Checkpoint synchronization: Difference between revisions
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The checkpoint synchronization is a problem of synchronizing multiple [[task]]s. Consider a workshop where several workers ([[task]]s) assembly details of some mechanism. When each of them completes his work they put the details together. There is no store, so a worker who finished its part first must wait for others before starting another one. Putting details together is the ''checkpoint'' at which [[task]]s synchronize themselves before going their paths apart. |
The checkpoint synchronization is a problem of synchronizing multiple [[task]]s. Consider a workshop where several workers ([[task]]s) assembly details of some mechanism. When each of them completes his work they put the details together. There is no store, so a worker who finished its part first must wait for others before starting another one. Putting details together is the ''checkpoint'' at which [[task]]s synchronize themselves before going their paths apart. |
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Revision as of 20:46, 12 August 2010
You are encouraged to solve this task according to the task description, using any language you may know.
The checkpoint synchronization is a problem of synchronizing multiple tasks. Consider a workshop where several workers (tasks) assembly details of some mechanism. When each of them completes his work they put the details together. There is no store, so a worker who finished its part first must wait for others before starting another one. Putting details together is the checkpoint at which tasks synchronize themselves before going their paths apart.
The task
Implement checkpoint synchronization in your language.
Make sure that the solution is race condition-free. Note that a straightforward solution based on events is exposed to race condition. Let two tasks A and B need to be synchronized at a checkpoint. Each signals its event (EA and EB correspondingly), then waits for the AND-combination of the events (EA&EB) and resets its event. Consider the following scenario: A signals EA first and gets blocked waiting for EA&EB. Then B signals EB and loses the processor. Then A is released (both events are signaled) and resets EA. Now if B returns and enters waiting for EA&EB, it gets lost.
When a worker is ready it shall not continue before others finish. A typical implementation bug is when a worker is counted twice within one working cycle causing its premature completion. This happens when the quickest worker serves its cycle two times while the laziest one is lagging behind.
If you can, implement workers joining and leaving.
Ada
<lang Ada>with Ada.Calendar; use Ada.Calendar; with Ada.Numerics.Float_Random; use Ada.Numerics.Float_Random; with Ada.Text_IO; use Ada.Text_IO;
procedure Test_Checkpoint is
protected Checkpoint is
entry Deliver;
entry Join (Label : out Character);
entry Leave;
private
Signaling : Boolean := False;
Ready_Count : Natural := 0;
Worker_Count : Natural := 0;
Unused_Label : Character := 'A';
entry Lodge;
end Checkpoint;
protected body Checkpoint is
entry Join (Label : out Character) when not Signaling is
begin
Label := Unused_Label;
Unused_Label := Character'Succ (Unused_Label);
Worker_Count := Worker_Count + 1;
end Join;
entry Leave when not Signaling is
begin
Worker_Count := Worker_Count - 1;
end Leave;
entry Deliver when not Signaling is
begin
Ready_Count := Ready_Count + 1;
requeue Lodge;
end Deliver;
entry Lodge when Ready_Count = Worker_Count or Signaling is
begin
Ready_Count := Ready_Count - 1;
Signaling := Ready_Count /= 0;
end Lodge;
end Checkpoint;
task type Worker;
task body Worker is
Dice : Generator;
Label : Character;
Shift_End : Time := Clock + 2.0; -- Trade unions are hard!
begin
Reset (Dice);
Checkpoint.Join (Label);
loop
Put_Line (Label & " is working");
delay Duration (Random (Dice) * 0.200);
Put_Line (Label & " is ready");
Checkpoint.Deliver;
exit when Clock >= Shift_End;
end loop;
Checkpoint.Leave;
end Worker;
Set : array (1..4) of Worker;
begin
null; -- Nothing to do here
end Test_Checkpoint;</lang> Sample output:
A is working B is working C is working D is working A is ready C is ready B is ready D is ready D is working C is working A is working B is working D is ready B is ready A is ready C is ready C is working B is working D is working A is working D is ready A is ready C is ready B is ready D is working A is working C is working B is working A is ready D is ready B is ready C is ready A is working D is working C is working B is working A is ready D is ready C is ready B is ready A is working C is working D is working B is working A is ready C is ready D is ready B is ready A is working D is working C is working B is working D is ready C is ready B is ready A is ready A is working D is working C is working B is working D is ready C is ready B is ready A is ready D is working C is working B is working A is working C is ready B is ready D is ready A is ready C is working B is working D is working A is working C is ready B is ready A is ready D is ready B is working A is working D is working C is working B is ready A is ready D is ready C is ready C is working D is working B is working A is working C is ready D is ready B is ready A is ready C is working D is working B is working A is working C is ready B is ready D is ready A is ready C is working B is working D is working A is working C is ready D is ready B is ready A is ready C is working D is working A is working B is working C is ready D is ready B is ready A is ready C is working D is working A is working B is working C is ready B is ready A is ready D is ready B is working A is working D is working C is working B is ready D is ready C is ready A is ready
PureBasic
PureBasic normally uses Semaphores and Mutex’s to synchronize parallel systems. This system only relies on semaphores between each thread and the controller (CheckPoint-procedure). For exchanging data a Mutex based message stack could easily be added, both synchronized according to this specific task or non-blocking if each worker could be allowed that freedom. <lang PureBasic>#MaxWorktime=8000 ; "Workday" in msec
- Structure that each thread uses
Structure MyIO
ThreadID.i Semaphore_Joining.i Semaphore_Release.i Semaphore_Deliver.i Semaphore_Leaving.i
EndStructure
- Array of used threads
Global Dim Comm.MyIO(0)
- Master loop synchronizing the threads via semaphores
Procedure CheckPoint()
Protected i, j, maxthreads=ArraySize(Comm())
Protected Worker_count, Deliver_count
Repeat
For i=1 To maxthreads
With Comm(i)
If TrySemaphore(\Semaphore_Leaving)
Worker_count-1
ElseIf TrySemaphore(\Semaphore_Deliver)
Deliver_count+1
If Deliver_count=Worker_count
PrintN("All Workers reported in, starting next task.")
