trip within the same data center?

    <p>500,000</p> Signup and view all the answers

    What is the latency of sending a packet from California to the Netherlands and back to California?

    <p>150,000,000</p> Signup and view all the answers

    What is the primary application of MapReduce programming in the context of warehouse-scale computing?

    <p>Distributed storage systems</p> Signup and view all the answers

    What is the primary reason for the increase in latency from the local node to the array?

    <p>The array switch hardware/software</p> Signup and view all the answers

    What is the unit of measurement for bandwidth in Table 10.2?

    <p>MiB/s</p> Signup and view all the answers

    What is the primary factor that collapses differences in bandwidth?

    <p>The network overhead</p> Signup and view all the answers

    What is the purpose of the core router in Fig. 10.3?

    <p>To operate in the Internet backbone</p> Signup and view all the answers

    What is the metric used to evaluate the efficiency of a WSC?

    <p>Power Utilization Effectiveness (PUE)</p> Signup and view all the answers

    What is the latency of DRAM in the local node according to Table 10.1?

    <p>0.1 µs</p> Signup and view all the answers

    What is the additional level in the networking hierarchy when a WSC needs more than one array?

    <p>One more level of arrays</p> Signup and view all the answers

    What does the 'four nines' refer to in the context of Warehouse-scale Computers?

    <p>A level of reliability in WSCs, where 99.99% uptime is guaranteed</p> Signup and view all the answers

    What is the primary benefit of 'easy' parallelism, also known as request-level parallelism (RLP), in WSCs?

    <p>Reduced need for communication and synchronization among independent efforts</p> Signup and view all the answers

    What is a key factor in determining the location of a Warehouse-scale Computer (WSC)?

    <p>All of the above</p> Signup and view all the answers

    What is the primary concern regarding operational costs in WSCs?

    <p>Energy consumption and power distribution</p> Signup and view all the answers

    What type of parallelism is crucial in WSCs, especially in web crawlers?

    <p>Data-level parallelism</p> Signup and view all the answers

    What is the main benefit of using a hierarchical switch architecture in WSCs?

    <p>Increased dependability and reduced failure rates</p> Signup and view all the answers

    What is a characteristic of massively parallel batch programs in WSCs?

    <p>They are used to compute lots of metadata</p> Signup and view all the answers

    What is the primary impact of the big scale of WSCs on their dependability?

    <p>Decreased dependability due to bigger failure rates</p> Signup and view all the answers

    What is the primary concern in terms of energy efficiency in Warehouse-scale Computers?

    <p>Increasing the computing power per joule</p> Signup and view all the answers

    What is the primary goal of dependability in Warehouse-scale Computers?

    <p>Providing 99.99% availability</p> Signup and view all the answers

    What is the primary benefit of reducing costs in Warehouse-scale Computers?

    <p>Saving millions of dollars</p> Signup and view all the answers

    What is the typical number of servers housed in a Warehouse-scale Computer?

    <p>50,000 to 100,000</p> Signup and view all the answers

    What is the main goal of the networking equipment in a Warehouse-scale Computer?

    <p>Connecting and housing servers</p> Signup and view all the answers

    What is the primary focus of Warehouse-scale Computers today?

    <p>Providing information technology for the world</p> Signup and view all the answers

    What is the role of software redundancy in achieving dependability in Warehouse-scale Computers?

    <p>It plays an important role alongside hardware redundancy</p> Signup and view all the answers

    What is the primary motivation for building Warehouse-scale Computers using commodity components?

    <p>Not provided in the content</p> Signup and view all the answers

    What is the primary purpose of the Power Utilization Effectiveness (PUE) metric?

    <p>To evaluate the efficiency of a WSC</p> Signup and view all the answers

    What is the main reason for the increase in latency from the local node to the array?

    <p>The network overhead and switch hardware/software</p> Signup and view all the answers

    What is the unit of measurement for latency in Table 10.1?

    <p>µs</p> Signup and view all the answers

    What is the purpose of the core router in Fig. 10.3?

    <p>To operate in the Internet backbone</p> Signup and view all the answers

    Why do DRAM and Disk numbers are only available for the local node in Table 10.2?

    <p>Because the complete numbers were computed only considering the Flash memory</p> Signup and view all the answers

    What is the primary factor that collapses differences in bandwidth?

    <p>The network overhead</p> Signup and view all the answers

    What is the additional level in the networking hierarchy when a WSC needs more than one array?

    <p>A regular 'Layer 3' router</p> Signup and view all the answers

    What is the primary impact of the big scale of WSCs on their dependability?

    <p>Not specified in the content</p> Signup and view all the answers

    What is the purpose of the map part in MapReduce?

    <p>To apply a programmer-supplied function to each input</p> Signup and view all the answers

    What is the storage capacity of each node in the array?

    <p>2,048 GiB Disk</p> Signup and view all the answers

    What is the purpose of the reduce part in MapReduce?

    <p>To collapse intermediate results by using a programmer-defined function</p> Signup and view all the answers

    What is the network bandwidth of each node in the array?

