Post on 26-Mar-2015
.NET Programming Language Research @ MSR Cambridge
Nick BentonMicrosoft ResearchCambridge
MS.NET days March 2002
MSR Cambridge Programming Principles and Tools Group Luca Cardelli
Nick Benton Cedric Fournet Andy Gordon Tony Hoare Andrew Kennedy Simon Peyton Jones Don Syme
Simon Marlow Claudio Russo Mark Shields + visitors, PhD students, interns,…
MS.NET days March 2002
.NET in MSR Programming Principles and Tools Group Verifier spec (POPL) CLR design feedback Project 7 (Babel) SML.NET (ICFP,ICFP,HOOTS) Generics for C# and .NET (PLDI) Extended IL (Babel) Polyphonic C# (FOOL, ECOOP) Stack-Walking Security analysis (POPL) Assertions
Part 1SML.NET
Nick Benton, Andrew Kennedy and Claudio Russo
MS.NET days March 2002
Advanced Programming Languages on the CLR .NET and the CLR provide a great opportunity
for programming language designers and implementers: The runtime provides services (execution engine,
garbage collection,…) which make producing a good implementation of your language easier
The frameworks & libraries mean you can actually do useful things with your new language (graphics, networking, database access, web services,…)
Multi-language component-based programming makes it much more practical for other people to use your language in their own projects
MS.NET days March 2002
Our favourite language: Standard ML
A safe, modular, strict, higher-order, functional, polymorphic programming language with compile-time type checking and type inference, garbage collection, exception handling, immutable data types, pattern-matching and updatable references, abstract data types, and parametric modules.
with several efficient implementations and a formal definition with a proof of
soundness. (Scheme+types+real syntax, Haskell with call by
value evaluation)
MS.NET days March 2002
Some SMLdatatype order = LESS | GREATER | EQUAL
datatype 'a Tree = Empty | Node of 'a * ('a Tree) * ('a Tree)
fun contains compare (x, Empty) = false | contains compare (x, Node(v, left, right)) = (case compare (x,v) of LESS => contains compare (x,left) | GREATER => contains compare (x, right) | EQUAL => true )
contains : ('a * 'a -> order) -> ('a * 'a Tree) -> bool
MS.NET days March 2002
What is SML.NET? Compiler for SML that targets verifiable CIL Research Issues:
Can we compile a polymorphic functional language to a monomorphic, object-oriented runtime?
Yes How can we make it produce fast, compact code?
Whole-program optimising compiler. Monomorphisation. Representation tricks. Novel typed intermediate language using monads to track side-effects.
How can we make cross-language working easy? We extend SML to support all of the .NET CLS (Common Language
Specification), providing smooth bidirectional interoperability with .NET framework libraries & other .NET languages
MS.NET days March 2002
Previous approaches to interop1. Bilateral interface with marshalling and explicit
calling conventions (e.g. JNI, O’Caml interface for C).
• Awkward, ugly, tied to one implementation, only for experts
2. Multilateral interface with IDL (e.g. COM, CORBA) together with particular language mappings (e.g. H/Direct, Caml COM, MCORBA).
• Have to write IDL and use tools, tend to be lowest-common denominator, memory management often tricky
MS.NET days March 2002
Interop in .NET
Languages share a common higher-level infrastructure (CLR): shared heap means no tricky cross-heap pointers
(cf reference counting in COM) shared type system means no marshalling
(cf string<->char* marshalling for Java<->C) shared exception model supports cross-language
exception handling
MS.NET days March 2002
SML.NET interop Sounds great, but SML is not object-oriented
So we are going to have to do some work… Possible approaches to interop:
do not extend language; instead provide wrappers that give a functional view of a CLS library (Haskell, Mercury).
Powerful functional type systems can go a very long way towards modelling OO type systems
redesign the language (OO-SML?) Our approach – a middle way:
re-use existing features where appropriate (non-object-oriented subset)
extend language for convenient interop when “fit” is bad (object-oriented features)
live with the CLS at boundaries: don’t try to export complex ML types to other languages (what would they do with them?)
