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Mel Write Attributes
The other side of a mel read which we explore in the previous chapter, is a mel write. Let's consider these two mel write statements:
A mel write statement is not only about accessing the remote mel element, but also about waiting for the task's turn to access in one of the writer's queues, then waiting for an empty slot in the mel buffer from the writer's end, and finally depositing the new value to that available slot. The last action will allow any reader task that is waiting for the mel element to be filled, to read out the new value and make the mel buffer slot available again for a new write.
Instead of depositing a new value, a writer task can overwrite the last written (and still un-read) value in the mel buffer, by using the
Mel Writer Mode
A mel write operation accesses a mel variable at the writer's end of the mel buffer. A fully descriptive mel write operation specifies the
Mel Write Waits
In the following Producer/Consumer example, the
The purpose of the
Mel Write Process
This is the behind-the-scene process for the statement
Mel Write Conditions
As a summary, the following conditions must be met before a task can write in a mel value:
The second condition is relevant when there are multiple queues vying for access to the same mel element. A queue with a higher priority is granted access to the mel element more often than lower priority queues.
The third condition is relevant when the mel element is full. The writer task must wait for a reader task to remove a value in the mel buffer to make room for the deposit.
Mel Write Buffers
The purpose of a mel write request is to deposit a new value to the mel element. If the mel element is not buffered -- which is the default for a mel declaration, the writer task has to wait for a reader task to empty the mel element to make room. If the mel element is buffered, the writer task can deposit into an empty slot in the buffer until the buffer is full.
The availability of a mel buffer differentiates a mel write from a mel read. A read can only work with only the top value of a mel. This also how a write works if the mel is not buffered. If a programmer buffers a mel variable, a writer can deposit in the empty slots of the buffer without waiting for a reader to empty the top slot. With enough resources assigned to the buffer, a mel write wait can be greatly minimized.
Mel Writeover Mode
In addition to the default
- Mel Write Attributes
- Mel Writer Mode
- Mel Write Waits
- Mel Writeover Mode
- Mel OR Writes
- Mel AND Writes
- Mel LIST Writes
Mel Write Attributes
The other side of a mel read which we explore in the previous chapter, is a mel write. Let's consider these two mel write statements:
<?>store1 = c1;
<? priority timeout mode="writeover">store2 = c2;
| Operation | Attributes | Synopsis |
|---|---|---|
<?>store = c |
Mel write operation on the mel variable store |
|
priority=n |
Access the mel element for writing with priority n | |
timeout=msec |
Wait for the mel buffer to have an empty slot for writing for msec milliseconds before aborting | |
mode=setting |
Request write access to the mel with this mode setting:
writer: request a write access (default)
writeover: request an overwrite access
|
|
method=<keyword> |
Method used for mel multiple writes: mel OR writes and mel AND writes | |
A mel write statement is not only about accessing the remote mel element, but also about waiting for the task's turn to access in one of the writer's queues, then waiting for an empty slot in the mel buffer from the writer's end, and finally depositing the new value to that available slot. The last action will allow any reader task that is waiting for the mel element to be filled, to read out the new value and make the mel buffer slot available again for a new write.
Instead of depositing a new value, a writer task can overwrite the last written (and still un-read) value in the mel buffer, by using the
mode attribute writeover. This feature can be used by a producing task to keep improving the interim result until a reader task takes it away.
Mel Writer Mode
A mel write operation accesses a mel variable at the writer's end of the mel buffer. A fully descriptive mel write operation specifies the
writer mel buffer access mode. For example to write to the mel variable store the value of the local variable c:
<? mode=writer> store = c;mode keyword can always be omitted:
<?writer> store = c;writer can also be omitted:
<?> store = c;= operator, a mel write operation can be identified at compile time without the verbose use of the writer mel buffer access mode. However, when we come upon mel writer zones in the future, we'll see the writer mode is necessary to differentiate them from mel reader zones.
