Reference Implementation of the Process Management Interface Exascale (PMIx) standard
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Extension of Tool Interaction Support
An increasingly diverse collection of tools have expressed interest in connecting to PMIx servers to obtain information on application processes (placement, status, identification) and queue status, as well as debugger support. This RFC contains a significant update of the prior tool support described in RFC0001 that impacts all three levels of interfaces (client, server, and host resource manager).
[EXTENSION][CLIENT-API][SERVER-API][RM-INTERFACE]
[APPROVED]
Copyright (c) 2016 Intel, Inc. All rights reserved.
This document is subject to all provisions relating to code contributions to the PMIx community as defined in the community’s LICENSE file. Code Components extracted from this document must include the License text as described in that file.
An increasingly diverse collection of tools have expressed interest in connecting to PMIx servers to obtain information on application processes (placement, status, identification) and queue status, as well as debugger support. Supporting this range of activity requires the following changes to the PMIx library:
Add support for tool connections to a specified system-level PMIx server. Although not required, environments can choose to launch a set of PMIx servers to support a given allocation – these servers will (if so instructed) provide a tool rendezvous point that is tagged with their pid and typically placed in an allocation-specific temporary directory to allow for possible multi-tenancy scenarios. However, login and other support nodes may not be utilized in allocations, and thus would not host a PMIx server – yet a PMIx-based tool operating on such nodes could be used to query system status, request job launch, and other system functions.
Supporting such operations requires that a system-level PMIx connection be provided which is not associated with a specific user or allocation. A new key has been added to direct the PMIx server to expose a rendezvous point specifically for this purpose. Only one system-level rendezvous point (not tagged with a pid) is permitted per node, and placed in a system-level temporary directory to facilitate easier discovery. Keys for specifying the system- and allocation-specific temporary directories have been provided.
As tools may find themselves in an environment where both system and allocation level PMIx servers are present, keys have been provided to direct the tool connection procedure to select the desired type of server (system or allocation). In the absence of direction, the tool will first look for a system-level connection, and then seek a session-level connection, returning an error if multiple allocation server rendezvous points are discovered and the caller failed to specify a pid.
Redefine the rank value from int to uint32. The expansion of requests for information complicated the need to specify the processes whose information was being requested. Large arrays of values raise questions of scalability, and created a desire for the definition of special rank values that could convey a particular well-defined pattern – e.g., definition of a rank value to represent all ranks on a given node. Additional concerns were raised regarding future demands on overall application sizes. Thus, the rank type was changed from the current int to uint32_t and given a dedicated typedef in case there are any future changes. Special rank values indicating grouping will be taken from the top of this range, beginning with PMIX_RANK_WILDCARD at UINT32_MAX-1.
NOTE: “Future proofing” the new definition requires strict enforcement of the pmix_rank_t type definition within the PMIx library code, particularly during use of pack/unpack routines. Thus, data provided by the host resource manager (e.g., in the call to PMIx_Register_nspace) will likewise be subject to these strict rules.
Add process state definitions for returning the state of a queried process. An initial, hardly exhaustive, set of definitions has been provided.
Note: This is a “best-fit” approximation of the actual process state based on fitting the actual resource manager-defined state to the closest corresponding PMIx definition. It is expected that the RM community will request refinement of these defined states over time to better reflect the actual process state in response to user requests.
pmix_data_array_t – used to pass an arbitrary array of data to/from the client. Queries for information frequently result in multiple answers for a given key. For example, a query for PMIX_QUERY_PROC_TABLE will return an array of pmix_proc_info_t structures, one for each process in the specified namespace. Thus, the returned value for the query will contain a single pmix_info_t structure containing the PMIX_QUERY_PROC_TABLE key, and a value consisting of a pmix_data_array_t filled with pmix_proc_info_t structures. The pmix_data_array_t structure contains:
NOTE: addition of the generalized pmix_data_array_t structure resulted in deprecation of the pmix_info_array_t definition as being a special case of the more general definition
Modify the PMIx_Query_nb interface to take an array of the new query structures. The callback function used to return the results of the request will consist of a pmix_info_t corresponding to each input pmix_query_t structure, with the pmix_value_t field containing either a unique value or a pmix_data_array_t of values. New query-related PMIx-standard key definitions have also been added.
For example, a request for the list of peers executing on a node (key=PMIX_LOCAL_PEERS) requires that one also specify the node being referenced – the nodeid would therefore be passed as a qualifer. Note that a similar request for the list of peers on multiple nodes can be accomplished by providing an array of qualifiers that specifies the nodeid of the nodes, as shown below:
pmix_query_t query;
uint32_t nodeid; size_t n;
PMIX_QUERY_CONSTRUCT(&query, 1); // initialize the structure, creating space for 1 key
query.keys[0] = strdup(PMIX_LOCAL_PROCS); // request the procs running on a node
PMIX_INFO_CREATE(query.qualifiers, 2); // specify two nodes whose info we want
nodeid = 5;
PMIX_LOAD_INFO(&query.qualifiers[0], PMIX_NODEID, &nodeid, PMIX_UINT32);
nodeid = 7;
PMIX_LOAD_INFO(&query.qualifiers[1], PMIX_NODEID, &nodeid, PMIX_UINT32);
PMIx_Query_info_nb(&query, 1, results_cbfunc, &query); // execute the query
The callback function for this request would receive a single pmix_info_t structure containing a key of PMIX_LOCAL_PROCS and a value that contained a pmix_data_array_t of pmix_proc_t structures, each structure containing the namespace and rank of a proc on one of the specified nodes. Note that the results would not indicate which node each proc was on – to obtain that level of detail, the requestor should have provided two query structures, each with a single qualifier specifying the node of interest.
NOTE: the blocking form of the PMIx_Query API has been remove from the PMIx standard. This action was taken due to the difference between the pmix_query_t input structures and the array of pmix_info_t structures returned by the query.
NOTE: The query interface between the PMIx server library and the host RM has been modified to pass the new pmix_query_t structure.
Add a bit-mapped uint32 directives field to the pmix_info_t. The expanded use of pmix_info_t structures as directives required addition of at least a required vs optional stipulation. Provision for future additional uses was provided by extending the field to a uint32_t size.
Replace the enum types for persistence, scope, data range, data type, and status with #define values. This enables extension of the PMIx-standard values by 3rd parties to support environment-specific capabilities.
NOTE: “Future proofing” these new definitions requires strict enforcement of their definitions within the PMIx library code, particularly during use of pack/unpack routines. Thus, values passed to/from the host resource manager will likewise be subject to these strict rules.
Add new “pretty-print” support functions for proc state, scope, persistence, data range, info directives, and data types. Returning of simple integer values can make debugging code difficult by requiring constant references to the pmix_common.h header. The provided functions translate the given value to a corresponding, user-friendly string suitable for printing.
Split the handling of PMIX_HOSTNAME requests to more clearly articulate what is being requested and returned. The PMIX_HOSTNAME key returned a value that depended on the rank of the process in the PMIx_Get request. This proved to be confusing, and therefore the definition of the key has been streamlined to support only one use-case, and new keys added to handle the other use-cases.
Code examples for debugger launch/attach and for a generic PMIx-base application launch tool have been added to the PMIx library’s “example” directory to help developers understand the use of these new capabilities.
The PMIx library implementation is covered in the Extend the PMIx_Tool support pull request. The prototype has been tested against Open MPI as referenced in an upcoming pull request.
Ralph H. Castain
Intel, Inc.
Github: rhc54