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QoS in Clustered Environments. Ahmad Faraj Faraj@cs.fsu.edu. Overview. Introduction Routing Mechanisms Approaches to QoS in Clusters Conclusions. Introduction. Networked applications inject different mixes of traffic in the network. Some classes of traffic require QoS treatment.

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QoS in Clustered Environments

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QoS in Clustered Environments

Ahmad Faraj

Faraj@cs.fsu.edu


Overview

  • Introduction

  • Routing Mechanisms

  • Approaches to QoS in Clusters

  • Conclusions


Introduction

  • Networked applications inject different mixes of traffic in the network.

  • Some classes of traffic require QoS treatment.

  • Traditional best-effort model cannot handle such QoS demand


Cluster Systems

  • Cost effective for high performance environment

    • Used in scientific computing, web servers, multimedia servers, commercial applications

  • Two switch/router design to build clusters:

    • Virtual cut-through (includes wormhole)

    • Packet switching


  • Virtual cut-through

    • Designed for multicomputers

    • Offers low latency and high bandwidth for best-effort traffic

    • To support QoS, must modify the switch

  • Packet switching: Ex. ATM

    • QoS support available for real time traffic

    • Can not handle best-effort efficiently due to high message latency (compared to virtual cut-through)


Bottom Line

  • Must reevaluate and optimize the network architecture to handle both types of traffic, best-effort and QoS, in clustered environments.


Routing mechanisms

  • Virtual cut-through & wormhole:

    • Packets is composed of small flits

    • A header flit leads and middle flits follow in a pipelined manner

    • Once header is received at the switch, it is forwarded to the outgoing channel

    • If channel is busy:

      • Virtual cut-through: store whole packet at the switch

      • Wormhole: store a few flits across several switches

    • Each worm carries routing info:

      • Can support multiple connections on a virtual channel


  • Virtual channels and physical links are shared resources

  • Real time application require predictable scheduling of such resources

  • Must enforce a global priority ordering among competing messages

  • Example of limitation:

    • assume a message with highest priority p at time t occupies a virtual channel

    • If another message arrives with p` > p, it must wait till p message release the channel

    • Limitation: with v virtual channels, can only enforce v level priority ordering, although message priority levels may be more


  • Pipelined circuit switching:

    • Similar to wormhole in terms of flits

    • Connection oriented: header flit tries to reserve the path first

    • If path is blocked, must backtrack and find another

    • Middle flits follow if path is available

    • If not, a connection is dropped


Classification of QoS Approaches

  • Virtual circuits

    • Paths are virtualized & controlled locally at switches

    • Based on QoS parameters, a separate VC is created where buffer and link bandwidth are reserved

    • To guarantee end-to-end QoS, switches are responsible to schedule packets

    • Flexible in terms of providing QoS

    • Large buffer, complex scheduling algorithms

    • Increases hardware complexity of switches


  • Physical circuits:

    • No virtualization  simpler design of switches

    • Link arbitration policy is used to implement some control on delay and bandwidth

    • Policy merges multiple streams at a physical link

    • This causes coupling between streams sharing the link

    • A QoS stream depends on other traffic flows sharing the link

    • inflexible to manage network resources to support QoS


  • Global Scheduling:

    • Complexity is moved out from switches to network interfaces

    • Switches are much simpler and fast

    • Network interfaces augmented with special hardware responsible for:

      • Routing

      • Timing packets injected into the network

      • Negotiation of shared resources with other NICs

    • Relatively new approach

      • Issues of practicality, scalability, cost of synchronization, and scheduling are open subjects to discuss


QoS in Packet Switching Networks

  • Rotating Combined Queuing RCQ:

    • Low cost queuing & scheduling algorithm

    • Provides QoS support in multicomputers and point to point LANs

    • Switch model supports:

      • Connection based switching: decide routing and reserve bandwidth at connection setup time

      • Output queuing: packets arriving simultaneously at an output link are queued and scheduled for transmission (reduces head-of-line blocking)


  • RCQ:

    • Reduce traffic cost by combing multiple decoupled queues

      • Combine queues allocated for a few connection with small traffic and large delay bounds

    • Support best-effort traffic using multiple FIFO queues per port

    • Uses frame-based scheduling

      • Connection is allocated number of packet slots in a period of time

    • Extra queues enable sender to send at higher rate more than reserved

    • Queuing structure allows real time traffic to bypass best-effort traffic

    • Permits best-effort traffic to utilize unused bandwidth by other connections


How Does It Work?

  • Enqueue arriving packets into one of the queue pointed by the current input queue pointer for a specific connection

  • If maximum number of allowed packets per connection is reached in the current queue, then move the pointer to the next queue

  • For each idle cycle of the output channel, send any pending packets

  • Else if there are no packets to transmit, move output queue pointer to the next queue and do the same

  • Idle connections change their input queue pointer to always point to the current output queue pointer

  • If QoS packet arrives, it is enqueued in the queue that is also pointed by the current output queue pointer, incurring a delay of the packets in front of it in the queue

  • Guarantees a worst-case delay of one frame time

  • End-to-end worst-case delay is bounded by the distance multiplied by the frame time


QoS in PCS Networks

  • Wormhole switching may suffer from message blockage while PCS does not

  • PCS is connection oriented

    • Can reserve bandwidth at connection setup

  • Requires a VC per connection

    • Thus, it demands for large number of virtual channels per PC for high link bandwidth

    • Switch hardware must support VCs ≥ Max simultaneous streams in the network

    • Else, new connection are not guaranteed

    • Streams may be dropped


  • To support QoS in PCS, use a preemption protocol for real time traffic

  • Higher priority messages can preempt lower priority message on a virtual channel

  • Blockage only occurs for low priority message competing with a high priority one


QoS in Wormhole-Switched Networks

  • SuperNet project:

    • QoS using a separate subnet:

      • Costly in terms of number host interfaces

    • Imposing synchronous structure over asynchronous network

      • Large overhead for small messages

      • Costly in terms of number host interfaces

    • Virtual Channels:

      • Better than the two above

      • Requires complex scheduling and buffer space at switches


Continued

  • MediaWorm:

    • Wormhole based router to support QoS

    • Supports two traffic: best-effort and QoS

    • Unlike FIFO, uses rate-based algorithm called Virtual Clock to schedule network resources

    • Virtual Clock regulates bandwidth of each connection by assigning virtual clock value vtick that ticks at each packet arrival

    • High bandwidth is represented by smaller vtick


  • Example:

    • Message requires 50K flits/s

    • Header flit carries a vtick set to 1/50K

    • Header flit asks this value at all routers it passes till it reaches the destination

    • Thus, no need for explicit connection setup

    • For best-effort traffic, vtick is set to ∞ since it has the maximum slack

    • Virtual Clock algorithm can improve QoS delivered to real time traffic compared to FIFO


  • MediaWorm can achieve as good performance as a PCS router without dropping any connections

  • PCS is expected to perform better since it is connection oriented. Yet, dropping of connections occurs


Conclusions

  • For cluster systems, wormhole-like routings seem to be popular

  • To support QoS is a challenge

  • Several approaches are overviewed

  • Use of virtual channels with a preemptive protocol to enforce priority among network traffic is a promising technique


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