Deliver_count=0
For j=1 To maxthreads
SignalSemaphore(Comm(j)\Semaphore_Release)
Next j
EndIf
ElseIf TrySemaphore(\Semaphore_Joining)
PrintN("A new Worker joined the force.")
Worker_count+1: SignalSemaphore(\Semaphore_Release)
ElseIf Worker_count=0
ProcedureReturn
EndIf
Next i
EndWith
ForEver
StartAll=0
EndProcedure
- A worker thread, all orchestrated by the Checkpoint() routine
Procedure Worker(ID)
Protected EndTime=ElapsedMilliseconds()+#MaxWorktime, n
With Comm(ID)
SignalSemaphore(\Semaphore_Joining)
Repeat
Repeat ; Use a non-blocking semaphore check to avoid dead-locking at shutdown.
If ElapsedMilliseconds()>EndTime
SignalSemaphore(\Semaphore_Leaving)
PrintN("Thread #"+Str(ID)+" is done.")
ProcedureReturn
EndIf
Delay(1)
Until TrySemaphore(\Semaphore_Release)
n=Random(1000)
PrintN("Thread #"+Str(ID)+" will work for "+Str(n)+" msec.")
Delay(n): PrintN("Thread #"+Str(ID)+" delivering")
SignalSemaphore(\Semaphore_Deliver)
ForEver
EndWith
EndProcedure
- User IO & init
If OpenConsole()
Define i, j
Repeat
Print("Enter number of workers to use [2-2000]: ")
j=Val(Input())
Until j>=2 And j<=2000
ReDim Comm(j)
For i=1 To j
With Comm(i)
\Semaphore_Release =CreateSemaphore()
\Semaphore_Joining =CreateSemaphore()
\Semaphore_Deliver =CreateSemaphore()
\Semaphore_Leaving =CreateSemaphore()
\ThreadID = CreateThread(@Worker(),i)
EndWith
Next
PrintN("Work started, "+Str(j)+" workers has been called.")
CheckPoint()
Print("Press ENTER to exit"): Input()
EndIf</lang>
Enter number of workers to use [2-2000]: 5 Work started, 5 workers has been called. A new Worker joined the force. A new Worker joined the force. A new Worker joined the force. A new Worker joined the force. A new Worker joined the force. Thread #5 will work for 908 msec. Thread #3 will work for 405 msec. Thread #1 will work for 536 msec. Thread #2 will work for 632 msec. Thread #4 will work for 202 msec. Thread #4 delivering Thread #3 delivering Thread #1 delivering Thread #2 delivering Thread #5 delivering All Workers reported in, starting next task. Thread #2 will work for 484 msec. Thread #4 will work for 836 msec. Thread #5 will work for 464 msec. Thread #3 will work for 251 msec. Thread #1 will work for 734 msec. Thread #3 delivering Thread #5 delivering Thread #2 delivering Thread #1 delivering Thread #4 delivering All Workers reported in, starting next task. Thread #3 will work for 864 msec. Thread #1 will work for 526 msec. Thread #5 will work for 145 msec. Thread #2 will work for 762 msec. Thread #4 will work for 283 msec. Thread #5 delivering Thread #4 delivering Thread #1 delivering Thread #2 delivering Thread #3 delivering All Workers reported in, starting next task. Thread #2 will work for 329 msec. Thread #4 will work for 452 msec. Thread #1 will work for 176 msec. Thread #5 will work for 702 msec. Thread #3 will work for 500 msec. Thread #1 delivering Thread #2 delivering Thread #4 delivering Thread #3 delivering Thread #5 delivering All Workers reported in, starting next task. Thread #5 will work for 681 msec. Thread #3 will work for 71 msec. Thread #2 will work for 267 msec. Thread #1 will work for 151 msec. Thread #4 will work for 252 msec. Thread #3 delivering Thread #1 delivering Thread #4 delivering Thread #2 delivering Thread #5 delivering All Workers reported in, starting next task. Thread #5 will work for 963 msec. Thread #3 will work for 378 msec. Thread #1 will work for 209 msec. Thread #4 will work for 897 msec. Thread #2 will work for 736 msec. Thread #1 delivering Thread #3 delivering Thread #2 delivering Thread #5 delivering Thread #4 delivering All Workers reported in, starting next task. Thread #2 will work for 44 msec. Thread #4 will work for 973 msec. Thread #1 will work for 700 msec. Thread #3 will work for 505 msec. Thread #5 will work for 256 msec. Thread #2 delivering Thread #5 delivering Thread #3 delivering Thread #1 delivering Thread #4 delivering All Workers reported in, starting next task. Thread #2 will work for 703 msec. Thread #4 will work for 296 msec. Thread #1 will work for 702 msec. Thread #3 will work for 99 msec. Thread #5 will work for 114 msec. Thread #3 delivering Thread #5 delivering Thread #4 delivering Thread #1 delivering Thread #2 delivering All Workers reported in, starting next task. Thread #3 will