Flow Java: Declarative
Concurrency for Java
Frej Drejhammar
SICS - Swedish Institute of Computer Science and
KTH - Royal Institute of Technology, Stockholm, Sweden
Christian Schulte (KTH), Per Brand(SICS), and Seif Haridi (KTH, SICS)
Flow Java: Declarative Concurrency for Java – p.1/29
Constraints for
Concurrency
Constraints
describe data structures
used for control (in contrast to Prolog)
Logic variables as dataflow variables
unconstrained
suspension
constrained
resumption
automatic synchronization
Well established idea
entailment
resumption [Maher, 87]
CCP [Saraswat, 90]
Flow Java: Declarative Concurrency for Java – p.2/29
Technology Transfer
Automatic synchronization for Java
starting point: logic variables
Constraint part limited (so far)
token equality
no structural equality
Why here: see where ideas emerging from
constraint programming can have impact
Flow Java: Declarative Concurrency for Java – p.3/29
Goals
Automatic synchronization
Flow Java: Declarative Concurrency for Java – p.4/29
Goals
Automatic synchronization
single assignment variables (logic
variables)
Flow Java: Declarative Concurrency for Java – p.4/29
Goals
Automatic synchronization
single assignment variables (logic
variables)
futures (read only variants) for security
Flow Java: Declarative Concurrency for Java – p.4/29
Goals
Automatic synchronization
single assignment variables (logic
variables)
futures (read only variants) for security
Conservative extension
Flow Java: Declarative Concurrency for Java – p.4/29
Goals
Automatic synchronization
single assignment variables (logic
variables)
futures (read only variants) for security
Conservative extension
concurrency model
Flow Java: Declarative Concurrency for Java – p.4/29
Goals
Automatic synchronization
single assignment variables (logic
variables)
futures (read only variants) for security
Conservative extension
concurrency model
model of object-orientation
Flow Java: Declarative Concurrency for Java – p.4/29
Goals
Automatic synchronization
single assignment variables (logic
variables)
futures (read only variants) for security
Conservative extension
concurrency model
model of object-orientation
parameter passing
Flow Java: Declarative Concurrency for Java – p.4/29
Overview
Flow Java
Single assignment variables
Binding
Synchronization
Futures
Aliasing
Implementation
Runtime
Compilation
Optimizations
Evaluation
Related Work
Conclusion
Flow Java: Declarative Concurrency for Java – p.5/29
Single Assignment
Variables
single Object s;
Introduce s as a single assignment variable
(SAV) of type Object
Flow Java: Declarative Concurrency for Java – p.6/29
Single Assignment
Variables
single Object s;
Introduce s as a single assignment variable
(SAV) of type Object
SAV typed
Flow Java: Declarative Concurrency for Java – p.6/29
Single Assignment
Variables
single Object s;
Introduce s as a single assignment variable
(SAV) of type Object
SAV typed
place holder for object
Flow Java: Declarative Concurrency for Java – p.6/29
Single Assignment
Variables
single Object s;
Introduce s as a single assignment variable
(SAV) of type Object
SAV typed
place holder for object
SAV initially unbound
Flow Java: Declarative Concurrency for Java – p.6/29
Binding
SAV of type t bound to objects of type t
(more detail in paper)
Flow Java: Declarative Concurrency for Java – p.7/29
Binding
SAV of type t bound to objects of type t
(more detail in paper)
Binding @=
Object o = new Object();
s @= o;
Flow Java: Declarative Concurrency for Java – p.7/29
Binding
SAV of type t bound to objects of type t
(more detail in paper)
Binding @=
Object o = new Object();
s @= o;
s equivalent to o in any subsequent
computation
Flow Java: Declarative Concurrency for Java – p.7/29
Binding
SAV of type t bound to objects of type t
(more detail in paper)
Binding @=
Object o = new Object();
s @= o;
s equivalent to o in any subsequent
computation
s bound (determined )
Flow Java: Declarative Concurrency for Java – p.7/29
Binding
SAV of type t bound to objects of type t
(more detail in paper)
Binding @=
Object o = new Object();
s @= o;
s equivalent to o in any subsequent
computation
s bound (determined )
Already bound: exception, if bound to
different object
Flow Java: Declarative Concurrency for Java – p.7/29
Synchronization
Accessing unbound SAV suspends executing
thread
Flow Java: Declarative Concurrency for Java – p.8/29
Synchronization
Accessing unbound SAV suspends executing
thread
field access
Flow Java: Declarative Concurrency for Java – p.8/29
Synchronization
Accessing unbound SAV suspends executing
thread
field access
method invocation
Flow Java: Declarative Concurrency for Java – p.8/29
Synchronization
Accessing unbound SAV suspends executing
thread
field access
method invocation
Automatic resumption when bound
Flow Java: Declarative Concurrency for Java – p.8/29
Restricting SAV to
Objects
Ease of implementation
Flow Java: Declarative Concurrency for Java – p.9/29
Restricting SAV to
Objects
Ease of implementation
Follows Java philosophy
Flow Java: Declarative Concurrency for Java – p.9/29
Restricting SAV to
Objects
Ease of implementation
Follows Java philosophy
No fundamental restriction
Flow Java: Declarative Concurrency for Java – p.9/29
Restricting SAV to
Objects
Ease of implementation
Follows Java philosophy
No fundamental restriction
int
Integer
Flow Java: Declarative Concurrency for Java – p.9/29
Example: Spawning
a Computation
class Spawn implements Runnable {
private single Object result;
private Spawn(single Object r) {
result = r;
}
public void run() {
result @= computation();
}
...