MS.NET days March 2002
Re-use SML features
static field
static method
namespace
void
null
multiple args
mutability ref
open
unit
structure
NONE
val binding
fun binding
tuple
CLS SML
using
private fields local decls
delegate first-class function
MS.NET days March 2002
Extend language
instance method invocation
instance field access
custom attributes
casts
class definitions
type test
exp :> ty
attributes in classtype
classtype
obj.#meth
obj.#fld
cast patterns
CLS
SML
MS.NET days March 2002
open System.Windows.Forms System.Drawing System.ComponentModel
fun selectXML () = let val fileDialog = OpenFileDialog() in fileDialog.#set_DefaultExt("XML"); fileDialog.#set_Filter("XML files (*.xml) |*.xml"); if fileDialog.#ShowDialog() = DialogResult.OK then case fileDialog.#get_FileName() of NONE => () | SOME name => replaceTree (ReadXML.make name, "XML file '" ^ name ^ "'") else () end
Extract from WinForms interop
instance method invocation
CLS Namespace = ML structure
static method = ML function
null value = NONE
static constant field = ML value
no args = ML unit value
CLS string = ML string
And note that there are no explicit types in this code
MS.NET days March 2002
Ray tracing in ML
ICFP programming competition: build a ray tracer in under 3 days
39 entries in all kinds of languages: C, C++, Clean, Dylan, Eiffel, Haskell, Java, Mercury,
ML, Perl, Python, Scheme, Smalltalk
ML (Caml) was in 1st and 2nd place
MS.NET days March 2002
Ray tracing in SML.NET
Translate winning entry to SML Add WinForms interop Run on .NET CLR Performance on this example twice as good as
popular optimizing native compiler for SML (though on others we’re twice as bad)
MS.NET days March 2002
Visual Studio Integration
Bootstrap the compiler to produce a .NET component for parsing, typechecking, etc. of SML
Use interlanguage working extensions to expose that as a COM component which can be glued into Visual Studio
Write new ML code to handle syntax highlighting, tooltips, error reporting, etc.
Part 2Generics in the CLR and C#
Andrew Kennedy and Don Syme
MS.NET days March 2002
Note: This is not in V1 of the CLR
It will be in V2
MS.NET days March 2002
Part 1: Introduction
MS.NET days March 2002
What are generics? Types which are parameterized by other types,
e.g. Stack<int>, Stack<string> Generics, templates, parametric polymorphism Ada, Eiffel, C++, ML, Haskell, Mercury,
Component Pascal,… Promote code reuse, increase type safety, better
performance Good for collections, higher-order programming
and generally building higher-level, reusable abstractions
MS.NET days March 2002
Generic code in C# today (1)class Stack {
private Object[] items; private int nitems; Stack() { nitems = 0; items = new Object[50]; }
Object Pop() { if (nitems == 0) throw new EmptyException(); return items[--nitems]; }
void Push(Object item) { ... return items[nitems++]; }}
MS.NET days March 2002
Generic code in C# today (2)Stack s = new Stack();
s.Push(1);
s.Push(2);
int n = (int)(s.Pop()) + (int)(s.Pop());
What’s wrong with that?• It’s inexpressive. Type system doesn’t document what should be in a particular stack.• It’s unsafe. Type errors (s.Push(“2”); ) lead to runtime exceptions instead of being caught by compiler• It’s ugly. Casts everywhere.• It’s slow. Casts are slow, and converting base types (e.g. ints) to Objects involves expensive boxing.
MS.NET days March 2002
Generic code in C# tomorrow (1)class Stack<T> {
private T[] items; private int nitems; Stack() { nitems = 0; items = new T[50]; }
T Pop() { if (nitems == 0) throw new EmptyException(); return items[--nitems]; }
void Push(T item) { if (items.Length == nitems) { T[] temp = items; items = new T[nitems*2]; Array.Copy<T>(temp, items, nitems); } items[nitems++] = item; }}
MS.NET days March 2002
Generics: large design space
Parameterized classes, interfaces, and methods e.g.// two-parameter class
class Dict<K,D> { ... }
// parameterized interface interface IComparable<T> { ... }
// parameterized struct struct Pair<A,B> { ... }
// generic method T[] Slice<T>(T[] arr, int start, int count)What goes in the
CLR? What goes in the compilers?
