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Reasoning about Hardware and Software Memory Models. Abhik Roychoudhury School of Computing National University of Singapore. Salient Points. Memory Model – What ? Hardware and Software Memory Models – Why ? Memory Model: Formal Executable Specs. Comparing Memory Models for efficiency - PowerPoint PPT Presentation
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ICFEM 2002, Shanghai
Reasoning about Hardware and Software Memory Models
Abhik Roychoudhury
School of Computing
National University of Singapore
ICFEM 2002, Shanghai
Salient Points
Memory Model – What ? Hardware and Software Memory Models – Why ? Memory Model: Formal Executable Specs. Comparing Memory Models for efficiency MM comparison for reasoning about programs Concluding Remarks
ICFEM 2002, Shanghai
Memory Model
Order in which memory operations (read / write) appear to execute to the programmer.
For uni-processors, the MM is s.t.– Memory Read / Write appear to execute one at a
time (atomic)– They execute in the program order.
ICFEM 2002, Shanghai
Memory Model
Traditionally used to describe allowed behaviors of shared memory multiprocessors.
….Proc 1 Proc 2 Proc n
SHARED MEMORY
ICFEM 2002, Shanghai
Hardware Memory Models
Initially Data1 = 0 Data2 = 0 Flag = 0
Data1 := 42;
Data2 := Data1
Flag := 1;
||While (Flag != 1) {};
Register := Data2
Programmer’s intuition:
Register should be 42, not 0
SEQUENTIAL CONSISTENCY
ICFEM 2002, Shanghai
Sequential Consistency
Simple extension of Uni-processor model. Each processor executes in program order. Operations appear to execute one at a time in some
sequential order. Other relaxed Memory models also possible.
– Certain (not all ) operations in a processor may be executed out-of-order.
ICFEM 2002, Shanghai
Relaxed Memory Models
Initially Data1 = 0 Data2 = 0 Flag = 0
Data1 := 42;
Data2 := Data1;
Flag := 1;
While (Flag != 1) {};
Register := Data2
Data1 := 42; Data2 := Data1 and Flag := 1 can be re-ordered e.g. SPARC PSO
Register = 0 is also possible
ICFEM 2002, Shanghai
Multithreaded programming
Multithreaded programming on multiprocessors is difficult due to the hardware memory model.
The execution platform supports behaviors other than Sequential Consistency.
Shared memory parallelism is becoming important because of Symmetric Multiprocessors (SMP).
Multithreaded program popularized by Java. Your multithreaded Java program may run diff.
threads on diff. processors without you knowing it !
ICFEM 2002, Shanghai
Java Multithreading
Can structure different parts of the program as diff. threads e.g. the User Interface as separate thread.
Threads communicate via shared variables. Threads can run on uni- or multi-processors. Semantics called the Java Memory Model (JMM) JMM describes all possible allowed behaviors of a
pgm. on any implementation (uni- / multi-processor). Supports locking of shared variables via
synchronized statements.
ICFEM 2002, Shanghai
Java Memory Model
Weaker than S.C. (to allow compiler/hardware opt.) Each shared variable has
– Master copy– Local copy in each thread
Threads read/write local/master copies on actions. Imposes ordering constraints on actions. But does
not impose total order on actions in a thread. Being considered for revision. Future JMMs are also
bound to be weaker than S.C.
ICFEM 2002, Shanghai
Compiler
Multithreaded Java Pgm
JVM
(Introduce barriers ?)
Hardware MM
(Abstraction of multiprocessor)
Bytecode
Hardware Instructions
…
Should respect JMM
ICFEM 2002, Shanghai
Hardware and Software MM
MM1 > MM2
– Behaviors allowed by MM1 Behaviors allowed by MM2
– Memory barriers introduced by JMM to prevent certain behaviors if the hardware MM is too weak.
If we prove: JMM > Hardware MM– JVM can be tuned to avoid inserting any memory barrier.
If we have: JMM > HW MM1 > HW MM2
– Certain multithreaded Java programs can behave differently on different multiprocessor platforms !!