single Object r;
new Thread(new Spawn(r)).start();
System.out.println(r);
}
Flow Java: Declarative Concurrency for Java – p.10/29
Flaw in Spawn
Thread using r can compromise abstraction:
unintentionally / maliciously bind r
Flow Java: Declarative Concurrency for Java – p.11/29
Flaw in Spawn
Thread using r can compromise abstraction:
unintentionally / maliciously bind r
Solution: Use futures
read-only variant of SAVs
Flow Java: Declarative Concurrency for Java – p.11/29
Futures
Each SAV x has associated future f,
invariants:
Flow Java: Declarative Concurrency for Java – p.12/29
Futures
Each SAV x has associated future f,
invariants:
f bound
x bound
f bound to o
x bound to o
x of type single t
f of type t
Flow Java: Declarative Concurrency for Java – p.12/29
Futures
Each SAV x has associated future f,
invariants:
f bound
x bound
f bound to o
x bound to o
x of type single t
f of type t
Synchronization as on SAVs
Flow Java: Declarative Concurrency for Java – p.12/29
Futures
Each SAV x has associated future f,
invariants:
f bound
x bound
f bound to o
x bound to o
x of type single t
f of type t
Synchronization as on SAVs
Bind a @= b suspends if b is undetermined
Flow Java: Declarative Concurrency for Java – p.12/29
Obtaining Futures
Explicit type conversion
single t
t
Flow Java: Declarative Concurrency for Java – p.13/29
Obtaining Futures
Explicit type conversion
single t
t
Implicit type conversion
void m(Object o) ...
single Object s;
m(s)
Flow Java: Declarative Concurrency for Java – p.13/29
Obtaining Futures
Explicit type conversion
single t
t
Implicit type conversion
void m(Object o) ...
single Object s;
m(s)
Essential for integration with non Flow Java
code
Flow Java: Declarative Concurrency for Java – p.13/29
Secure Spawn
Add method to Spawn
public static Object spawn() {
single Object r;
new Thread(new Spawn(r)).start();
return r; // implicit conversion
}
Flow Java: Declarative Concurrency for Java – p.14/29
Aliasing
A SAV can be aliased to other SAVs
Flow Java: Declarative Concurrency for Java – p.15/29
Aliasing
A SAV can be aliased to other SAVs
While still unbound
Flow Java: Declarative Concurrency for Java – p.15/29
Aliasing
A SAV can be aliased to other SAVs
While still unbound
x and y SAV: x @= y aliases x and y
Flow Java: Declarative Concurrency for Java – p.15/29
Aliasing
A SAV can be aliased to other SAVs
While still unbound
x and y SAV: x @= y aliases x and y
Binding either x or y to o binds both SAVs
to o
Flow Java: Declarative Concurrency for Java – p.15/29
Aliasing
A SAV can be aliased to other SAVs
While still unbound
x and y SAV: x @= y aliases x and y
Binding either x or y to o binds both SAVs
to o
Associated futures as well
Flow Java: Declarative Concurrency for Java – p.15/29
Aliasing
A SAV can be aliased to other SAVs
While still unbound
x and y SAV: x @= y aliases x and y
Binding either x or y to o binds both SAVs
to o
Associated futures as well
equality test == extended
Flow Java: Declarative Concurrency for Java – p.15/29
Aliasing
A SAV can be aliased to other SAVs
While still unbound
x and y SAV: x @= y aliases x and y
Binding either x or y to o binds both SAVs
to o
Associated futures as well
equality test == extended
x == y immediately returns true if x and y
aliased
Flow Java: Declarative Concurrency for Java – p.15/29
Aliasing
A SAV can be aliased to other SAVs
While still unbound
x and y SAV: x @= y aliases x and y
Binding either x or y to o binds both SAVs
to o
Associated futures as well
equality test == extended
x == y immediately returns true if x and y
aliased
Otherwise suspends until both are bound
Flow Java: Declarative Concurrency for Java – p.15/29
Overview
Flow Java
Single assignment variables
Binding
Synchronization
Futures
Aliasing
Implementation
Runtime
Compilation
Optimizations
Evaluation
Related Work
Conclusion
Flow Java: Declarative Concurrency for Java – p.16/29
Implementation
Based on GCJ
Flow Java: Declarative Concurrency for Java – p.17/29
Implementation
Based on GCJ
Moderate extension
lines
