Presented by Rukmini and Diksha Chauhan

Virginia Tech2nd May, 2007

Movement-Based Checkpointing and Logging for Recovery in Mobile Computing

Systems

Sapna E.George, Ing-Ray Chen & Ying Jin

AgendaRelated workMobile Computing SystemProposed Movement-based Checkpointing

and LoggingRecovery SchemesPerformance AnalysisConclusion

Properties of Mobile ComputingInherent properties

Host MobilityDisconnectionsWireless bandwidth LimitationBattery LifeStorageHardware failureSoftware Failure

MotivationPropose an efficient failure recovery scheme

Distributed SystemsFault-tolerance schemes

LoggingCheckpointingRollback Recovery

Definition Domino Effect

inter-process dependencies - cascading rollbacks

Asynchronous Recovery

Related WorkAcharya et al. in [1] describes a distributed

uncoordinatedcheckpointing scheme, where multiple MHs can arrive at a global consistent checkpoint without coordination messages.

The paper does not describe how failure recovery is achieved nor does it address the issue of recovery information management in the face of MH movement.

Underlying Model

Basic DefinitionsMobile

Mobile Host(MH)

Mobile support Systems(MSS)Infrastructue machinesHigh speed Static wired n/w

Basic DefinitionsCell

Local MSS

Communication Between MH and MSS-Constraints

Process of Communication between MH’sTwo one-hop wireless transmissionsArbitrary hops

Basic DefinitionsHandoff

Instantaneously ProcessMH crosses a cell boundary

MH disconnect(MSS1) voluntarily from network to conserve power and reconnect(MSS2) at a later time.

MH sends the ID of MSS1 to the new MSS2-initiates handoff procedures.

Processes and StatesThree States

Normal Execution Application-related Computation Sending or receiving messages Logging

SaveRecovery

Write EventMessage received from other MH or serverUser Input or Local Computation

Movement-Based Checkpointing & Message LoggingCheckpoints after a certain number of host migrations

across cells rather than periodically.

Recovery SchemeCombines independent checkpointing and optimistic

message logging enabling asynchronous recovery of a MH upon failure.

Application recovery mechanisms - optimize recovery cost (failure-free operational cost), recovery time Storage requirements for recovery related information

Movement-Based Checkpointing & Message LoggingScheme uses distance or number of handoffs

Parameter to trigger information consolidation MH crosses a distance threshold from the location

of the latest checkpoint, the recovery information is collected and transferred to the MH’s local MSS.

Recovery protocol – proactively controls no. of checkpoints and logs by movement-based checkpointing strategyadditional overhead of unnecessary checkpoints

and log consolidation during failure-free operation is avoided.

Checkpointing & Message Logging

m is a f (user’s mobility rate, the failure rate and log arrival rate ) –Adaptation to user and Application behaviour

Movement-based Checkpointing and LoggingMH –Stored variables

cp_seq -stores the sequence number of the latest checkpoint and

cp_loc -stores the ID of the MSS that has recorded the latest checkpoint.

MSScp- Latest MSS Handoff_counter to 0MSSlogs (log_set) - IDs of MSSs

ActivityCheckpointsLogging

Independent RecoveryIndependent – without Coordination with other

hosts.Recovery process

MH sends MSS cp_seq, cp_loc and log_setMSS initiates (requests) data collectionMSS compiles

Logs into list ordered by time Checkpoints

Once recovery is completed successfully, a checkpoint of the current state is taken and sent to the MSS and the local variables are reset.

Storage Management at MSS MH’s Disk

Unstable Limited

MSS’s Disk Stable storage Considerably large

storage at MSSs –depleted1. Temporarily halt –Perform Garbage Collection2. Alternative Storage3. Deleting outdated recovery Information

SPN Model Parameters

SPN Model

Transition firing rate

Checkpoint Rate of MH

During checkpointing:

(a) MH takes a snapshot of its current state

(b) MH sends the checkpoint to the current MSS through the wireless channel.