Mel Write Waits
In the following Producer/Consumer example, the
Producer task makes use of the mel write access:
main () {
<mel> int store;
<!> Producer (store);
<!> Consumer (store);
}
void Producer (<mel> int store) {
while ( 1 ) {
try { <?>store = Produce(); }
catch ( store<TIMEDOUT> ) {
printf ("Timeout - try again");
<resume>;
}
catch ( store<CLOSED> ) {
printf ("Store closed - get out");
break;
}
catch ( store<...> ) {
printf ("Unexpected error [%d] due to [%s]", store<error>, store<why>);
break;
}
}
}
void Consumer (<mel> int store) {
int maxconsump = 500;
while ( maxconsump-- ) {
Consume(<?>store);
}
<close>store;
}
Consumer task only Consumes 500 items. Then it ends the while loop, closes the mel store, and ends itself.
The purpose of the
Producer task is to keep Produce'ing the items for Consumer until the latter is done with consumption.
Mel Write Process
This is the behind-the-scene process for the statement
<? timeout>store = Produce();- The
ProducertaskProduces a new product and puts itself into one of many possible writers' queues of the melstore.
- The writers' queue
Producerselects to stand in line is based on the priority of the mel write operation. Here, no priority attribute is explicit so the default writers' queue is used.
- If there are other writer tasks in the queue, the
Producertask moves up the queue as the front tasks get off the queue, until it reaches the top position of the queue.
- At the top position, the
Producerwaits for the queue to have WRITE access to the melstore. In this example,Produceris the only writer task, there is no vying for the access from higher priority queues. Therefore, although the default writers' queue has the lowest priority to write to the mel buffer, it is granted WRITE access.
- If the mel buffer is vacant, the
Producertask will deposit its product into the mel buffer from the writer's end, increases the sequence number, and gets off the melstoredefault writers' queue. The mel buffer is now filled. The slot with the new product eventually moves up the mel buffer to the top position on the reader's end and becomes available for retrieval by theConsumerreader task.
- If the mel buffer is fullProducer keeps the top position in the queue and waits until the mel buffer of
storebecomes vaant. Then it processes as above.
- If
Producercannot write its product before the timeout, theTIMEDOUTexception is raised.Producerwill get off the melstorewriters' queue, and processes the<resume>operation to try to deposit again.If we were using
continueinstead of <resume>, we would loop back thewhileloop, which would cause a new product to beProduced. The product that got timed out would be lost. If the logic of the program were to skip any timed out item (perhaps because it had become obsolete), thencontinuewould be the right choice. - If
Consumerhas processed500items, it will close the mel elementstore. On the next iteration of thewhileloop, theProducerwill get hit with theCLOSEDexception, when it tries to write to the melstore.
- When the
Producerreceives theCLOSEDexception, it will get off the melstorewriters' queue, and processes theCLOSEDcatchhandler. Here, itbreaks out of thewhileloop, and with nothing else to do, it will end itself.
store, the Producer detects an exception other than TIMEDOUT or CLOSED, it will get off the mel writers' queue, and jumps to the catch-all handler:
catch ( store<...> )Unexpected exception statement with the error code and cause of the exception, and then breaks off the while loop. Without the catch-all handler for store, any unexpected and thus unprocessed exception from store will cause the Producer task to abort abruptly, instead of ending graciously.
Mel Write Conditions
As a summary, the following conditions must be met before a task can write in a mel value:
- The task is at the top position of a writers' queue
- The writers' queue has been granted WRITE access to the mel element
- The mel element must have a vacant slot for the task to deposit its product
The second condition is relevant when there are multiple queues vying for access to the same mel element. A queue with a higher priority is granted access to the mel element more often than lower priority queues.
The third condition is relevant when the mel element is full. The writer task must wait for a reader task to remove a value in the mel buffer to make room for the deposit.
Mel Write Buffers
The purpose of a mel write request is to deposit a new value to the mel element. If the mel element is not buffered -- which is the default for a mel declaration, the writer task has to wait for a reader task to empty the mel element to make room. If the mel element is buffered, the writer task can deposit into an empty slot in the buffer until the buffer is full.
The availability of a mel buffer differentiates a mel write from a mel read. A read can only work with only the top value of a mel. This also how a write works if the mel is not buffered. If a programmer buffers a mel variable, a writer can deposit in the empty slots of the buffer without waiting for a reader to empty the top slot. With enough resources assigned to the buffer, a mel write wait can be greatly minimized.
