Hardware Support for Mutual Exclusion

7/31/2019 Hardware Support for Mutual Exclusion

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Hardware supports for mutual

Exclusion

Lince Sebastian

S4Mca

Roll: 26


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Pessimistic approaches

Optimistic approaches


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Pessimistic approaches

It tends to assume worst possible case and to

defend against it by rather austere measures

that often end up limiting concurrency


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Pessimistic Solution

Block everything that could possibly interfere,

so their should nothing to interfere..

Update the global variable

Unblock


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Optimistic approaches

Optimistic approaches are based on

assumption that no or few conflicts are likely

to experienced by any particular user of

shared recourse


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Optimistic Solution

Read the value of the global variable andprepare the tentative local update based onthat value the global variable remains

accessible to others users Compare the current value of global variable

to the value used to prepare the tentativeupdate. If value of global variable isunchanged, the tentative local update toglobal variable , discard the tentative update..


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Disable/ enable interrupt (P)

The basic idea is quite simple ..it follows the

principle that shared resources must be

exclusively obtained by the process wishing to

enter its critical section subsequently released

for the use of others


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while (true) {

/* disable interrupts */

/* critical section *//* enable interrupts */

/* remainder */

}


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Test and set instruction(P)

In this assigning the global variable to FREE

(0)when the guarded shared resource is

available

Check the status of global variable.

If s=0, then resource is available

If s=1, denotes busy state.

Initially s will be 0


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The function of the test and set instruction may be presented as the

following function:

boolean testset (int i){

if (i == 0)

{

i = 1;return true;

}

else

{

return false;

}

}


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Compare and swap instruction(o)

Updating the global variable in the presence

of concurrent activity and without any support

from OS or the interrupt mechanism.

Here copying the value of global variable to

local space and using the value to calculate

the tentative update.


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The value of global variable is not disturbed by

any other process during the calculation of the

update, the tentative update is applied to the

global variable and thus made it permanent.

Independent from interrupt mechanism .

Can used in multiprocessor environment.

Running process is always capable of making

progress towards its completion.


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No Claps please

Thank you

Hardware Support for Mutual Exclusion

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