work for 97 msec. Thread #5 will work for 192 msec. Thread #2 will work for 762 msec. Thread #1 will work for 232 msec. Thread #4 will work for 484 msec. Thread #3 delivering Thread #5 delivering Thread #1 delivering Thread #4 delivering Thread #2 delivering All Workers reported in, starting next task. Thread #1 will work for 790 msec. Thread #5 will work for 602 msec. Thread #3 will work for 105 msec. Thread #2 will work for 449 msec. Thread #4 will work for 180 msec. Thread #3 delivering Thread #4 delivering Thread #2 delivering Thread #2 is done. Thread #4 is done. Thread #3 is done. Thread #5 delivering Thread #5 is done. Thread #1 delivering Thread #1 is done. Press ENTER to exit
Tcl
This implementation works by having a separate thread handle the synchronization (inter-thread message delivery already being serialized). The alternative, using a read-write mutex, is more complex and more likely to run into trouble with multi-core machines. <lang tcl>package require Tcl 8.5 package require Thread
namespace eval checkpoint {
namespace export {[a-z]*}
namespace ensemble create
variable members {}
variable waiting {}
variable event
# Back-end of join operation
proc Join {id} {
variable members variable counter if {$id ni $members} { lappend members $id } return $id
}
# Back-end of leave operation
proc Leave {id} {
variable members set idx [lsearch -exact $members $id] if {$idx > -1} { set members [lreplace $members $idx $idx] variable event if {![info exists event]} { set event [after idle ::checkpoint::Release] } } return
}
# Back-end of deliver operation
proc Deliver {id} {
variable waiting lappend waiting $id
variable event if {![info exists event]} { set event [after idle ::checkpoint::Release] } return
}
# Releasing is done as an "idle" action to prevent deadlocks
proc Release {} {
variable members variable waiting variable event unset event if {[llength $members] != [llength $waiting]} return set w $waiting set waiting {} foreach id $w { thread::send -async $id {incr ::checkpoint::Delivered} }
}
# Make a thread and attach it to the public API of the checkpoint proc makeThread Template:Script "" {
set id [thread::create thread::wait] thread::send $id { namespace eval checkpoint { namespace export {[a-z]*} namespace ensemble create
# Call to actually join the checkpoint group proc join {} { variable checkpoint thread::send $checkpoint [list \ ::checkpoint::Join [thread::id]] } # Call to actually leave the checkpoint group proc leave {} { variable checkpoint thread::send $checkpoint [list \ ::checkpoint::Leave [thread::id]] } # Call to wait for checkpoint synchronization proc deliver {} { variable checkpoint # Do this from within the [vwait] to ensure that we're already waiting after 0 [list thread::send $checkpoint [list \ ::checkpoint::Deliver [thread::id]]] vwait ::checkpoint::Delivered } } } thread::send $id [list set ::checkpoint::checkpoint [thread::id]] thread::send $id $script return $id
}
# Utility to help determine whether the checkpoint is in use
proc anyJoined {} {
variable members expr {[llength $members] > 0}
}
}</lang> Demonstration of how this works.
<lang tcl># Build the workers foreach worker {A B C D} {
dict set ids $worker [checkpoint makeThread {
proc task {name} { checkpoint join set deadline [expr {[clock seconds] + 2}] while {[clock seconds] <= $deadline} { puts "$name is working" after [expr {int(500 * rand())}] puts "$name is ready" checkpoint deliver } checkpoint leave thread::release; # Ask the thread to finish }
}]
}
- Set them all processing in the background
dict for {name id} $ids {
thread::send -async $id "task $name"
}
- Wait until all tasks are done (i.e., they have unregistered)
while 1 {
after 100 set s 1; vwait s; # Process events for 100ms
if {![checkpoint anyJoined]} {
break
}
}</lang> Output:
A is working C is working B is working D is working B is ready A is ready D is ready C is ready B is working A is working D is working C is working D is ready A is ready C is ready B is ready B is working D is working A is working C is working D is ready C is ready B is ready A is ready D is working C is working B is working A is working D is ready A is ready C is ready B is ready D is working C is working A is working B is working C is ready A is ready B is ready D is ready