Operating Systems, 2011, Danny Hendler & Amnon Meisels 1 Distributed Mutual Exclusion ...

Operating Systems, 2011, Danny Hendler & Amnon Meisels 1

Distributed Mutual Exclusion

Introduction

Ricart and Agrawala's algorithm

Raymond's algorithm

This presentation is based on the book: “Distributed operating-systems & algorithms” by Randy Chow and Theodore Johnson

2Operating Systems, 2011, Danny Hendler & Amnon Meisels

Distributed mutual exclusion required (e.g.) for transaction processing on replicated data

We assume there are no failureso Processors do not failo Communication links do not fail

It is easy to implement mutual exclusion using totally-ordered timestampso The first algorithm we show may use Lamport's timestamps

Distributed mutual exclusion: introduction

Ricart and Agrawal's algorithm: high-level ideas

When you want to enter your CSo Record your timestampo Ask everyone else whether they “permit”

When asked for a permissiono Halt response if in CSo Halt response if in entry code with a smaller timestamp

(we need total order between timestamps)o Otherwise, “permit”

Upon exit from CSo Send halted responses (if any)

Ricart and Agrawal's algorithm: data-structures

Per processor variables

timestamp current_timeTimestamp my_timestampinteger reply_countboolean isRequestingboolean reply_deferred[M]

Processor’s current Lamport timestamp

The timestamp of the processor’s current request

timestamp current_timeTimestamp my_timestampinteger reply_countboolean isRequestingboolean reply_deferred[M] The number of permissions

that the processor still need to collect before entering the CS

True iff this processor is requesting or using the CS

Entry j is true iff this processor deferred replying to processor j’s request

Ricart and Agrawal's algorithm: entry-codeRequest_CS:1 my_timstamp current_time2 isRequesting TRUE3 Reply_count M-14 for every other processor j5 send(REMOTE_REQUEST; my_timestamp)6 wait until reply_count = 0

Set the (Lamport) timestamp of my request

Mark that this processor is requesting entry to CS

Need to receive replies from all other processors

Request permission from all other processors

Ricart and Agrawal's algorithm: entry codeRequest_CS:1 my_timstamp current_time2 isRequesting TRUE3 Reply_count M-14 for every other processor j5 send(REMOTE_REQUEST; my_timestamp)6 wait until reply_count = 0

When all other processors reply – may enter the CS

Ricart and Agrawal's algorithm: monitoringCS_monitoring:Wait until a REMOTE_REUQUEST or REPLY message is received

REMOTE_REQUEST(sender; request_time)1. Let j be the sender of the REMOTE_REQUEST message2. if (not is_requesting or my_timestamp > request_time)3. send(j, REPLY)4. else5. reply_deferred[j]=TRUE

REPLY4. reply_count reply_count-1

Listener thread to respond to protocol messages at all times

Ricart and Agrawal's algorithm: monitoring

Upon receipt of remote request

CS_monitoring:Wait until a REMOTE_REUQUEST or REPLY message is received

If should grant processor j’th request

Send reply to processor j

Otherwise, defer replying to this request

Upon receiving a reply, decrement reply_count

Ricart and Agrawal's algorithm: exit sectionRelease_CS_monitoring:1. is_requesting false2. For j=1 through M (other than this processor's ID)3. if reply_deferred[i]=TRUE4. send(j, REPLY)5. reply_deferred[j]=FALSE

No longer requesting CS

Ricart and Agrawal's algorithm: exit sectionRelease_CS_monitoring:1. is_requesting false2. For j=1 through M (other than this processor's ID)3. if reply_deferred[i]=TRUE4. send(j, REPLY)5. reply_deferred[j]=FALSE

For each processor awaiting a reply from this processor, send reply and mark that there are no more deferred replies.

Ricart and Agrawal's algorithm: comments

What is the number of messages required for each

passage through the critical section?2(M-1) messages.

Let's see a more message-efficient algorithm…

Distributed Mutual Exclusion

Introduction

Ricart and Agrawala's algorithm

Raymond's algorithm

Raymond's algorithm: high-level ideas

There is a single token in the systemo Only the holder of the token may enter the CS

Processors communicate by using a static tree structureo Requests for the token are sento The token itself is sent when available and requested

Processors maintain FIFO request queues to prevent starvation

If processors request entry “one at a time” – at most a logarithmic number of messages per entryo When there are many simultaneous requests, the number of messages

per entry typically decreases

Raymond's algorithm: high-level ideas (cont'd) Algorithm invariant: tree is always oriented towards token

holder

Token holder

Raymond's algorithm: data-structures

Boolean token_holderBoolean inCScurrent_dirrequests_queue

True iff this processor currently holds the token

Boolean token_holderBoolean inCScurrent_dirrequests_queue True iff this processor is

currently in the critical section

The neighbor that is in the direction of the token (or self if this processor holds the token)

FIFO queue holding IDs of neighbors from which requests for the token arrived (may also contain self)

Raymond's algorithm: entry and exit codeRequest_CS:1 If not token_holder2 if requests_queue.isEmpty( )3 send(current_dir, REQUEST)4 requests_queue.enqueue(self)5 wait until token_holder is true6 inCS true

Release_CS:7. inCS false8. If not requests_queue.isEmpty( )9. current_dir requests_queue.dequeue( )10. send(current_dir, TOKEN)11. token_holder false12. if not requests_queue.isEmpty( )13. send(current_dir, REQUEST)

If this processor currently holds the token it immediately enters CS. Otherwise…

If requests queue is empty, send a request for the token. (If queue is non-empty, a request for the token was already sent.)

Enqueue ‘self’ to requests queue since this request is on behalf of this processor

When token_holder is set, this processor has the token and may enter the CS

No longer in critical section

If requests are waiting…

Dequeue the next hop for the earliest request and send the TOKEN to it. Also, update orientation of the token.

Release_CS:7. inCS false8. If not requests_queue.isEmpty( )9. current_dir requests_queue.dequeue( )10. send(current_dir, TOKEN)11. token_holder false12. if not requests_queue.isEmpty( )13. send(current_dir, REQUEST) This processor no longer holds token

If there are more requests in this processor’s queue, send another request for the token

Raymond's algorithm: monitoringMonitor_CS:1 while (true)2 wait for a REQUEST or a TOKEN message REQUEST3. if token_holder4. if inCS5. requests_queue.enqueue(sender)6. else7. current_dir sender8. send(current_dir, TOKEN)9. token_holder false10. else11. if requests_queue.isEmpty()12. send(current_dir,REQUEST)13. requests_queue.enqueue(sender) TOKEN14. current_dir requests_queue.dequeue( )15. if current_dir = self16. token_holder true17. else18. send(current_dir, TOKEN)19. if not requests_queue.isEmpty( )20. send(current_dir, REQUEST)

Listener thread to respond to protocol messages at all times

Upon a request.If current processor holds token…

If current processor in CS then request must wait, enqueue the direction of requesting processor

Otherwise current processor holds the token but is not in CS, hence requests queue is empty.

Send token to where the request came from, mark that current processor no longer holds token, and the new orientation of the otken…

Otherwise current processor does not hold the token…

If requests queue is empty, send request in the direction of the token…

Enqueue the direction of this request…

Upon the arrival of the token…

Dequeue oldest request and set new orientation of the token to its direction

If request was by this processor, mark that it currently has the token and may enter the CS

Otherwise, send the token in the direction of the request

If the queue is non-empty, send another request for the token

Raymond's algorithm: execution scenario

Operating Systems, 2011, Danny Hendler & Amnon Meisels 1 Distributed Mutual Exclusion ...

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