Difference between revisions of "Fault Tolerance"
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=== REVOKE ===
=== REVOKE ===
Revision as of 19:02, 9 March 2015
The work done to allow fault tolerance was started by Darius Buntinas and continued by Wesley Bland.
In order to use the fault tolerance features of MPICH, users need to enable a flag at configure time:
This will allow the MPICH implementation to correctly check the status of communicators when calling MPI operations.
Users will also need to enable a runtime flag for the Hydra process manager:
This will prevent the process manager from automatically killing all processes when any process exits abnormally.
Within the application, the most basic code required to take advantage of any fault tolerance features is to change the error handler of the user's communicators to at least MPI_ERRORS_RETURN. At the moment, fault tolerance is only implemented for the ch3:tcp device. The other devices will require some changes in order to correctly return errors up through the stack.
Local failures are detected by Hydra, the process manager via the usual Unix methods (closed local socket). If a process terminates abnormally, it is detected by the process manager in and SIGUSR1 is used to notify the MPI application of the failure. This notification is also sent to the PMI server to be broadcast to all other processes so they can also raise SIGUSR1.
As mentioned previously, TCP is currently the only netmod that supports fault tolerance. It is done by detecting that a socket is closed unexpectedly. When that happens, the netmod calls the tcp cleanup function (MPID_nem_tcp_cleanup_on_error) and returns an error (MPIX_ERR_PROC_FAILED) via the usual MPICH error handling methods.
TODO: Add more details here.
When a failure is detected and the signal SIGUSR1 is raised, it is caught by the CH3 progress engine. This is done by comparing the number of known SIGUSR1 signals to the current number. If it has changed, the progress engine calls into the cleanup functions. The first function is MPIDI_CH3U_Check_for_failed_procs. In this function, CH3 gets the most recent list of failed processes from the PMI server. Currently, value is replicated on all PMI instances, but in the future, it could do something more scalable and do a reduction to generate this value with local knowledge.
If some new failures are detected, they are handled by a series of calls, first MPIDI_CH3I_Comm_handle_failed_procs. This function loops over each communicator to determine if it contains a failed process. If it does, it is marked as not anysource_enabled. This means that future recv (or non-blocking receive) calls will probably return with a failure indicating that they could not complete. However, they will be given one chance through the progress engine to match so it's possible that they might still finish. Also during that function, the progress engine will be signalled to indicate that a request has completed. This will allow the progress engine to return control back to the MPI level where a decision can be made about whether to return an error or return to the progress engine to continue waiting for completion.
After cleaning up the communicator objects, the next function is terminate_failed_VCs. This function closes all of the VC connections for failed processes. This will also clean out the nemesis send queue to remove all messages to the failed processes.
Deep in the call stack, the function MPIDI_CH3U_Complete_posted_with_error is called which cleans up the posted receive queue. This is part of the process of cleaning up failed VCs so only requests which match the failed VCs are removed (when on is removed, it's printed to the log to make it easy to track).
FAILURE_ACK / FAILURE_GET_ACKED
The failure acknowledgement functions both end up implemented in the same file in CH3: src/mpid/ch3/src/mpid_comm_failure_ack.c. They are pretty straightforward. The reason that these functions work correctly is because of the way PMI works. If that mechanism changes, these functions will have to be updated.
PMI always returns a string containing a list of failed processes when asked. This string is only appended, never deleted or re-ordered. Because of this, we can just keep track of the last failed process which we've acknowledged (when we acknowledge a failure) and use that as the basis to generate the group of failed processes (when we ask for the acknowledged failures.
The value of the last acknowledged failure is stored in MPIDI_CH3I_comm as last_ack_rank. This value is accessed via comm_ptr.dev->anysource_enabled. In the acknowledgement function, this value is set based on the last known failed process (MPIDI_last_known_failed) which is set in MPIDI_CH3U_Check_for_failed_procs.
At the same time that we set the last acknowledged failed process, we also set a flag indicating that it's now safe to process receive operations involving MPI_ANY_SOURCE by setting the flag anysource_enabled in the MPIDI_CH3I_comm object.
The function to get the group of failed process works in much the same way. Internally, it uses MPIDI_CH3U_Get_failed_group to parse the string of failed processes from PMI. When it reaches the last acknowledged failure, it quits processing the string and creates a group, which is returned to the user. That group is intersected with the group of the communicator itself and returned to the user.
This function is a series of of calls to generate the group of failed processes, determine whether the group is consistent for all ranks, and create a new communicator from the failed processes. The group of failed processes is generated with the call to MPID_Comm_get_all_failed_procs with data from PMI. The inverse of that group is then used with MPIR_Comm_create_group to attempt to create a new communicator containing only the processes that are alive. After the new communicator is created, the processes do an MPIR_Allreduce_group with the errflag value to ensure that everyone returned correctly. If any of those parts fail, the process is started again from the beginning up to 5 times. If the operation still can't complete after 5 attempts, the machine should be thrown away and an error is returned.
The agreement function is similar to the shrink function. However, it also determines if any unacknowledged failures exist. It starts by getting the acknowledged failed group from MPID_Comm_failure_get_acked and then calls MPID_Comm_get_all_failed_procs to get the entire list of failed processes. These groups are compared with MPIR_Group_compare_impl and if they are not equal, a flag is marked. Because failure knowledge is local, this flag will not initially be equivalent on all processes, meaning the collective portion of the algorithm must still be completed. This is done with two calls to MPIR_Allreduce_group. The first collectively decides the return value of the success flag, the second does a bitwise and (MPI_BAND) over the user supplied integer value. If the allreduce fails, then a failure is known at all processes and they will all return an error collectively. Otherwise, they will return MPI_SUCCESS with the new flag value included.
Revoke is the most complex function. It is implemented primarily in MPID_Comm_revoke. This function has two major code paths depending on whether the communicator has already been revoked or not.
If it has not yet been revoked, it will internally mark the communicator as revoked with the MPID_Comm.revoked flag. It will do the same for the node_comm and the node_roots_comm. It will also mark a counter in the device portion of the communicator to track how many other processes need to be informed about the revoke (more accurately, how many other processes have sent a message saying that they have revoked). Next, the function grabs a reference count of the communicator to prevent it from being destroyed if the user does something like revoke and immediately free the communicator. Then a message is sent to all other processes to inform them of the revoke. If that message fails, we decrement the counter and ignore the failure. Then we clean the receive queues (unexpected and posted) using MPIDI_CH3U_Clean_recvq to remove any messages by marking them as MPIX_ERR_REVOKED.
If the communicator had already been revoked before calling (as will happen when a message arrives to trigger the revoke message handler), the only part that happens is to decrement the waiting_for_revoke counter and release the reference to the communicator if necessary.
When the revoke message arrives at a process, it triggers a packet handler to avoid going through the message queue. The handler (MPIDI_CH3_PktHandler_Revoke) simply finds the appropriate communicator and calls MPID_Comm_revoke.