Do we have exact runtime type information?if (x is Set<int>)
....
But can type instantiations can be non-reference?
Set<int> List<float>Do you erase
types at runtime?
Do you share code?
Constraints? class C<T: IFoo, IBar> { ... }
MS.NET days March 2002
Generic interfaces interface IDictionary<K,D> { D Lookup(K); ... }
class Dictionary<K,D> : IDictionary<K,D> { D Lookup(K); ... }
Dictionary<String,String>
MS.NET days March 2002
Generic methods
static void Sort<T> (T[]) { ... }
int[] x = { 5,4,3,2 }; Sort(x);
A generic method is just a family of methods, indexed by type.
MS.NET days March 2002
Explicit Constraints
interface IComparable<T> { static int Compare(T,T); }
class BinaryTree<T : IComparable<T> >{ void insert(T x) { switch (x.Compare(y)) {
...
} } T y;
Tree<T> left;
Tree<T> right;}
Static constraints are available via interfaces.
MS.NET days March 2002
Part 2: Implementing Generics for the .NET CLR
MS.NET days March 2002
Generics: Our design
Parameterized classes, interfaces, structs, methods, delegates Yes
Exact runtime types: Yesif (x is Set<string>) { ... }
Instantiations at value types: YesSet<String> Set<int> List<float>
Constraints: Yes (by interfaces)class C<T: IComponent>
Variance: NoSet<String> is not subtype-related to Set<Object>
MS.NET days March 2002
How does it work?Stack<int> si = new Stack<int>();
Stack<int> si2 = new Stack<int>();
Stack<string> ss = new Stack<string>();
Stack<object> so = new Stack<object>();
si.Push(5);
ss.Push(“Generics”);
so.Push(myObj);
1a. Look for Stack<int>1b. Nothing exists yet, so create structures for Stack<int>
2a. Look for Stack<int>2b. Type already loaded!
3a. Look for Stack<string>3b. <string> not compatible with <int>, so create structures for Stack<string>
4a. Look for Stack<object>4b. <object> compatible with <string>, so re-use Stack<string>
MS.NET days March 2002
How does it work?Stack<int> si = new Stack<int>();
Stack<int> si2 = new Stack<int>();
Stack<string> ss = new Stack<string>();
Stack<object> so = new Stack<object>();
si.Push(5);
ss.Push(“Generics”);
so.Push(myObj);
Compile code for Stack<int>.Push
Compile code for Stack<string>.Push
Re-use code from Stack<string>.Push
MS.NET days March 2002
Implementing Generics (0)
Dynamic loading & JIT compilation change many fundamental assumptions Can consider code generation at runtime Can use more dynamic data structures
The current challenge is to implement all of: exact runtime types + code sharing +
dynamic loading + non-uniform instantiations
MS.NET days March 2002
Implementing Generics (1)
Our implementation:1. Dynamic code expansion and sharing
Instantiating generic classes and methods is handled by the CLR.
Generate/compile code as necessary Use JIT compiler Share code for “compatible” instantiations Compatibility is determined by the implementation
MS.NET days March 2002
Implementing Generics (2)
Our implementation:1. Dynamic code expansion and sharing
2. Pass and store runtime type information Objects carry runtime type information
We record this information by duplicating vtables. Other options are possible.
Generic methods take an extra parameter The active type context is always recoverable
MS.NET days March 2002
Implementing Generics (3)
Our implementation:1. Dynamic code expansion and sharing
2. Pass and store runtime type information
3. Optimized use of runtime type information
MS.NET days March 2002
Generics: Performance1. Instantiations at value types
Stack with repeated push and pops of 0...N elements
(a) List_int
(b) List (i.e. existing collection class)
(c) List<int> 5X speedup (c) v. (b)
2. No casts 20% speed on similar micro-benchmarks
3. Runtime type computations Generic allocation ~10% slower in generic code
MS.NET days March 2002
Generics: Design Comparison
C++
GJBracha, Odersky,Stoutamire, Wadler
NextGen Cartwright, Steele
PolyJ Bank, Liskov, Myers
Agesen, Freund, Mitchell
CLRKennedy,Syme
Dynamic Load
Efficient non-reference instantiations
½ ½
Exact run-time types ½ Efficient via fewer runtime type tests ½ Code Sharing ½ Ship without source Debug (type pars visible at runtime) ½ Safe, efficient co-variance ½
MS.NET days March 2002
Comparison with C++ templates What C++ gets wrong:
Modularity need to see the template at link time
Efficiency nearly all implementations code-expand
Safety not checked at declaration & use, but at link time
Simplicity very complicated
MS.NET days March 2002
Comparison with C++ templates What we get right:
Modularity: no need to see the generic IL until runtime
Efficiency: code expansion managed by the VM
Safety: check at declaration & use
Simplicity: there are some corners, but the mechanism is simple to
explain & use
MS.NET days March 2002
Summary A world first: cross-language generics. A world first: generics in a high-level virtual machine
design.
