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Cooperative Task Management without Manual Stack Management
or, Even-driven Programming is Not the Opposite of Threaded Programming
A. Adya, J. Howell, M. Theimer, W. Bolosky and J. Douceur
Presented by Will Landecker
1
Outline
● Introduction: threads vs events?● Task management● Stack management● Hybrid stack management● Conclusions
2
Threads vs. events?
Events
● Explicit yielding
● Asynchronous I/O
– No blocking
● Continuations
Threads
● Preemptive scheduling
● Synchronous I/O
– Blocking
● Save state, resume automatically
3
Threads vs. events?
Events
● Explicit yielding
● Asynchronous I/O
– No blocking
● Continuations
Threads
● Preemptive scheduling
● Synchronous I/O
– Blocking
● Save state, resume automatically
4
Threads vs. events?
Events
● Explicit yielding
● Asynchronous I/O
– No blocking
● Continuations
Threads
● Preemptive scheduling
● Synchronous I/O
– Blocking
● Save state, resume automatically
Task management
Stack management
5
Outline
● Introduction: threads vs events?● Task management● Stack management● Hybrid stack management● Conclusions
6
Task management● A task encapsulates a control flow● Preemptive
● Serial
● Cooperative
time
A B A C B ...
time
A CB ...
time
A CB ...B
YIE
LD!
7
Task management● A task encapsulates a control flow● Preemptive
– Processes interleave / overlap on uni/multiprocessor– Shared state, need synchronization / protection– Synchronous I/O
● Serial
● Cooperative
time
A CB ...
time
A CB ...B
YIE
LD!
8
Task management● A task encapsulates a control flow● Preemptive
– Processes interleave / overlap on uni/multiprocessor– Shared state, need synchronization / protection– Synchronous I/O
● Serial– Task runs to completion– Easier to guarantee integrity of data– I/O must be asynchronous
● Cooperative time
A CB ...B
YIE
LD!
9
Task management● A task encapsulates a control flow● Preemptive
– Processes interleave / overlap on uni/multiprocessor– Shared state, need synchronization / protection– Synchronous I/O
● Serial– Task runs to completion– Easier to guarantee integrity of data– I/O must be asynchronous
● Cooperative– Explicit yield: similar guarantees about data integrity– I/O must still be asynchronous
10
Outline
● Introduction: threads vs events?● Task management● Stack management● Hybrid stack management● Conclusions
11
Stack management
● Manual stack management– Rip tasks into pseudo-atomic event handlers– Explicitly save task state to heap– Messages, continuations– Code becomes difficult to maintain and read as
software evolves
● Automatic stack management– 1 task 1 procedure– Can ignore calls to I/O– Control returned automatically– Maintainable, readable code
12
Outline
● Introduction: threads vs events?● Task management● Stack management● Hybrid stack management● Conclusions
13
Hybrid stack management
Claim: combine maintainability and readability of automatic stack management with reasoning benefits of cooperative task management.
In reality: New system allows both manual and automatic stack management.
Task management
Sta
ck m
anag
emen
t
cooperative preemptive
man
ual
auto
mat
ic
event-driven
multithreaded“sweet spot”
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Hybrid stack management
New system allows both manual and automatic stack management. What could go wrong?
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Hybrid stack management
New system allows both manual and automatic stack management. What could go wrong?
Manual calling automatic
Given a manual stack management procedure A, and an automatic stack management procedure B.
A calls B, B blocks on I/O.
A
B
… I/O ...
B
A
time
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Hybrid stack management
Fibers (Windows) are scheduled cooperatively.
Adaptors switch between fibers.
Main fiber runs a scheduler. Procedures expecting automatic stack management must run on a different fiber.
Each code style (manual vs. automatic stack management) is unaware of the other.
B
… I/O ...
B
time
A
A
Main fiber otherFiber
C
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Hybrid stack management
Automatic calling manual
Given a manual stack management procedure A, and an automatic stack management procedure B.
B calls A, A requests I/O.
A
B
B
time
A
… I/O ...
B
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Hybrid stack management
Solution:
Execute B and A on a new fiber.
A sees asynchronous I/O. Scheduler ensures B only gets one return.
A
B
B
Main fiber
otherFiberSched.
Sched.
Sched.
A
I/Orequest
I/Ocomplete
19
Outline
● Introduction: threads vs events?● Task management● Stack management● Hybrid stack management● Conclusions
20
Conclusions
● Threads vs events? Or stack management vs task management?
● Cooperative task management is good for reasoning about concurrency.
● Manual stack management can be cumbersome.
● A “hybrid” stack management system uses fibers to allow both manual and automatic stack management.
Task management
Sta
ck m
anag
emen
tcooperative preemptive
man
ual
auto
mat
ic
event-driven
multithreaded“sweet spot”
21
