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