Note that we have been using the term mel buffer even on mel elements that are not buffered. In these cases the mel buffer is considered to be of size one (i.e. having one slot).
Mel Writeover Mode
In addition to the default
writer mode which requires the task to wait for a vacant mel buffer, the mel write operation supports the writeover mode that does not require such wait. This mode behaves like the writer mode if the mel buffer is vacant, but if the buffer is full or filled, it will overwrite the last written item without looking for an empty slot.
Let's create a new Producer that creates a "good-enough" product first for impatient consumers. It then produces a "better" product to replace the "good-enough" product if the latter has not been retrieved by any consumer.
void Producer (<mel> int store) {
int c;
while ( 1 ) {
if ( c = ProduceGood() ) break;
<?>store = c;
if ( c = ProduceBetter() ) break;
<? mode=writeover>store = c;
}
<close>store;
}
The mode keyword is optional. The Producer could also make this statement:
<?writeover>store = c;
The above Producer makes two write operations to the mel variable store. The first one is a normal write operation using the default writer mode where it has to wait for a vacant slot in the mel buffer. The second write overwrites the value from the first write if it has not been picked up by a consumer; otherwise it just deposits the "better" product for the next consumer.
This is the behind-the-scene processing of a generic task doing a writeover to a mel element. The first steps are the same as those for a writer mode:
- The task generates a new product and puts itself into one of many possible writers' queues of the mel element.
- The writers' queue the task selects to stand in line is based on the priority of the mel write operation. If no priority attribute is explicit, the task joins the default writers' queue.
- If there are other writer tasks in the queue, the task stays in the queue, and moves up to the front of the queue as the tasks in front of it get off the queue.
- At the front position, the task waits for the queue to have WRITE access to the mel buffer due to competition from higher priority queues, if any.
- After its queue is granted WRITE access, the task checks for the status of the mel buffer. This is where the writeovr mode can differ from the default writer mode:
- If the mel buffer is empty (i.e., all mel buffer slots are vacant), the task deposits its product at the writer's end of the mel buffer. This is the same as in a writer mode.
- If the mel buffer filled (i.e., some slots are vacant and some not), the task overwrites the last written slot with its own product. This differs from the writer mode where the task deposits its product on an available vacant slot.
- If the mel buffer is full (i.e., none of the slots are vacant), the task overwrites the last written slot with its own product. This differs from the writer mode where the task waits for a vacant slot to be made available.
- The task increases the sequence number of the deposited or overwritten item, and gets off the mel queue with no error reported.
- If there is an error during the writeover process, the task will get off the mel queue without making any new deposit or overwrite in the mel buffer, and raises an exception.
The above Producer attempts to overwrite a "good-enough" product with a "better" product, but a fast Consumer task can still retrieve the "good-enough" product before Producer has time to overwrite it. In addition, if there is another writer task that inserts its own product right after Producer writes in its "good-enough" product, the value that Producer overwrites with its "better" product will be of the other writer task since this value is the last written value.
There are several uses of a writeover mode in NERWous C:
- Ad-hoc use like in the above example, to fit the logical need of a program
- As the write mode for a read-only mel
- As the write mode for a release statement
- As the write mode for exclusive zones
Mel OR Writes
Like the mel OR read, there is a corresponding mel OR write. Let's consider a Producer task that can send its product to either store1 or store2 mel elements, depending on which one is available to receive the new value. There are two methods that such a task can use to traverse the mel elements:
Mel OR Write Methods Method Value Description Serial The task checks the mel elements serially based on the order in the OR list. Random This is the default method when no method is specified. A random generator dictates which mel element in the OR list is checked first. The task then circulates through the remaining mel elements in the list. This method is more compute intensive but gives a fairer chance to all mel elements in the OR list.
Serial OR Writes
Let's start with a Producer that uses the Serial method:
void Producer (<mel> int store1, <mel> int store2) {
while ( <? method=serial>(store1 || store2) = Produce() );
<close>store1;
<close>store2;
}
The keyword method can be omitted:
while ( <? serial>(store1 || store2) = Produce() );
This is the behind-the-scene process of the Serial mel OR write statement:
<? serial>(store1 || store2) = Produce()
- The
Producer task Produces a product, and puts itself onto store1 default writers' queue.
