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SYNCHRONISATION
HARDWARETARUN KUMAR [email protected]
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CS Problem DynamicsWhen a process executes code that manipulates shareddata (or resource), we say that the process is in it’s CriticalSection (for that shared data).The execution of critical sections must be mutuallyexclusive: at any time, only one process is allowed toexecute in its critical section (even with multiple processors).So each process must first request permission to enter itscritical section.
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CS Problem Dynamics (cont..)
The section of code implementing this request is called theEntry Section (ES).
The critical section (CS) might be followed by a Leave/ExitSection (LS).
The remaining code is the Remainder Section (RS).
The critical section problem is to design a protocol that theprocesses can use so that their action will not depend onthe order in which their execution is interleaved (possibly on
many processors).
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General structure of
process P i (other is P j )do {critical section
remainder section} while (TRUE);
Processes may share some common variables to synchronize
their actions.
entry section
leave section
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Solution to CS Problem
There are 3 requirements that must stand for a correctsolution:
1. Mutual Exclusion
2. Progress
3. Bounded WaitingWe can check on all three requirements in each proposedsolution, even though the non-existence of each one ofthem is enough for an incorrect solution.
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Mutual Exclusion
If process P i is executing in its critical section, then no otherprocesses can be executing in their critical sections.
Implications:
Critical sections better be focused and short.
Better not get into an infinite loop in there.If a process somehow halts/waits in its critical section, itmust not interfere with other processes.
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Progress
If no process is executing in its critical section andthere exist some processes that wish to enter theircritical section, then the selection of the process thatwill enter the critical section next cannot be postponedindefinitely:
• If only one process wants to enter, it should beable to.
• If two or more want to enter, one of them shouldsucceed.
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Bounded Waiting
A bound must exist on the number of times that otherprocesses are allowed to enter their critical sectionsafter a process has made a request to enter its criticalsection and before that request is granted.
• Assume that each process executes at a nonzero speed.
• No assumption concerning relative speed of then processes.
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Types of solutions to CS Problem
Software solutions –• algorithms who’s correctness does not rely on any other
assumptions.Hardware solutions –• rely on some special machine instructions.
Operating System solutions –• provide some functions and data structures to the
programmer through system/library calls.Programming Language solutions –
• Linguistic constructs provided as part of a language.
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Synchronization Hardware
Many systems provide hardware support for criticalsection codeUniprocessors – could disable interrupts
Currently running code would execute withoutpreemptionGenerally too inefficient on multiprocessorsystems
• Operating systems using this not broadlyscalable
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Synchronization Hardware (c
Modern machines provide special atomic hardwareinstructions
• Atomic = non-interruptibleEither test memory word and set value
Or swap contents of two memory words
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Solution using TestAndSet()
Shared Boolean variable lock, initialized to false.Solution:
do {while ( TestAndSet (&lock ))
; // do nothing// critical section
lock = FALSE;// remainder section
} while (TRUE);
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Solution using Swap()Shared Boolean variable lock initialized to FALSE ; Eachprocess has a local Boolean variable keySolution:
do {key = TRUE;while ( key == TRUE)
Swap (&lock, &key );// critical section
lock = FALSE;// remainder section
} while (TRUE);
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Bounded-waiting Mutual
Exclusion with TestandSet()do {waiting[i] = TRUE;key = TRUE;
while (waiting[i] && key)key = TestAndSet(&lock);
waiting[i] = FALSE;// critical section
j = (i + 1) % n;
while ((j != i) && !waiti j = (j + 1) % n;
if (j == i)
lock = FALSE;else
waiting[j] = FALSE;// remainder section
} while (TRUE);
