QoS in Clustered Environments

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QoS in Clustered Environments

• Ahmad Faraj
• Faraj_at_cs.fsu.edu

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Overview

• Introduction
• Routing Mechanisms
• Approaches to QoS in Clusters
• Conclusions

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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

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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

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• 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)

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Bottom Line

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

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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

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• 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 gt 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

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• 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

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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

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• 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

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• 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

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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)

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• 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

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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

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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

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• 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

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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

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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

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• 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 8 since
  it has the maximum slack
• Virtual Clock algorithm can improve QoS delivered
  to real time traffic compared to FIFO

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• 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

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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