- If the mel WRITE conditions are satisfied on
store1, Producer will deposit its product into store1 buffer at the writer's end, increases the sequence number on store1, gets off the mel store1 default writers' queue, and exits the mel OR write statement. The store1 mel element is now filled for a reader task.
- If the mel WRITE conditions are not met at
store1, the Producer continues to wait in the store1 writers' queue, and also puts itself to store2 default writers' queue.
- If the mel WRITE conditions are satisfied on
store2, Producer will deposit its product into store2 buffer at the writer's end, increases the sequence number on store2, removes itself from both mel writers' queues, and gets off the mel OR write statement. The store2 mel element is now filled for a reader task.
- Otherwise,
Producer will wait on both queues, until the mel WRITE conditions become true for either one. The Producer then deposits its product to the identified mel buffer, increases the sequence number on that mel element, removes itself from both queues, and gets off the mel OR write statement. Either store1 or store2 has been filled, but Producer does not know which one.
- If it happens that the mel WRITE conditions become true at both mel elements at the same time, the CHAOS runtime will ensure that
Producer will write to one or the other mel element , and not both.
In the above example, Producer keeps producing and depositing its product to either store1 or store2 whichever is vacant at that time, until it Produces a zero value. It then breaks off the while loop and closes both mel channels.
Random OR Writes
Although either mel element can be selected, there is a bias for the first mel element in the Serial OR write method, due to it being looked on first when the OR write starts. If this first mel element is always vacant, perhaps due to it having an ample buffer or working with a fast reader task, it will keep being picked up, shunting the rest from ever be. If the logic of the program demands that all mel elements in the mel OR write have a fair chance, then the Random method is used:
void Producer (<mel> int store1, <mel> int store2) {
while ( <? method=random>(store1 || store2) = Produce() );
<close>store1;
<close>store2;
}
The keyword method can be omitted:
<? random>(store1 || store2) = Produce()
Since the Random method is the default method for mel OR writes, the keyword value random can also be omitted:
<?>(store1 || store2) = Produce()
The behind-the-scene process for the Random mel OR write starts with the task picking a mel element in the mel OR list in a "fairly random" manner. NERWous C does not define a particular strategy for "fairly random" as long all the mel elements in the mel OR list has a fair chance to be picked first.
Once the starting element is picked, the Random method follows the Serial method. After the last mel element in the OR list is checked and found not available for writing, the Random method wraps around and checks the first element in the mel OR write list.
OR Write Exceptions
The above Producer assumes no error to happen in the mel OR write operation. The following Producer is hardened with exceptions to handle possible errors:
void Producer (<mel> int store1, <mel> int store2) {
while ( 1 ) {
try {
int c = Produce();
if ( !c ) break;
<? timeout=100>(store1 || store2) = c );
}
catch ( (store1 || store2)<TIMEDOUT as=store> ) {
printf ("The mel element [%s] has timed out", store<name>);
<resume>;
}
catch ( (store1 && store2)<CLOSED> ) {
printf ("Both mel elements have closed");
break;
}
catch ( (store1 || store2)<... as=store> ) {
printf ("The mel element [%s] has error [%d]", store<name>, store<error>);
if ( store<error> == NERW_ERROR_CLOSED ) <resume>;
break;
}
<close>store1;
<close>store2;
}
The exception rules for mel OR writes are similar to the mel OR reads rules:
- Skipping rule: if an exception happens to a mel element, and there is no specific handler for that mel exception, the CHAOS runtime will remove the task from waiting on that mel element. The mel OR write continues with the mel elements that have not had any exception.
- Aborting rule: if there is a specific handler for that mel exception, the mel OR write is aborted. The task gets off all the waiting mel queues, including those that belong to mel elements that do not have exceptions; and the task will process the identified handler.
- Precedence rule: individual exception handlers take precedence over combination exception handlers. An AND combination exception handler takes precedence over an OR combination exception handler.
Based on the above rules, we can simplify Producer as follows:
void Producer (<mel> int store1, <mel> int store2) {
while ( 1 ) {
try {
<? timeout=100>(store1 || store2) = Produce() );
}
catch ( (store1 && store2)<...> ) {
if ( store1<error> == NERW_ERROR_TIMEDOUT || store2<error> == NERW_ERROR_TIMEDOUT)
<resume>;
break;
}
}
<close>store1;
<close>store2;
}
In this new Producer, if one mel element gets into an error, it will be automatically skipped; and the mel OR write continues with the other mel element. There is no need for an explicit handler for this built-in behavior.
