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Avoiding Scheduler Subversion using Scheduler-Cooperative Locks
Yuvraj Patel, Leon Yang, Leo Arulraj, Andrea Arpaci-Dusseau, Remzi Arpaci-Dusseau, Michael Swift University of Wisconsin-Madison
Avoiding Scheduler Subversion using Scheduler - Cooperative Locks - - PowerPoint PPT Presentation
Avoiding Scheduler Subversion using Scheduler - Cooperative Locks - - PowerPoint PPT Presentation
Avoiding Scheduler Subversion using Scheduler - Cooperative Locks Yuvraj Patel , Leon Yang , Leo Arulraj , Andrea Arpaci - Dusseau , Remzi Arpaci - Dusseau , Michael Swift University of Wisconsin - Madison Once upon a time .... Processes P0 P1
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P0 P1
Accessing a kernel service Default priority
1 Processes
Once upon a time ....
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P0 P1
Accessing a kernel service Default priority
1 Processes
Once upon a time ....
*Linux CFS Scheduler
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Schedulers are important for resource sharing
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Let us now introduce a lock....
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A ticket lock reduces wait-time and avoids starvation
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If P0 holds the lock twice as long as P1
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If P0 holds the lock twice as long as P1
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P1 P0
Time
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If P0 holds the lock twice as long as P1
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P1 P0
Time
While in critical section, P0 uses twice much CPU as P1
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If P0 holds the lock twice as long as P1
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P1 P0
Time
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If P0 holds the lock twice as long as P1
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P1 P0
Time
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If P0 holds the lock twice as long as P1
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P1 P0
Time
Every time P0 enters critical section, it uses twice as much CPU as P1
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Despite 50:50 allocation, P0 uses twice as much CPU as P1's
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Locks determine the CPU allocation instead of the scheduler
7 Scheduler Subversion
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When does scheduler subversion happen?
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Two reasons
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Different critical section lengths
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Different critical section lengths
Thread dwelling longer in critical section becomes dominant user of CPU
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Majority locked run time
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Majority locked run time
Lock–Unlock: Is That All? A Pragmatic Analysis of Locking in Software
- Systems. ACM Trans. Comput. Syst.,36(1), March 2019.
Remote Core Locking: Migrating Critical-Section Execution to Improve the Performance of Multithreaded Applications. USENIX ATC 2012 Understanding Manycore Scalability of File Systems, USENIX ATC 2016 Non-scalable locks are dangerous. Linux Symposium, 2012 1. 2. 3. 4.
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Threads have to spend a lot
- f time in the critical section
and it is often a case
1, 2, 3, 4
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Every concurrent system can be prone to scheduler subversion
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Remedy
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Track lock usage of users
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Three design components
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Track lock usage of users
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P1 P0 x Track the time spent by each process in critical section
Example
Three design components
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Track lock usage of users
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P1 P0 x 2x Track the time spent by each process in critical section
Example
Three design components
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P1 P0 x 2x
Track lock usage of users Penalize dominant users Example
Three design components
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Track lock usage of users Penalize dominant users
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Penalize while acquiring the lock again P1 P0 x 2x
Example
Three design components
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Track lock usage of users Penalize dominant users
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P1 P0 x 2x x
Example
Three design components
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P1 P0 x 2x x
Track lock usage of users Penalize dominant users Provide appropriate
- pportunity to each user
Example
Three design components
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P1 P0 x 2x x x Penalizing P0 creates enough opportunity to let P1 acquire the lock multiple times
Track lock usage of users Penalize dominant users Provide appropriate
- pportunity to each user
Example
Three design components
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P1 P0 x 2x x x Opportunity depends on the scheduling goals
Track lock usage of users Penalize dominant users Provide appropriate
- pportunity to each user
Example
Three design components
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Scheduler-Cooperative Locks (SCL)
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Ticket Lock SCL
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Conclusion
We introduce the scheduler subversion problem We introduce scheduler- cooperative locks (SCL) to mitigate scheduler subversion
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