Need to formally specify and analyze MMs
ICFEM 2002, Shanghai
Developing Exec. Spec. – Hard !
JMM imposes ordering constraints on read/write actions of shared variables.
The ordering constraints are given informally as set of rules which must never be violated.
Past work: Operational exec. spec. of JMM [ICSE02]– Th1 || Th2 || … || Thn || Memory (Asynchronous Composition)
– Thi executes read/write actions which are modeled as guarded commands
Executable Spec. of Hardware MMs (e.g. TSO/PSO from Sun SPARC) developed earlier [Dill et. al. 93]
ICFEM 2002, Shanghai
Total Store Order (TSO)
One of the hardware memory models in Sun SPARC. All instructions complete in program order, Except can complete as
Write completion can be delayed. store u; store v cannot be re-ordered. store u; load u cannot be re-ordered.
Store u;
Load vLoad v;Store u
ICFEM 2002, Shanghai
Comparing JMM and TSOOnly re-ordering allowed is store load
Initially u = v = 0
Store u,1
Load v, reg1
Store v,1
Load u, reg2
Seq. Consistency: reg1 = 1 reg2 = 1
TSO : reg1 = 0 or 1, reg2 = 0 or 1
JMM > TSO (Proved by JMM checker)
JVM can execute on TSO without barriers.
ICFEM 2002, Shanghai
Comparing Memory Models
1. Develop formal executable specifications of software and hardware memory models Ms, Mh
2. Select terminating test programs {P1,…,Pk} which exploit the re-orderings allowed by Mh
– Based on informal understanding of Mh
– Typically lifted from multiprocessor architecture manuals.
3. Verify that the set of selected test programs {P1,…,Pk} is complete w.r.t. the re-orderings of Mh
– Formal reasoning step. Automated if the test programs are selected methodically.
ICFEM 2002, Shanghai
Comparing Memory Models
4. Perform reachable state space exploration (as in Model Checking) of test programs {P1,…,Pk} on executable models Mh and Ms
– Use formal executable specs of Mh and Ms
– Generate all possible behaviors of test pgms on Mh and Ms
– Automated formal reasoning step
5. Check whether the re-orderings of Mh exposed by programs {P1,…,Pk} are also allowed by Ms
Combines formal and informal reasoning.
ICFEM 2002, Shanghai
Another Application
Unsynchronized programs: Multithreaded programs where threads access shared variables without locks.
Allow more behaviors under relaxed hardware MMs. Use executable specification of JMM to find all
possible behaviors. If any of these is “unexpected” Can be manifested
in certain (not all) Multiprocessor platforms !! Verify presence / absence of this behavior by using
executable versions of Hardware memory models.
ICFEM 2002, Shanghai
Unsynchronized Programs
Allowed behaviors differ on different HW Mem. Models.
Used in low-level libraries which are executed often.
Data1 := 42;
Data2 := Data1
Flag := 1;
||While (Flag != 1) {};
Register := Data2
Absence of locks allows write re-orderings in a thread to be visible in other threads.
ICFEM 2002, Shanghai
Idiom for lazy instantiation of a singleton – Double checked Locking idiom.
Check whether garbage data-fields can be returned
If (inst == null)
synchronized(Singleton.class){
if (inst == null) {
inst = new Singleton();
}
Return inst;
Case Study
Run by
each thread
ICFEM 2002, Shanghai
Results from DC Locking
Allowed behaviors from JMM checker show– Incompletely instantiated object can be returned.– Due to re-ordering of writes within constructor.
Allowed behaviors from TSO checker show– Incompletely instantiated obj. cannot be returned.– Proved by State Space Exploration in 0.01 s
Other hardware MM (such as Partial Store Order) can show this behavior : proved by PSO checker.
ICFEM 2002, Shanghai
Concluding Remarks
Formal understanding of MMs was necessary for low-level multiprocessor code.
Advent of Java multithreading and SMP requires us to understand hardware MMs for running low-level multithreaded Java code.
Language level memory model in Java (JMM) necessitates comparison of JMM with hardware MMs
Accomplished by formal Executable Specifications and formal analysis techniques (Model Checking).