Scheduler Activations :
Effective Kernel Support for the User-Level Management of
Parallelism
陳建村
[email protected]
Reference
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Anderson,T.,Bershad,B.,Lazowska,E.,and Levy,H.,"Scheduler
Activations:Effective Kernel Support for the User-Level
Management of Parallelism", ACM Transactions on Computer
Systems,Vol.10,.No.1,February 1992,pp.53-79.
Outline
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Motivation
Solution
Example
Performance
Conclusion
Q&A
Motivation
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User-level threads
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good performance and flexibility
Thread behavior can be distorted by OS activity
(scheduling, I/O, etc)
Kernel threads
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Worse performance and poor flexibility but not as
restricted
Motivation (cont.)
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Kernel thread used by programmer
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Poor performance and poor flexibility
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Cost of accessing thread management operation
The cost of generality
User-level thread on top of kernel thread
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Poor behavior, because I/O, multiprogramming
=> SOLVE THIS PROGRMAS
Motivation (cont.)
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Source of poor integration user thread & kernel
thread
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Kernel threads block, resume, and are preempted without notification
to the user level
Kernel threads are scheduled obliviously with respect to the user-level
thread
Solve
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New kernel interface between kernel and user-level thread
Scheduler Activation
Solution
Design goal
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Combine the functionality of the kernel threads
with performance and flexibility of user-level
thread
The difficulty in achieving the goals
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The necessary control and scheduling
information is distributed between the kernel
and each application’s address space
The Approach
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Scheduler activations
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It serves as a execution context, for running userlevel threads, in exactly the same way that a kernel
thread does
It notifies the user-level thread system of a kernel
event
It provides space in the kernel for saving the
processor context of the activation’s current userlevel thread, when the thread is stopped by the
kernel
Key Characteristics of Scheduler
Activations
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Processors are allocated to job’s by the kernel
A user-level thread scheduler controls which
threads run on a job’s allocated processors
The user-level notifies the kernel of changing
demand for processors
The kernel notifies the user-level scheduler of
system events that affect the job
Key Characteristics of Scheduler
Activations
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The kernel never time-slices scheduler
activations
Application programs can remain unmodified
Scheduler Activation
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Vectored events cause user-level scheduler to
reconsider its scheduling
Execution context for running user threads
Notify the user-level thread of a kernel events – upcall
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Add this processor
Processor has been preempted
Scheduler activation has blocked
Scheduler activation has unblocked
Scheduler Activation (cont.)
Create new
scheduler activation
Upcall
by using
new scheduler activation
Kernel Event (above list)
User-level
scheduler decide
Scheduler Activation (cont.)
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Notifying the kernel of user-level events
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Small subsets of user-level operation affects
Processor Allocation
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Add more processors
This processors is idle
Scheduler Activation (cont.)
Critical SectionRecovery method used
2-2
Put thread back
on the ready list
no
Check
is thread
in critical section ?
yes
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4
upcall
0
2-1
Temporarily
continue via user-level
context switch
If complete,relinquish control back
3
Example : I/O request/completion
Performance
Thread Operation Latencies (μsec)
Speedup of N-Body application
versus number of processors,
100% of memory available
Execution time of N-Body
application versus amount of
available memory, 6 processor
Speedup of N-Body application,
Multiprogramming Level = 2, 6
Processors, 100% of memory available
Conclusion
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It solve user-level thread and kernel thread integration
problems by new kernel interface – scheduler
activation
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Processor allocation is done by kernel
Thread scheduling is done by each address space
The kernel notifies the address space thread scheduler of
every event affecting the address space
The address space notifies the kernel of the sub set of userlevel events that can affect processor allocation decisions
Q&A
Note
User-Level Threads On Top of
Scheduler Activations
user-level threads
user-level scheduling
blocked
active
user
kernel
scheduler activation
kernel-level scheduling
physical processor
blocked active