    <p>1 Gbit/s</p> Signup and view all the answers

    How many nodes are there in each rack?

    <p>80</p> Signup and view all the answers

    What is the purpose of the EmitIntermediate function in MapReduce?

    <p>To produce a key-value pair</p> Signup and view all the answers

    How many racks are there in each array?

    <p>30</p> Signup and view all the answers

    What is the storage hierarchy of each computing node?

    <p>DRAM, Flash, Disk</p> Signup and view all the answers

    Study Notes

    Latency in WSC

    • Latency in WSC increases due to networking software and switch overhead in the rack and array switch hardware/software.
    • Latency numbers for DRAM, Flash, and Disk are:
      • Local Node: 0.1µs (DRAM), 100µs (Flash), 10,000µs (Disk)
      • Rack: 300µs (DRAM), 400µs (Flash), 11,000µs (Disk)
      • Array: 500µs (DRAM), 600µs (Flash), 12,000µs (Disk)

    Bandwidth in WSC

    • Bandwidth numbers for DRAM, Flash, and Disk are:
      • Local Node: 20,000 MiB/s (DRAM), 200 MiB/s (Disk)
      • Rack: 1,000 MiB/s (Flash), 100 MiB/s (Flash)
      • Array: 10 MiB/s (Flash)

    Networking Hierarchy

    • The network overhead increases latency and bandwidth differences between the local node and the array.
    • In some cases, WSC needs more than one array, which adds an extra level to the networking hierarchy.

    Power Utilization Effectiveness (PUE)

    • PUE is a metric to evaluate the efficiency of a WSC.
    • Example values for power usage: 0.5 (L1 cache reference), 5 (branch mispredict), 7 (L2 cache reference), 100 (main memory reference), 20,000 (send 2 KiB over 1 Gbps network), etc.

    Cloud Computing

    • John McCarthy predicted the concept of cloud computing as a public utility in 1961.
    • Large Internet companies like Amazon, Google, and Microsoft use warehouse-scale computers to fulfill the increasing demand of users.

    Warehouse-scale Computers (WSC)

    • WSC needs to consider interactive and batch processing workloads, such as searches and social networking.
    • Main requirements of WSC include:
      • Ample parallelism (data-level and request-level parallelism)
      • Operational costs (energy, power distribution, and cooling)
      • Location (inexpensive electricity, proximity to Internet backbone optical fibers, and human resources)
      • Big scale trade-offs (less dependability, bigger failure rates)

    Nodes, Racks, and Switches

    • WSC consists of nodes, racks, and switches, with an Ethernet switch on top of each rack and an array switch connecting multiple racks.
    • Figure 10.1 illustrates the hierarchy of switches in WSC.

    Warehouse-Scale Computers (WSC)

    • A WSC is considered the foundation of Internet services used by billions of people daily, acting as one giant machine.
    • It costs hundreds of millions of dollars for building, electrical and cooling infrastructure, servers, and networking equipment.
    • The target is providing information technology for the world, instead of high-performance computing - HPC only for scientists and engineers.

    Main Goals

    • The main goals of WSC include the cost-performance relation, energy efficiency, and dependability.
    • Cost-performance ratio: the work done per dollar is critical, with small cost reductions saving millions of dollars.
    • Energy efficiency: the work done per joule is critical, with concerns about power and cooling.
    • Dependability: achieved via redundancy, with a target of 99.99% availability, i.e., less than one hour of downtime per year.

    Networking Hierarchy

    • The network overhead considerably increases latency, and the network collapses differences in bandwidth.
    • Some WSCs need more than one array, adding an extra level in the networking hierarchy.
    • Regular "Layer 3" routers connect arrays together and to the Internet, with the core router operating in the Internet backbone.

    Memory Hierarchy

    • Each computing node contains:
      • 16 GiB DRAM
      • 128 GiB Flash
      • 2,048 GiB Disk
      • 1 Gbit/s Ethernet port
    • The rack holds 80 nodes, and the array has 30 racks.

    Latency and Bandwidth

    • Latency numbers:
      • DRAM: 0.1 µs (local node), 300 µs (rack), 500 µs (array)
      • Flash: 100 µs (local node), 400 µs (rack), 600 µs (array)
      • Disk: 10,000 µs (local node), 11,000 µs (rack), 12,000 µs (array)
    • Bandwidth numbers:
      • DRAM: 20,000 MiB/s (local node)
      • Flash: 1,000 MiB/s (local node), 100 MiB/s (rack), 10 MiB/s (array)
      • Disk: 200 MiB/s (local node)

    Power Utilization Effectiveness (PUE)

    • PUE is a metric to evaluate the efficiency of a WSC, given by Eq. (10.1).

    Workload: MapReduce

    • MapReduce is a popular framework for batch processing in WSC.
    • The map part applies a programmer-supplied function to each input, producing an intermediate result of key-value pairs.
    • The reduce part collects the output and collapses it using another programmer-defined function.
    • Example: words and documents indexing using MapReduce.

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    Description

    This quiz covers the numbers of latency in microseconds for different storage devices and network components. It includes the overhead of networking software, switch, and array switch hardware/software.

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