Implementation: The virtual machine (runtime) level is the right place to
implement generics for .NET Potential for:
Other implementation techniques (more aggressive sharing, some boxing, etc.)
Further extensions (type functions, variance, more expressive constraints)
Good performance
Part 3Polyphonic C#
Nick Benton, Luca Cardelli and Cedric Fournet
MS.NET days March 2002
Introduction
MS.NET days March 2002
Programming in a networked world
Developers now have to work in aConcurrentDistributedHigh latency (& low reliability, security sensitive, multi-everything)
environment. Which is hard And they’re mostly not very good at it
Try using Outlook over dialup
MS.NET days March 2002
Asynchronous communication
Distribution => concurrency + latency => asynchrony => more concurrency
Message-passing, event-based programming, dataflow models
For programming languages, coordination (orchestration) languages & frameworks, workflow
MS.NET days March 2002
Language support for concurrency
Make invariants and intentions more apparent (part of the interface)
Good software engineering Allows the compiler much more freedom to
choose different implementations Also helps other tools
MS.NET days March 2002
.NET Today Multithreaded execution environment with lock per object C# has “lock” keyword, libraries include traditional
shared-memory synchronization primitives (mutexes, monitors, r/w locks)
Delegate-based asynchronous calling model, events, messaging
Higher level frameworks built on that Hard to understand, use and get right
Different models at different scales Support for asynchrony all on the caller side – little help building
code to handle messages (must be thread-safe, reactive, and deadlock-free)
MS.NET days March 2002
Polyphonic C#
An extension of the C# language with new concurrency constructs
Based on the join calculus A foundational process calculus like the -calculus but
better suited to asynchronous, distributed systems A single model which works both for
local concurrency (multiple threads on a single machine)
distributed concurrency (asynchronous messaging over LAN or WAN)
MS.NET days March 2002
The Language
MS.NET days March 2002
In one slide: Objects have both synchronous and asynchronous methods. Values are passed by ordinary method calls:
If the method is synchronous, the caller blocks until the method returns some result (as usual).
If the method is async, the call completes at once and returns void.
A class defines a collection of synchronization patterns (chords), which define what happens once a particular set of methods have been invoked on an object: When pending method calls match a pattern, its body runs. If there is no match, the invocations are queued up. If there are several matches, an unspecified pattern is selected. If a pattern containing only async methods fires, the body runs in a new
thread.
MS.NET days March 2002
A Simple Bufferclass Buffer {
String get() & async put(String s) {
return s;
}
}
MS.NET days March 2002
A Simple Bufferclass Buffer {
String get() & async put(String s) {
return s;
}
}
•An ordinary (synchronous) method with no arguments, returning a string
MS.NET days March 2002
A Simple Bufferclass Buffer {
String get() & async put(String s) {
return s;
}
}
•An ordinary (synchronous) method with no arguments, returning a string
•An asynchronous method (hence returning no result), with a string argument
MS.NET days March 2002
A Simple Bufferclass Buffer {
String get() & async put(String s) {
return s;
}
}
•An ordinary (synchronous) method with no arguments, returning a string
•An asynchronous method (hence returning no result), with a string argument
•Joined together in a chord
MS.NET days March 2002
A Simple Bufferclass Buffer {
String get() & async put(String s) {
return s;
}
}
•Calls to put() return immediately (but are internally queued if there’s no waiting get()).
•Calls to get() block until/unless there’s a matching put()
•When there’s a match the body runs, returning the argument of the put() to the caller of get().
