Commit e6c3cd52 authored by David Goodell's avatar David Goodell
Browse files

[svn-r6923] fix multithreaded context ID allocation

Racy accesses don't happen any longer, although I think this code now runs
slower in some cases.  Also, unrelated to this change, this algorithm probably
scales poorly with the number of contending threads.

No reviewer.
parent cb936755
......@@ -684,6 +684,9 @@ int MPIR_Get_contextid_sparse(MPID_Comm *comm_ptr, MPIR_Context_id_t *context_id
MPI_INT, MPI_BAND, comm_ptr );
if (mpi_errno) MPIU_ERR_POP(mpi_errno);
/* MT FIXME 2/3 cases don't seem to need the CONTEXTID CS, check and
* narrow this region */
if (ignore_id) {
/* we don't care what the value was, but make sure that everyone
* who did care agreed on a value */
......@@ -692,7 +695,6 @@ int MPIR_Get_contextid_sparse(MPID_Comm *comm_ptr, MPIR_Context_id_t *context_id
else if (own_mask) {
/* There is a chance that we've found a context id */
/* Find_and_allocate_context_id updates the context_mask if it finds a match */
*context_id = MPIR_Find_and_allocate_context_id(local_mask);
......@@ -707,8 +709,8 @@ int MPIR_Get_contextid_sparse(MPID_Comm *comm_ptr, MPIR_Context_id_t *context_id
else {
/* else we did not find a context id. Give up the mask in case
there is another thread (with a lower context id) waiting for
there is another thread (with a lower input context id)
waiting for it.
We need to ensure that any other threads have the
opportunity to run. We do this by releasing the single
......@@ -720,35 +722,33 @@ int MPIR_Get_contextid_sparse(MPID_Comm *comm_ptr, MPIR_Context_id_t *context_id
mask_in_use = 0;
else {
/* As above, force this thread to yield */
/* FIXME: TEMP for current yield definition*/
/* Here is the test for out-of-context ids */
/* FIXME we may be able to "rotate" this a half iteration up to the top
* where we already have to grab the lock */
if ((testCount-- == 0) && (*context_id == 0)) {
int hasNoId, totalHasNoId;
/* We don't need to lock on this because we're just looking for
zero or nonzero */
/* FIXME [goodell@] I don't agree with the above comment, pthreads
* doesn't really give us any guarantees when outside of a locked region and
* there isn't much of a penalty for acquiring the CS in this case.
* It gets even cheaper if we "rotate" this check half way around to
* the top of this loop where we already acquire the CS anyway.
* In the GLOBAL case, we actually are holding a lock here, so it
* doesn't trigger helgrind/DRD warnings. */
hasNoId = MPIR_Locate_context_bit(context_mask) == 0;
/* we _must_ release the lock above in order to avoid deadlocking on
* this blocking allreduce operation */
mpi_errno = MPIR_Allreduce_impl( &hasNoId, &totalHasNoId, 1, MPI_INT,
MPI_MAX, comm_ptr );
if (mpi_errno) MPIU_ERR_POP(mpi_errno);
if (totalHasNoId == 1) {
/* Release the mask for use by other threads */
if (own_mask) {
mask_in_use = 0;
MPIU_ERR_SETANDJUMP(mpi_errno, MPI_ERR_OTHER, "**toomanycomm");
Markdown is supported
0% or .
You are about to add 0 people to the discussion. Proceed with caution.
Finish editing this message first!
Please register or to comment