If both mel elements get any kind of error (same or different errors), then the task gets off the mel OR write, and processes the AND combination catch-all exception handler. If one or both errors are a TIMEDOUT, the task does a resumption in order to try to deposit the current product from Produce again. The timed-out mel element or elements will be cleared on this error on exit of the handler, and can accept a write deposit. (If the logic of the program says to discard timed-out products because they are deemed "stale", the continue statement should be used to re-iterate the while loop for a new product, instead of the <resume> statement.)
If neither mel elements get a TIMEDOUT error (i.e. both get a CLOSED or some other error), the task breaks off the infinite while loop. The <close> operations that Producer does on the mel elements indicate to the other tasks in the program that Producer is done with the mel elements. These operations are extraneous if the mel elemnents are already closed by other tasks.
Mel AND Writes
While the mel OR writes allow writing into one of the listed mel elements, the mel AND writes are for writing to all of them. The AND mel write has several built-in methods to process the listed mel elements:
Mel AND Write Methods
Method Value Description Serial The task writes to the mel elements serially based on the order in the AND list. Parallel This is the default method when no method is specified. The task makes a parallel attempt to write to all the mel elements in the AND list. The writes occur individually whenever a mel element is available for writing by this task. InSimul The task makes a parallel attempt to write to all the mel elements in the AND list. The writes occur simultaneously when all mel elements are available for writing by this task.
Serial AND Writes
To illustrate the mel AND write Serial method, let's first replace the Produce function with ProduceTwo which returns two values:
void Producer (<mel> int store1, <mel> int store2) {
int c1, c2;
while ( 1 ) {
(c1, c2) = ProduceTwo();
<? timeout> store1 = c1;
<? timeout> store2 = c2;
if ( !c1 || !c2 ) break;
}
<close>store1;
<close>store2;
}
Once ProduceTwo generates the two products, they are stored in the local variables c1 and c2. The Producer task then writes the first product to the mel buffer store1. Once that successful, it writes the second product to the mel buffer store2. After both writes are done, Producer checks if any product is 0 which marks the end of the production; in that case, it breaks out of the while loop and lt;close> the mel buffers to signal to any reader tasks the end of production.
The above Producer code is now simplified with the mel AND write statement:
void Producer (<mel> int store1, <mel> int store2) {
while ( 1 ) {
try {
<? timeout serial>(store1 && store2) = ProduceTwo() );
if ( !store1<value> || !store2<value> ) break;
}
catch (store1 || store2)<TIMEDOUT as=store>) {
printf ("Can't put into %s in time", store<name>);
<resume> store;
}
catch (store1 || store2)<CLOSED as=store>) {
printf ("%s has closed", store<name>);
break;
}
}
<close>store1;
<close>store2;
}
The optional keyword method can be used if one desires to be explicit:
<? timeout method=serial>(store1 && store2) = ProduceTwo() );
Let's look at the behind-the-scene process of the Serial method for a mel AND write:
- The
Producer task runs ProduceTwo and generates two new items.
- The
Producer task puts itself to store1 default writers' queue with the default timeout value.
- If the
Producer cannot put its item into store1 buffer before the timeout, it aborts the mel wait on store1, exits the mel AND write, and triggers the TIMEDOUT exception on store1.
The handler
catch ( (store1 || store2)<TIMEDOUT as=store> )
will pick up this exception. It printfs out a message then invokes <resume> to put itself back at the bottom of the store1 writers' queue to try to deposit the first produced item again.
- If the mel WRITE conditions are met at
store1 before the timeout, the Producer deposits the first produced item into the mel buffer, and drops out from that mel writers' queue. This allows other writer tasks to write to the mel element store1.
- The
Producer task now puts itself to store2 default writers' queue with the default timeout value.
- If the
Producer cannot put the second produced item into store2 buffer before the timeout, it aborts the mel wait on store2, exits the mel AND wait, and triggers the TIMEDOUT exception on store2.