•Exactly which pairs of calls are matched up is unspecified.
MS.NET days March 2002
A Simple Bufferclass Buffer {
String get() & async put(String s) {
return s;
}
}•Does example this involve spawning any threads?
•No. Though the calls will usually come from different pre-existing threads.
•So is it thread-safe? You don’t seem to have locked anything…
•Yes. The chord compiles into code which uses locks. (And that doesn’t mean everything is synchronized on the object.)
•Which method gets the returned result?
•The synchronous one. And there can be at most one of those in a chord.
MS.NET days March 2002
Reader/Writer …using threads and mutexes in Modula 3
An introduction to programming with threads. Andrew D. Birrell, January 1989.
MS.NET days March 2002
class ReaderWriter { void Exclusive() & private async Idle() {} void ReleaseExclusive() { Idle(); }
void Shared() & private async Idle() { S(1); } void Shared() & private async S(int n) { S(n+1); } void ReleaseShared() & private async S(int n) { if (n == 1) Idle(); else S(n-1); }
ReaderWriter() { Idle(); }}
A single private message represents the state: none Idle() S(1) S(2) S(3) …
Reader/Writer in five chords
MS.NET days March 2002
Asynchronous Service Requests and Responses
The service might export an async method which takes parameters and somewhere to put the result: a buffer, or a channel, or a delegate (O-O function pointer)
delegate async IntCB(int v);
class Service {
public async request(String arg, IntCB callback) {
int result;
// do something interesting…
callback(result);
}
}
MS.NET days March 2002
Asynchronous Service Requests and Responses - join
class Join2 { void wait(out int i, out int j)& async first(int r1)& async second(int r2) {
i = r1; j = r2; return;}
}// client code:int i,j;Join2 x = new Join2();service1.request(arg1, new IntCB(x.first));service2.request(arg2, new IntCB(x.second));// do something useful// now wait until both results have come backx.wait(i,j);// do something with i and j
MS.NET days March 2002
Asynchronous Service Requests and Responses - select
class Select { int wait()& async reply(int r) {
return r;}
}// client code:int i;Select x = new Select();service1.request(arg1, new IntCB(x.reply));service2.request(arg2, new IntCB(x.reply));// do something useful// now wait until one result has come backi = x.wait();// do something with i
MS.NET days March 2002
Extending C# with chords Classes can declare methods using generalized
chord-declarations instead of method-declarations.
chord-declaration ::= method-header [ & method-header ]* body
method-header ::= attributes modifiers [return-type | async] name (parms)
Interesting well-formedness conditions:1. At most one header can have a return type (i.e. be synchronous).2. The inheritance restriction.3. “ref” and “out” parameters cannot appear in async headers.
MS.NET days March 2002
Implementation
MS.NET days March 2002
Example: Sum of Squares
SumOfSquares
total(0,8)
add(1)
add(4)
add(9)
add(64)
MS.NET days March 2002
Sum of Squares
SumOfSquares
total(1,7)
add(4)
add(9)
add(64)
MS.NET days March 2002
Sum of Squares Codeclass SumOfSquares {
private async loop(int i) {if (i > 0) {
add(i * i);loop(i - 1);
}}
private int total(int r, int i) & private async add(int dr) {int rp = r + dr;if (i > 1) return total(rp, i - 1);return rp;
}
public SumOfSquares(int x) {loop(x);int i = total(0, x);System.Console.WriteLine("The result is {0}.", i);
}}
MS.NET days March 2002
Sum of Squares Translationusing System;using System.Collections;using System.Threading;class SyncQueueEntry{ public int pattern; public System.Threading.Thread mythread; public System.Object joinedentries;}class SumOfSquares{ Queue Q_totalint_int_ = new Queue(); Queue Q_addint_ = new Queue(); class loopint__runner{
SumOfSquares parent;int field_0;public loopint__runner(SumOfSquares p_p,int p_0) { parent = p_p; field_0 = p_0; Thread t = new Thread(new ThreadStart(this.doit)); t.Start();}void doit() { parent.loopint__worker(field_0);}
