By: Swetha Kendyala Software Rejuvenation

When software applications execute continuously for long periods of time, the processes corresponding to the software in execution age or slowly degrades with respect to the effective usage of their system resources. Process aging will affect the performance and eventually cause the application to fail. Introduction

What is Software Rejuvenation? The act of gracefully terminating an application and immediately restarting Goal: Prevents unexpected error termination by terminating the program before it suffers an error

Intended Use Software rejuvenation is primarily indicated for servers where applications are intended to run indefinitely without failure

Why do applications fail? Process Aging: gradual degradation of application performance, over time, that may lead to premature program termination

Causes Memory leaks Unreleased file locks File descriptor leaking Etc.

Software Rejuvenation Periodic preemptive rollback of continuously running applications to prevent failures in the future

Transition Model For SW without Rejuvenation Transition Model For SW with Rejuvenation

Downtime and cost without rejuvenation P f = Downtime w/o r (L) = P f * L Cost w/o r (L) = P f * L * c f

Downtime and cost with rejuvenation P p = P f = P r = P 0 = Downtime w r (L) = (P f + P r ) * L Cost w r (L) = (P f * c f + P r * c r ) * L

Thresholds - Goal Goal is to stay in S 0 for the longest amount of time

Thresholds cont. To see how r 4 affects downtime and cost, lets differentiate the previous equations with respect to r 4

Thresholds cont. Downtime: If r 3 is dominant, the derivative becomes negative and downtime decreases when r 4 increases thus rejuvenate at state S p If r 3 is small, slow recovery from S R, downtime increases as r 4 increases

Thresholds cont. Cost = When c r is dominant, cost increases as r 4 increases, implies no rejuvenation benefit When c r is small, cost decreases as r 4 increases

Thresholds cont. Overall, costs need to be calculated for individual programs For best results: perform rejuvenation at state S P (r 4 = ∞) or don’t perform rejuvenation (r 4 = 0)

Example 1 MTBF = 12 months; = 1/(12*30*24) Takes 30 min to recover from unexpected error; r 1 = 2 Base Longevity is seven days; r 2 =1/(7*24) If rejuvenation is performed, mean repair time after rejuvenation is 20 minutes; r 3 = 3 Ave. Cost of unscheduled downtime due to failure, c f, is $1,000/hour Ave. Cost of scheduled downtime during rejuvenation, c r, is $40/hour

Software Rejuvenation No rejuvenation (r 4 = 0) Once Every three Week r 4 = 1/(2*7*24) Once Every Two Weeks r 4 =1/(1*7*24) Hours of Downtime Cost of Downtime

Software Rejuvenation No rejuvenation (r 4 = 0) Once Every month r 4 = 1/(20*24) Once Every Two Weeks r 4 =1/(4*24) Hours of Downtime Cost of Downtime 3.6k2.48k1.11k

Example 2 MTBF = 3 months; = 1/(3*30*24) Takes 30 min to recover from unexpected error; r 1 = 2 Base Longevity is three days; r 2 =1/(3*24) If rejuvenation is performed, mean repair time after rejuvenation is 10 minutes; r 3 = 6 Ave. Cost of unscheduled downtime due to failure, c f, is $5,000/hour Ave. Cost of scheduled downtime during rejuvenation, c r, is $5/hour

Software Rejuvenation No rejuvenation (r 4 = 0) Once Every three Week r 4 = 1/(11*24) Once Every Two Weeks r 4 =1/(4*24) Hours of Downtime Cost of Downtime

Example 3 MTBF = 3 months; = 1/(3*30*24) Takes 2 min to recover from unexpected error; r 1 = 0.5 Base Longevity is 10 days; r 2 =1/(10*24) If rejuvenation is performed, mean repair time after rejuvenation is 10 minutes; r 3 = 6 Ave. Cost of unscheduled downtime due to failure, c f, is $5,000/hour Ave. Cost of scheduled downtime during rejuvenation, c r, is $5/hour

Implementation Implementation of Software Rejuvenation is fairly easy. Cron Jobs can be set to restart the application at various intervals watchd can be used to detect if applications have failed and restart them

Real World Examples BILL-DATS II Collector –Billing collection system used by AT&T long- distance network –Set to rejuvenate after 1 week –Hasn’t prematurely failed after several year

“S” Scientific Speech synthesis system Long running scientific application Used to process several hundred sentences over the course of many days Found to fail after 100 sentences Rejuvenates after 15

Conclusions: Decision to use Software Rejuvenation depends on predetermined failure rates and associated costs. r 4 = 0, No rejuvenation r 4 = ∞, Rejuvenation

Questions???