The handler
catch ( (store1 || store2)<TIMEDOUT as=store> )
picks up this exception. It printfs out a message then invokes <resume> operation. This operation knows that the first produced item has been successfully deposited in store1. It therefore puts the Producer task back to the store2 writers' queue (at the bottom position) to try to deposit the second produced item again.
- If the mel WRITE conditions are met at
store2 before the timeout, the Producer deposits the second produced item into the store2 mel buffer, and drops out from that mel writers' queue. This allows other writer tasks to write to the mel element store2.
- The
Producer task exits the mel AND write statement with both produced items deposited into the corresponding mel elements, one after the other as dictated by the Serial AND write method.
- If the
Producer task encounters a mel closure from any mel element during the mel waits, the CLOSED exception will be raised, and handled by:
catch ( (store1 || store2)<CLOSED as=store> )
This exception handler does a printf on the closing mel element and breaks out of the while loop.
What would happen if a <resume> or continue were used instead of the break? The <resume> operation would clear all exceptions, including the CLOSED exceptions, retry the mel AND write on produced item that have not been written by sending a request to the remote mel elemnt(s), get back the piggy-backed status that the mel elements have been closed, jump into the CLOSED exception handler again, and keep repeating the same process, causing the Producer task to livelock -- i.e. to spin endlessly on the same produced items.
The continue would also cause a livelock, but in a different way. This statement would not clear the exceptions, then cause the Producer to loop back to the while loop, ProduceTwo two new items, attempt to write them to the closed mel elements, detect locally that the mel elements already have the CLOSED status, replace the sending a write request to the remote mel elements with the jump into the CLOSED exception handler. This spinning is then repeated endlessly, each time producing and discarding two new products.
In summary, the Serial method for mel AND writes is just a more compact way to write the one-at-a-time program.
Parallel AND Writes
Let's now change Producer again so that it still add the products of ProduceTwo to both mel buffers, but we will remove the requirement that the additions to be done at the same time. Instead, they will be added at the same time. This AND write method is therefore referred to as the Parallel method:
void Producer (<mel> int store1, <mel> int store2) {
while ( 1 ) {
try {
<?>(store1 && store2) = ProduceTwo() );
if ( !store1<value> || !store2<value> ) break;
}
catch (store1 || store2)<TIMEDOUT as=store>) {
printf ("Can't put into %s in time", store<name>);
<resume> store;
}
catch (store1 || store2)<CLOSED as=store>) {
printf ("%s has closed", store<name>);
break;
}
}
<close>store1;
<close>store2;
}
The Parallel method is the default method for the mel AND write, and is assumed when no method is requested, as in the example above. A programmer can choose to be explicit:
<? timeout method=parallel>(store1 && store2) = ProduceTwo() );
Like always, the keyword method can be omitted:
<? timeout parallel>(store1 && store2) = ProduceTwo() );
This is the behind-the-scene processing for a mel AND write with the Parallel method:
- The
Producer task runs ProduceTwo and generates two new items.
- The
Producer task puts itself to both store1 and store2 writers' queues.
- Whenever the mel WRITE conditions are satisfied for one mel element,
Producer will deposit the corresponding product into that mel buffer, and removes itself from that mel writers' queue.
- Whenever the mel WRITE conditions are satisfied on the remaining mel element,
Producer will do the same thing as above for this mel element.
- If the mel WRITE conditions are not satisfied in time for one of the mel queues, the
Producer task will get off the mel queue, and jumps into the handler
catch ( (store1 || store2)<TIMEDOUT as=store> )
It printfs out a message then invokes <resume> on that mel element. This operation will clear the exception on that mel element, puts the Producer task back at the bottom of that writers' queue to attempt to deposit the correspondent product again.
-
- Once the
Producer task has both products successfully written into their corresponding mel buffers, it gets off the mel AND write statement.