} private void loopint__worker(int i) {
if (i >= 1) {add(i * i);loop(i - 1);}
} static void Main() {
SumOfSquares s = new SumOfSquares();int thesum = s.sum(10);Console.WriteLine(thesum);
} public int sum(int x) {
loop(x);return total(0, x);
} private int total(int sync_p_0,int sync_p_1) {
SyncQueueEntry qe = new SyncQueueEntry();int matchindex=0;System.Threading.Monitor.Enter(Q_totalint_int_);if (!(Q_addint_.Count ==0)) { qe.joinedentries = (int) (Q_addint_.Dequeue()); System.Threading.Monitor.Exit(Q_totalint_int_); matchindex = 0; goto joinlabel;
}// enqueue myself and sleep;qe.mythread = Thread.CurrentThread;Q_totalint_int_.Enqueue(qe);System.Threading.Monitor.Exit(Q_totalint_int_);
try { Thread.Sleep(Timeout.Infinite);} catch (ThreadInterruptedException) {}// wake up herematchindex = qe.pattern;
joinlabel:switch (matchindex) {case 0: int r = sync_p_0; int i = sync_p_1; int dr = (int)(qe.joinedentries); int rp = r + dr;
if (i > 1) {return total(rp, i - 1); }
return rp;}throw new System.Exception();
} private void add(int p_0) {
Object qe = p_0;System.Threading.Monitor.Enter(Q_totalint_int_);if (!(Q_totalint_int_.Count ==0)) { SyncQueueEntry sqe = (SyncQueueEntry) (Q_totalint_int_.Dequeue()); sqe.joinedentries = qe; System.Threading.Monitor.Exit(Q_totalint_int_); sqe.pattern = 0; sqe.mythread.Interrupt(); return;
}Q_addint_.Enqueue(qe);System.Threading.Monitor.Exit(Q_totalint_int_);return;
} private void loop(int i) {
loopint__runner r = new loopint__runner(this,i); }}
MS.NET days March 2002
Sum of Squares Translationusing System;using System.Collections;using System.Threading;class SyncQueueEntry{ public int pattern; public System.Threading.Thread mythread; public System.Object joinedentries;}class SumOfSquares{ Queue Q_totalint_int_ = new Queue(); Queue Q_addint_ = new Queue(); class loopint__runner{
SumOfSquares parent;int field_0;public loopint__runner(SumOfSquares p_p,int p_0) { parent = p_p; field_0 = p_0; Thread t = new Thread(new ThreadStart(this.doit)); t.Start();}void doit() { parent.loopint__worker(field_0);}
} private void loopint__worker(int i) {
if (i >= 1) {add(i * i);loop(i - 1);}
} static void Main() {
SumOfSquares s = new SumOfSquares();int thesum = s.sum(10);Console.WriteLine(thesum);
} public int sum(int x) {
loop(x);return total(0, x);
} private int total(int sync_p_0,int sync_p_1) {
SyncQueueEntry qe = new SyncQueueEntry();int matchindex=0;System.Threading.Monitor.Enter(Q_totalint_int_);if (!(Q_addint_.Count ==0)) { qe.joinedentries = (int) (Q_addint_.Dequeue()); System.Threading.Monitor.Exit(Q_totalint_int_); matchindex = 0; goto joinlabel;
}// enqueue myself and sleep;qe.mythread = Thread.CurrentThread;Q_totalint_int_.Enqueue(qe);System.Threading.Monitor.Exit(Q_totalint_int_);
try { Thread.Sleep(Timeout.Infinite);} catch (ThreadInterruptedException) {}// wake up herematchindex = qe.pattern;
joinlabel:switch (matchindex) {case 0: int r = sync_p_0; int i = sync_p_1; int dr = (int)(qe.joinedentries); int rp = r + dr;
if (i > 1) {return total(rp, i - 1); }
return rp;}throw new System.Exception();
} private void add(int p_0) {
Object qe = p_0;System.Threading.Monitor.Enter(Q_totalint_int_);if (!(Q_totalint_int_.Count ==0)) { SyncQueueEntry sqe = (SyncQueueEntry) (Q_totalint_int_.Dequeue()); sqe.joinedentries = qe; System.Threading.Monitor.Exit(Q_totalint_int_); sqe.pattern = 0; sqe.mythread.Interrupt(); return;
}Q_addint_.Enqueue(qe);System.Threading.Monitor.Exit(Q_totalint_int_);return;
} private void loop(int i) {
loopint__runner r = new loopint__runner(this,i); }}
class SumOfSquares{ Queue Q_totalint_int_ = new Queue(); Queue Q_addint_ = new Queue();
…
MS.NET days March 2002
