Communicators and Context IDs
- 1 What Is A Context ID?
- 2 Context ID Mask
- 3 Sub-communicator Type Field
- 4 Context Type Suffix
- 5 Context ID API
- 6 When and How Context IDs Are Selected For Communicators
What Is A Context ID?
When MPI receives a message and matches it against MPI_Recv requests, it compares the message's envelope to the MPI_Recv's envelope. The envelope is the triple of (source, tag, communicator). The source and tag are explicitly integers, yet the communicator is a logical construct indicating a particular communication context. In MPICH2 this context is implemented via an additional tag field known as the context id. It's worth remembering that there is no wild card matching for communicators.
The MPICH2 context ID is a 16-bit integer field that is structured as follows:
In this crude diagram each character represents a bit. There are three fields of the context ID indicated by letter and color:
- Dynamic Process Context Bit (
- If 0, this is a traditional context ID allocated from the context ID mask (explained below). If 1 this is a context ID used for dynamic processes and is allocated in a different manner.
- Mask Word Index (
- This is the index into the context ID mask (explained below).
- Bit Index (
- This is which bit index within the mask word that this ID refers to.
- Is Localcomm(
- This bit is set to 1 if this communicator is a local communicator for an intercommunicator. This allows us to derive local communicator context IDs from the intercommunicator context ID without any communication.
- Sub-communicator Type (
- This is explained further below.
- Context Type Suffix (
- This is used to indicate different communication contexts within a communicator. For example, user point-to-point messages (MPI_Send/MPI_Recv) occur in a different context than collective messages (MPI_Bcast, etc). This also explained further below.
The actual type of a context ID is
MPIR_Context_id_t, which is
All members of a communicator use the same context ID for that communicator, but a context ID is not a globally unique ID. That is, the communicator's group information combined with the context ID constitute a unique ID. A great example of this is when a
MPI_Comm_split is called collectively each of the resulting disjoint communicators has the same context ID yet different group information.
Context ID Mask
The context ID mask is a bit vector that is used to keep track of which context IDs have been allocated. In the current code it is an array of
MAX_CONTEXT_MASK (128) 32-bit
unsigned ints for a total of 4096. Each process has its own mask and its state may vary from process to process depending on communicator membership patterns.
Mask Access And Multi-threading
Talk about critical sections, the local context mask, and
TODO finish this section In the mean time you can examine the code for a better explanation: src/mpi/comm/commutil.c:332
Problems and Gotchas
There are several issues and things to watch out for when working on the context ID code in
- The current code expects that
unsigned intvalues are 32-bits or larger. The comments imply that it needs exactly 32-bit
unsigned ints but it looks like we lucked out and it should work with larger sizes as well. This needs to be cleaned up in the current code.
- IDs are allocated from the lowest available mask integer index but the highest available bit index within that integer. This leads to a nice looking pattern when the mask is viewed as a hex string via the
MPIR_ContextMaskToStrfunction but a strange ordering of ID values (124, 120, 116, ..., 0, 252, 248, ..., 128, 380, etc).
- While new IDs are allocated in the fashion described just above, the three default communicators (
MPIR_ICOMM_WORLD) take up bits 0-2 of word 0 (prefixes 0, 8, and 16). In contrast, the first context ID allocated after
MPI_Initwill be bit 31 of word 0 (id prefix 124). This works out OK, it's just surprising when you are debugging and get
"03fffff8ffffffff..."when you print out the mask field. It wouldn't hurt to change this to something less surprising if we get the time.
- When allocating context IDs for intercommunicators it is important to allocate the
context_idfields correctly. By convention the
recvcontext_idis allocated from the local group's pool while the
context_idis allocated from the remote group's pool. It is important not to reverse this, since
MPIR_Free_contextidonly frees the
recvcontext_id. A reversal of these two fields will result in freeing unallocated IDs and leaking other context IDs in some cases.
- It is safe to call
MPIR_Free_contextidon "derived" context IDs (such as in localcomm IDs or topological sub-communicators), however those IDs will not truly be freed at that point. Since they share an entry in the context ID bit vector with the parent context ID they will be ignored. The assumption is that any child IDs will be freed first and then the parent IDs will be freed next, at which point the actual context ID will be released.
Sub-communicator Type Field
This is used to distinguish a "top-level" or "parent" communicator from any "sub-communicators" or "child communicators" it may have. Currently whenever a communicator is created we also create two child communicators for it: an internode comm and an intranode comm. These comms are used to implement SMP-aware collective operations. Rather than calling MPIR_Allreduce three times where we previously called it once, we derive context IDs for the sub-communicators from the parent communicator's context ID. A field value of 0 indicates a parent comm, 1 indicates an intranode communicator, and 2 indicates an internode communicator. See here for the relevant constants: src/include/mpiimpl.h:1368.