- If while waiting in one mel element queue, the
Producer task may get timed out in one (or both)
The randomness with the mel LIST write is that we don't know which mel buffer is written to first. If the order is important, then the Producer code must be explicit:
void Producer (<mel> int store1, <mel> int store2) {
int c1, c2;
while ( 1 ) {
if ( (c1, c2) = ProduceTwo() ) break;
/* Write to store1 then store2 */
<?>store1 = c1;
<?>store2 = c2;
}
<close>store1;
<close>store2;
}
InSimul AND Writes
A mel AND write is used when a programmer wants both store1 and store2 to be written to at the same time. Let's now change Producer so that it calls a function ProduceTwo which returns two values. These values will be respectively inserted to store1 and store2 mel buffers at the same time.
void Producer (<mel> int store1, <mel> int store2) {
while ( <?>(store1 && store2) = ProduceTwo() );
<close>store1;
<close>store2;
}
For example, we can think of a gaming program where a launching of nuclear missile requires a squeeze on a trigger and a press of a button to be received simultaneously. Of course for this program to work, the mel variables that represent the trigger and the button ought to be of buffer size 1, since it would be meaningless to buffer such requests.
The mel AND write operation behaves similarly to a mel AND read to prevent deadlocks in case of multiple tasks accessing the same mels for writing. The difference is that instead of waiting for a mel to be valued, a writer task waits for a mel to have an available slot at the mel buffer writer's end so it can deposit its product into. If the buffer is of size greater than 1, it is easier to satisfy a mel AND write than a mel AND read.
This is the behind-the-scene processing for a mel AND write:
- The task puts itself in both mel writer's queues.
- In due course, the task will be at the first position of one the mel writers' queues and joins the "top-of-the-queue" writers group of that queue. It then checks if it is also in the "top-of-the-queue" group at the other mel writer's queue. If it is not, it will wait for it to join the "top-of-the-queue" group at the other writers' queue.
- While waiting in only one of the "top-of-the-queue" group, if another writer task comes along on that writers' queue, the task will allow that later-coming task to "jump the line" and deposit its value to an available empty buffer slot if there is one.
- Eventually the task will be in the "top-of-the-queue" groups at both mel writers' queues. It will keep checking for empty slots at both mel buffers. If none is available, it will wait. If only one buffer is available, it will wait, but this time, it will not allow any later-coming writer task, that is not in the "top-of-the-queue" group, to "jump the line" and fill that available buffer slot.
- Eventually the task will find empty slots at both mel buffers. It will then fill the mel buffer slots with its products, increases the sequence number of both mel variables, removes itself from both writers' queues, and gets off the mel AND write.
There can be more than one task in a "top-of-the-queue" group. This group is reserved for tasks that are doing AND writes. When a task doing an AND write moves up to the first position in a writers' queue, it may not find all the conditions for a successful AND write at that moment. Instead of relinquishing the first position in the queue, it joins the "top-of-the-queue" group, and bides its wait there.
For example, we have a task T1 that does an AND write on mel resource R1 and R2. T1 is in the "top-of-the-queue" group on R2 but not yet on R1. Then a task T2 comes along and does a normal write on R2. If there is mel buffer slot available, T1 will allow T2 to "jump the line" and fills that slot. Eventually, T1 is in the "top-of-the-queue" groups of both R1 and R2. However one or both mel buffers are still full, so T1 waits. When the cyclic T2 comes back for another product deposit run, it cannot "jump the line" this time. It has to wait. The next available empty slot is already reserved to T1 since T1 has achieved the "top-of-the-queue" condition at both queues, and now needs the "available mel buffer slot" condition in order to fulfill its AND write.
Let's keep the above example but add a task T3 that does an AND write on resource R2 and R3. When T3 gets to the first position on R2 but not on R3, it will join T1 in the "top-of-the-queue" group of R2. Now if R2 becomes available, the CHAOS runtime automatically assigns it to T1 and not T3 since the former has attained "top-of-the-queue" in all the mels in the AND list, and not the latter.
Eventually T3 also joins the "top-of-the-queue" group of R3, and has achieved "top-of-the-queue" in all the mels in its AND list. Now if R2 becomes available, CHAOS will also give that slot to T1 because it has joined the "top-of-the-queue" group of R2 before T3. T1 now can satisfy all its AND write conditions, deposits its products and gets off both writers' queues. T3 has to wait for the next empty slot from R2 to go with the R3 slot it already has, before it can exit its AND write. The task T2 that is waiting for both T1 and T3 to be done with R2, can now access the next empty slot from R2.
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