Sum of Squares Translationusing System;using System.Collections;using System.Threading;class SyncQueueEntry{ public int pattern; public System.Threading.Thread mythread; public System.Object joinedentries;}class SumOfSquares{ Queue Q_totalint_int_ = new Queue(); Queue Q_addint_ = new Queue(); class loopint__runner{
SumOfSquares parent;int field_0;public loopint__runner(SumOfSquares p_p,int p_0) { parent = p_p; field_0 = p_0; Thread t = new Thread(new ThreadStart(this.doit)); t.Start();}void doit() { parent.loopint__worker(field_0);}
} private void loopint__worker(int i) {
if (i >= 1) {add(i * i);loop(i - 1);}
} static void Main() {
SumOfSquares s = new SumOfSquares();int thesum = s.sum(10);Console.WriteLine(thesum);
} public int sum(int x) {
loop(x);return total(0, x);
} private int total(int sync_p_0,int sync_p_1) {
SyncQueueEntry qe = new SyncQueueEntry();int matchindex=0;System.Threading.Monitor.Enter(Q_totalint_int_);if (!(Q_addint_.Count ==0)) { qe.joinedentries = (int) (Q_addint_.Dequeue()); System.Threading.Monitor.Exit(Q_totalint_int_); matchindex = 0; goto joinlabel;
}// enqueue myself and sleep;qe.mythread = Thread.CurrentThread;Q_totalint_int_.Enqueue(qe);System.Threading.Monitor.Exit(Q_totalint_int_);
try { Thread.Sleep(Timeout.Infinite);} catch (ThreadInterruptedException) {}// wake up herematchindex = qe.pattern;
joinlabel:switch (matchindex) {case 0: int r = sync_p_0; int i = sync_p_1; int dr = (int)(qe.joinedentries); int rp = r + dr;
if (i > 1) {return total(rp, i - 1); }
return rp;}throw new System.Exception();
} private void add(int p_0) {
Object qe = p_0;System.Threading.Monitor.Enter(Q_totalint_int_);if (!(Q_totalint_int_.Count ==0)) { SyncQueueEntry sqe = (SyncQueueEntry) (Q_totalint_int_.Dequeue()); sqe.joinedentries = qe; System.Threading.Monitor.Exit(Q_totalint_int_); sqe.pattern = 0; sqe.mythread.Interrupt(); return;
}Q_addint_.Enqueue(qe);System.Threading.Monitor.Exit(Q_totalint_int_);return;
} private void loop(int i) {
loopint__runner r = new loopint__runner(this,i); }}
private void add(int p_0) {System.Threading.Monitor.Enter(Q_totalint_int_);if (!(Q_totalint_int_.Count ==0)) { SyncQueueEntry sqe =
(SyncQueueEntry)(Q_totalint_int_.Dequeue()); sqe.joinedentries = p_0; System.Threading.Monitor.Exit(Q_totalint_int_); sqe.pattern = 0; sqe.mythread.Interrupt(); return;}Q_addint_.Enqueue(p_0);System.Threading.Monitor.Exit(Q_totalint_int_);return;
}
MS.NET days March 2002
Current Work Examples and test cases
Web combinators, adaptive scheduler, web services (Terraserver), active objects and remoting (stock trader)
Generally looking at integration with existing mechanisms and frameworks
Language designDirect syntactic support for timeouts
Solid Implementation
MS.NET days March 2002
Predictable Demo: Dining Philosophers
eating
eating
waitingto eat
waitingto eat thinking
MS.NET days March 2002
Code extractclass Room {
public Room (int size) { hasspaces(size); }
public void enter() & private async hasspaces(int n) {
if (n > 1) hasspaces(n-1);
else isfull();
}
public void leave() & private async hasspaces(int n) {
hasspaces(n+1);
}
public void leave() & private async isfull() { hasspaces(1); }
}
MS.NET days March 2002
Conclusions A clean, simple, new model for asynchronous
concurrency in C# Declarative, local synchronization Applicable in both local and distributed settings Efficiently compiled to queues and automata Easier to express and enforce concurrency
invariants Compatible with existing constructs, though they
constrain our design somewhat Solid foundations Works well in practice
MS.NET days March 2002
That’s all
For more information, contact me nick@microsoft.com or see
http://research.microsoft.com/~{nick,akenn,dsyme}
MS.NET days March 2002
TimeoutBufferclass TimeoutBuffer {
TimeoutBuffer(int delay) {
Timer t = new Timer(new TimerCallBack(this.tick), delay);
empty();
}
async empty() & void put(Object o) {has(o);}
async empty() & void tick() {timeout();}
async timeout() & void put(Object o) {timeout();}