(c) The MSS stores it in its stable storage.

 

 

 

where is the time required to transmit a checkpoint through wireless link.

 

Transition Firing RateRecovery Rate of MH i.e, inverse of Recovery Time

Recovery Time includes :

(a) time to send recovery information requests to the MSSs storing the latest checkpoint and all logs since the latest checkpoint

 (b) time to transmit the latest checkpoint from the MSS where it is stored (MSScp) to the MSS in which the MH has recovered (MSSrec) through the wired network and through the wireless channel to the MH 

Transition Firing Rate

c) time to transmit all the logs from the respective MSSs where they are located (MSSlogs) to the MSSrec through the wired network and through the wireless channel to the MH and

 

(d) time to rollback to the last checkpoint and apply all the logs at

the MH.

  

Variables & Represents the number of MSSs storing logs.At most its value is the number of handoffs before failure, i.e. i

Represents average hop count between MSScp and MSSrec.

Recovery TimeTime Spent on Recovery Requests:

Time spent on transmitting the latest checkpoint to MH:

Recovery Time contd..Time spent to transmit the logs to MH:

where n is the number of log entries since the last checkpointTime spent to rollback to the last checkpoint and apply the logs:

Total Recovery time after i movements:

Recovery Cost per failureThe SPN model’s underlying Markov model has 2M+1 states. The average recovery time per failure is given by:

The total failure-free operations cost(or time spent on checkpointing and logging before failure) is given by:

where denotes the number of checkpoints before failure and denotes the number of log entries before failure.

Recovery Cost per failure contd…Total Cost of Recovery per failure is the weighted sum of the

average recovery time and the total time spent on the checkpointing and logging per failure and is given by:

where w1 and w2 are the weights associated with recovery time and failure-free operation cost.

This paper uses w1 = w2 = 0.5 to account for the situation where

is equally proportional to and

Recovery Probability

The recovery probability is defined as the probability that recovery time is less than or equal to T

Results and AnalysisThe SPN model was implemented and analyzed using the SPNP s/w

The following parameter values were kept constant in all the runs.size of a log entry is 50B, size of a checkpoint is 2000B, bandwidth of the wired network is 2Mbps, ratio of bandwidth of wireless to wired network (r) is 0.1, Telog is 0.0001s. Tlog_w is 0.002s and Tckp_w is 0.08s.

Model parameters such as mobility rate, log arrival rate, failure rate, and movement threshold were varied across runs

Results and Analysis contd…

Recovery Probability vs. Recovery Time.

Results and Analysis contd…

Recovery Probability vs. Log Arrival Rate.

Results and Analysis contd…

Recovery Probability vs. Failure Rate.

Results and Analysis contd…

Recovery Probability vs. Movement Threshold.

Results and Analysis contd…

Recovery Time vs. Movement Threshold.

Results and Analysis contd…

Determining Optimal Movement Threshold that minimizes Recovery Cost Per Failure.

ApplicabilityResults can be applied in the following manner:

Build a Table at static time covering possible parameter values of the mobility rate and failure rate of the MH and log arrival rate of the mobile applications

List the optimal M value to minimize the recovery cost per failure for each parameter set.

Select optimal M dynamically at runtime based on the measured rates to minimize the recovery cost per failure.

Summary Implemented movement-based checkpointing and logging scheme which

checkpoints after M movements (mobility handoffs) as compared to current approaches where checkpoints are taken periodically.

A performance model developed based on stochastic Petri nets to identify the optimal M, given the failure, mobility and log arrival rates, to minimize the cost of recovery per failure.

The results of performance analysis and the sensitivity of recoverability to the various parameters were shown

Future WorkTo analyze and compare the proposed approach to existing

approaches, in terms of the gain achieved over the use of constant periodic checkpointing.

To extend the proposed work to MIPv6 environments.

QUESTIONS ??