Hfs file system fault tolerant features code#

The application restarts itself from the last successful data file written out. In the event of a failure, only computations from the last checkpoint are lost. You then have to modify the application to write out such a data file periodically. With this option you must understand the parallel application and modify it so that the application can restart from an input data file. For short-running applications this is the best option.ДЇor longer-running applications, checkpointing is a commonly used option. On average the time is less than twice the normal run time. It guarantees that the application will complete even if failures occur, although it does not guarantee how long this will take. This option is used by many batch systems because it requires no knowledge of the application. The first option is the simplest to implement but the most expensive in the amount of calculation that must be redone. The most commonly used options are restart from the beginning, roll back to the last checkpoint, or reassign the work of a failed task. Recovery depends heavily on the type of parallel algorithm used in the application. Recovery is the final step in building fault-tolerant programs. The difference is the number of notification messages that get sent in the event of a failure. So the monitoring overhead is the same with all these options. Remember that the PVM system is doing the monitoring, not the application tasks. Another common, but not particularly efficient, method is to have every task watch all the other tasks. Thus each task just watches one or two other tasks. A common method is to have each task watch its neighbor in a logical ring. Note that in the manager/worker scheme the manager often assumes the additional duty as watcher.ДЄ second option is for the application tasks to watch each other.

Hfs file system fault tolerant features code#

The advantage of this approach is that the application code can remain cleaner. One or more of these watcher tasks are spawned across the cluster and often have the additional responsibility of managing the recovery phase of the application. The first option is designing a separate watcher task. Given the flexibility of the pvm_notify command, there are several options for how the application can be designed to receive notification from the PVM system. The cnt argument is the number of tasks or node IDs in the ids list for which notification is requested. The msgtag argument specifies what message tag the task will be using to listen for events. A separate notify call must be made for each event type that the application wishes to be notified about. The EventType options are PvmTaskExit, PvmHostDelete, or PvmHostAdd. Int info = pvm_notify( int EventType, int msgtag, int cnt, int *ids )

In this case no ID can be specified, and the notification message returns the ID of the new node(s). The addition of one or more cluster nodes to the application's computational environment is also an event that PVM can notify an application about. And, as before, the notification message returns the ID of the failed node(s).

Again the requesting application task can specify a particular node, set of nodes, or all nodes. The failure or deletion of a node in the cluster is another notify event that can be requested. This approach provides more robustness in the first step of detection. Thus there is no need for the notified task and the failed task ever to have communicated in order to detect the failure. Unlike many detection systems, PVM's monitoring system is not based on the detection of a broken communication channel between the monitored and notifed task.

The notification message contains the ID of the task that failed. Or it can ask to be notified if any task within the application fails. The PVM task(s) requesting notification can specify a particular task or set of tasks to be monitored. The second step in building fault-tolerant applications is notification. These events include the failure of a task, the failure of a cluster node, or the availability of new nodes into the application. Any or all tasks in an application can ask to be notified of specific events. The PVM runtime system has a monitoring and notification capability built into it. Detection is typically the job of the runtime environment but when the runtime envoronment does not provide this capability, application developers can create their own set of monitoring tasks to oversee an application. The first step is being able to detect that something has gone wrong. 11.2 Building Fault-Tolerant Parallel ApplicationsДЇrom the application's view, three steps must be performed for fault tolerance: detection, notification, and recovery.