async timeout() & Object get() {timeout(); throw new TimeOutExn();}
async has(Object o) & Object get() {has(o); return o;}
async has(Object o) & void tick() {has(o);}
}
MS.NET days March 2002
JoCaml allows multiple synchronous methods to be joined, as in the following rendezvous
But in which thread does the body run? In C#, thread identity is “very” observable, since threads are the holders of particular re-entrant locks. So we rule this out in the interests of keeping & commutative. (Of course, it’s still easy to code up an asymmetric rendezvous in Polyphonic C#.)
Why only one synchronous method in a chord?
int f(int x) & int g(int y) {
return y to f;
return x to y;
}
MS.NET days March 2002
The problem with inheritance
We’ve “half” overridden f Too easy to create deadlock or async leakage
class C {
virtual void f() & virtual async g() {…}
virtual void f() & virtual async h() {…}
}
class D : C {
override async g() { …}
}
void m(C x) { x.g(); x.f();}
…
m(new D());
MS.NET days March 2002
The Inheritance Restriction Inheritance may be used as usual, with a restriction
to prevent the partial overriding of patterns:
For a given class, two methods f ang g are co-declared if there is a chord in which they are both declared.
Whenever a method is overriden,every codeclared method must also be overriden.
Hence, the compiler rejects patterns such as
public virtual void f() & private async g() {…}
In general, inheritance and concurrency do not mix well.Our restriction is simple; it could be made less restrictive.
MS.NET days March 2002
Types etc.
async is a subtype of void Allow covariant return types on those two:
An async method may override a void one A void delegate may be created from an async
method An async method may implement a void method in
an interface async methods are automatically given the [OneWay] attribute, so remote calls are non-blocking
MS.NET days March 2002
Compiling chords Since synchronization is statically defined for every class,
we can compile it efficiently (state automata). We cache the synchronization state in a single word. We use a bit for every (polyphonic) method. We pre-compute bitmasks for every pattern. Simple version just looks up queue state directly
For every polyphonic method, we allocate a queuefor storing delayed threads (or pending messages).
The compilation scheme can be optimized: Some states are not reachable. Empty messages only need to be counted. The content of (single, private) messages can be stored in local
variables. Requires some analysis.
MS.NET days March 2002
Implementation issues When compiling a polyphonic class, we add
private fields for the synchronization state and the queues; private methods for the body of asynchronous patterns; some initialization code.
The code handling the join patterns must be thread-safe. We use a single lock (from the first queue) to protect the state word
and all queues. This is independent from the object lock and only held briefly whilst
queues are being manipulated.
For asynchronous methods, there’s an auxiliary class for storing the pending messages.
MS.NET days March 2002
Adding synchronization code When an asynchronous method is called:
add the message content to the queue; if the method bit is 0, set it to 1 in the synchronization state
and check for a completed pattern: For every pattern containing the method,
compare the new state to the pattern mask. If there is a match, then wake up a delayed thread
(or start a new thread if the pattern is entirely asynchronous).
When a synchronous method is called: if the method bit is 0, set it to 1 in the synchronization state
and check for a completed pattern For every pattern containing the method,
compare the new state to the pattern mask. If there is a match, dequeue the asynchronous arguments,
adjust the mask, and run the body for that pattern. Otherwise, enqueue the thread, go to sleep, then retry.