Flow Java: Declarative Concurrency for Java – p.17/29
Implementation
Based on GCJ
Moderate extension
lines
Available from
http://www.sics.se/~frej/flow_java
Flow Java: Declarative Concurrency for Java – p.17/29
Implementation
Based on GCJ
Moderate extension
lines
Available from
http://www.sics.se/~frej/flow_java
Builds on Java’s framework for synchronized
methods
Flow Java: Declarative Concurrency for Java – p.17/29
Implementation
Based on GCJ
Moderate extension
lines
Available from
http://www.sics.se/~frej/flow_java
Builds on Java’s framework for synchronized
methods
SAV / future distinction maintained by
compiler, synchronization object at runtime
(SO)
Flow Java: Declarative Concurrency for Java – p.17/29
Unbound SO
o
sync. object
sync. vtab
vptr
class
rptr
method
UNB
method
void fsync() {
this = WaitDet(this);
jump(f);
}
Flow Java: Declarative Concurrency for Java – p.18/29
Bound SO
o
sync. object
vtab
vptr
class
rptr
method
vptr
rptr
method
void f() {
field
}
field
object
Flow Java: Declarative Concurrency for Java – p.19/29
Runtime System
Primitives @=, ==, !=
Flow Java: Declarative Concurrency for Java – p.20/29
Runtime System
Primitives @=, ==, !=
Suspension uses wait() and notifyAll()
of SO
Flow Java: Declarative Concurrency for Java – p.20/29
Runtime System
Primitives @=, ==, !=
Suspension uses wait() and notifyAll()
of SO
Aliasing: chains of SOs
Flow Java: Declarative Concurrency for Java – p.20/29
Runtime System
Primitives @=, ==, !=
Suspension uses wait() and notifyAll()
of SO
Aliasing: chains of SOs
Defined ordering:
atomic bind/alias
deadlock-free suspension/resumption
Flow Java: Declarative Concurrency for Java – p.20/29
Compilation
SOs initialized when entering scope
Flow Java: Declarative Concurrency for Java – p.21/29
Compilation
SOs initialized when entering scope
Field access wrapped to ensure suspension:
WaitDet(a).foo = 1
a.foo = 1
Flow Java: Declarative Concurrency for Java – p.21/29
Compilation
SOs initialized when entering scope
Field access wrapped to ensure suspension:
WaitDet(a).foo = 1
a.foo = 1
Static method calls wrap object reference (the
synchronization vtab not used)
Flow Java: Declarative Concurrency for Java – p.21/29
Optimizations
Path compression in primitives
Flow Java: Declarative Concurrency for Java – p.22/29
Optimizations
Path compression in primitives
this never wrapped
Flow Java: Declarative Concurrency for Java – p.22/29
Optimizations
Path compression in primitives
this never wrapped
CSE: WaitDet(), disregarding suspension,
is side effect free:
WaitDet(o).a + WaitDet(o).b
t = WaitDet(o);
t.a + t.b
Flow Java: Declarative Concurrency for Java – p.22/29
Overview
Flow Java
Single assignment variables
Binding
Synchronization
Futures
Aliasing
Implementation
Runtime
Compilation
Optimizations
Evaluation
Related Work
Conclusion
Flow Java: Declarative Concurrency for Java – p.23/29
Evaluation
Java Grande benchmark suite
Flow Java: Declarative Concurrency for Java – p.24/29
Evaluation
Java Grande benchmark suite
10% to 40% overhead
Flow Java: Declarative Concurrency for Java – p.24/29
Evaluation
Java Grande benchmark suite
10% to 40% overhead
Pathological cases 75%
Flow Java: Declarative Concurrency for Java – p.24/29
Evaluation
Java Grande benchmark suite
10% to 40% overhead
Pathological cases 75%
Synthetic benchmarks comparing Flow Java
to Java
Flow Java: Declarative Concurrency for Java – p.24/29
Evaluation
Java Grande benchmark suite
10% to 40% overhead
Pathological cases 75%
Synthetic benchmarks comparing Flow Java
to Java
Spawn 2% slower
Flow Java: Declarative Concurrency for Java – p.24/29
Evaluation
Java Grande benchmark suite
10% to 40% overhead
Pathological cases 75%
Synthetic benchmarks comparing Flow Java
to Java
Spawn 2% slower
Producer/Consumer 6% faster
Flow Java: Declarative Concurrency for Java – p.24/29
Related work
Concurrent Logic/Constraint Programming
Main difference
Flow Java: Declarative Concurrency for Java – p.25/29
Related work
Concurrent Logic/Constraint Programming