Context Type Suffix
The last bit of the ID is used to indicate different communication contexts within a communicator. Point-to-point and collective communication occur in separate contexts and use a different suffix to form different context IDs. Theoretically File or Win communication could use a separate context suffix value, but they don't. Because they don't, we only use 1 bit now instead of 2 as we did previously. The suffix values are named and can be found here: src/include/mpiimpl.h:1283.
These values are passed as the
context_offset argument to
MPID_Send and friends where it is added to the appropriate context ID stored in the communicator structure. This is the value that is actually used for matching.
MPIC_* family of functions are convenience functions that put the appropriate collective offset to the corresponding
MPID_* function. In practice this always amounts passing "1", but it leaves us the option for more flexible changes in the future.
Context ID API
static char MPIR_ContextMaskToStr(void)
Useful to dump the state of the context mask.
static void MPIR_Init_contextid(void)
Sets all of the bits of the context mask to 1 except for bits 0,1, and 2 of word 0.
static int MPIR_Locate_context_bit(uint32_t local_mask)
Finds the highest bit of the lowest word that is set in the given mask and returns the corresponding context ID.
static int MPIR_Allocate_context_bit(uint32_t mask, MPIR_Context_id_t id)
Clears the bit in
mask corresponding to the given context
static int MPIR_Find_and_allocate_context_id(uint32_t local_mask)
Finds the highest bit of the lowest word that is set in the given mask. It resets that bit in the
context_mask and returns the found ID prefix.
int MPIR_Get_contextid(MPID_Comm *comm_ptr, MPIR_Context_id_t *context_id)
Allocates a new context ID prefix collectively over the given communicator
comm_ptr. Returns the new context ID in
context_id. The core of the algorithm copies the current state of the mask to a local buffer and then performs an
NMPI_Allreduce with an
MPI_BAND operation to find the intersection of valid context IDs among all participating processes. The result of this reduction is fed to
MPIR_Find_and_allocate_context_id to determine the new context ID prefix.
int MPIR_Get_intercomm_contextid( MPID_Comm *comm_ptr, MPIR_Context_id_t *context_id, MPIR_Context_id_t *recvcontext_id)
MPIR_Comm_copy to get context IDs for a new intercommunicator from an old intercommunicator. Note that it returns a pair of IDs, one for sending and one for receiving.
When and How Context IDs Are Selected For Communicators
There are three predefined communicators that reserve context IDs at MPI_Init time:
MPI_COMM_WORLD(id prefix 0)
MPI_COMM_SELF(id prefix 4)
MPI_ICOMM_WORLD(id prefix 8)
This occurs here in the code: src/mpi/init/initthread.c:206
MPIR_Get_contextid(comm_ptr). This ID is the same across all the disjoint communicators that are created. That is, if
MPI_Comm_split is called such that three new communicators are created, the context ID will be the same in all three communicators (although the groups will obviously be different between communicators).
MPIR_Comm_copy which in turn calls
MPIR_Get_contextid over the source communicator. This new context ID is used for the duplicate communicator.
MPIR_Get_contextid(old_comm_ptr) and use that as the
recvcontext_id. Rank 0s of each group exchange their allocated context IDs and use the received values as the
context_id field value.
MPIR_Get_contextid(local_comm_ptr) to get the
recvcontext_id for the new communicator. Then roots of the groups exchange context IDs and then broadcast them to the rest of their local groups. This received value serves as the
context_id for the new communicator.
All communicators that result from a single collective split call have the same context IDs (but obviously different groups).
MPIR_Comm_copy which in turn calls
MPIR_Get_intercomm_contextid. Each group generates a
MPIR_Get_contextid. Then the roots exchange that value with each other and broadcast the result to the local group. The value received from the other side becomes the sending context ID (the field named
context_id in the
FIXME WRONG need Pavan to explain the new scheme here.
- Allocate a context ID via
MPIR_Get_contextid(comm_ptr)over the connecting communicator. This is the
recvcontext_idfor the new intercommunicator.
Then in the root:
- Connect to the port and create a temporary communicator (context ID 4095 (Why?)) to the root on the other side from this connection.
- Exchange global process group size, local communicator size and the context ID determined locally. This is sent via the temporary communicator.
- broadcast the received info on the local communicator
Just the root:
- exchange PG info with the accept side root
- store the received context ID as the
context_idfor the new intercommunicator.
Just the root:
- synchronize with the remote root
- free the temporary communicator
- barrier over the local communicator
The counterpart to the
connect algorithm above. It is essentially the same except the first step is to accept the connection instead of to initiate it.
This is simply implemented via a
PMI_Spawn_multiple followed by a
MPIR_Comm_connect/MPIR_Comm_accept under the hood.