Main difference
no terms/constraints
Flow Java: Declarative Concurrency for Java – p.25/29
Related work
Concurrent Logic/Constraint Programming
Main difference
no terms/constraints
futures and types
Flow Java: Declarative Concurrency for Java – p.25/29
Related work
Concurrent Logic/Constraint Programming
Main difference
no terms/constraints
futures and types
predefined concurrency model
Flow Java: Declarative Concurrency for Java – p.25/29
Related work
Concurrent Logic/Constraint Programming
Main difference
no terms/constraints
futures and types
predefined concurrency model
Close relative: Oz
Flow Java: Declarative Concurrency for Java – p.25/29
Related work
Concurrent Logic/Constraint Programming
Main difference
no terms/constraints
futures and types
predefined concurrency model
Close relative: Oz
also futures
Flow Java: Declarative Concurrency for Java – p.25/29
Related work
Concurrent Logic/Constraint Programming
Main difference
no terms/constraints
futures and types
predefined concurrency model
Close relative: Oz
also futures
no types: no implicit conversion from SAV
to future
Flow Java: Declarative Concurrency for Java – p.25/29
Language
Extensions
Multilisp: [Halstead 1985] futures and thread
creation is combined, no single-assignment
variables and no aliasing
Flow Java: Declarative Concurrency for Java – p.26/29
Language
Extensions
Multilisp: [Halstead 1985] futures and thread
creation is combined, no single-assignment
variables and no aliasing
Alice: [UdS] SML with single assignment
variables (without aliasing) and futures
Flow Java: Declarative Concurrency for Java – p.26/29
Language
Extensions
Multilisp: [Halstead 1985] futures and thread
creation is combined, no single-assignment
variables and no aliasing
Alice: [UdS] SML with single assignment
variables (without aliasing) and futures
Extended Ada: [Thornley 1994,1995] with
single assignment variables, not conservative
Flow Java: Declarative Concurrency for Java – p.26/29
Language
Extensions
Multilisp: [Halstead 1985] futures and thread
creation is combined, no single-assignment
variables and no aliasing
Alice: [UdS] SML with single assignment
variables (without aliasing) and futures
Extended Ada: [Thornley 1994,1995] with
single assignment variables, not conservative
CC++:
[Chandy, Kesselman, 93] No futures, no
aliasing, no thread creation. Call with SAV
arguments suspends
Flow Java: Declarative Concurrency for Java – p.26/29
Language
Extensions
Id: [Arvind ea, 1989] I-structures as arrays of
dataflow variables
Flow Java: Declarative Concurrency for Java – p.27/29
Extensions to Java/C
#
Decaf: [Sargeant 2000], logic variables as in
Flow Java but not futures. Not conservative.
Flow Java: Declarative Concurrency for Java – p.28/29
Extensions to Java/C
#
Decaf: [Sargeant 2000], logic variables as in
Flow Java but not futures. Not conservative.
Chords for C#: [Benton ea, 2002] Translation
into C# without Chords.
Flow Java: Declarative Concurrency for Java – p.28/29
Conclusions and
Future Work
Conservative extension
Flow Java: Declarative Concurrency for Java – p.29/29
Conclusions and
Future Work
Conservative extension
Simplifies concurrent programming
Flow Java: Declarative Concurrency for Java – p.29/29
Conclusions and
Future Work
Conservative extension
Simplifies concurrent programming
Extensions straightforward make few
assumptions of object representation
Flow Java: Declarative Concurrency for Java – p.29/29
Conclusions and
Future Work
Conservative extension
Simplifies concurrent programming
Extensions straightforward make few
assumptions of object representation
Moderate efficiency penalty
Flow Java: Declarative Concurrency for Java – p.29/29
Conclusions and
Future Work
Conservative extension
Simplifies concurrent programming
Extensions straightforward make few
assumptions of object representation
Moderate efficiency penalty
First step towards a platform for high-level,
efficient, accessible, distributed and parallel
programming
Flow Java: Declarative Concurrency for Java – p.29/29