Without the code you removed we face a deadlock situation. Multiple threads will pick different next_cid on each process and thy will never succeed with the reduce operation. And this is what we're trying to avoid with the test.
george. On Sep 11, 2007, at 10:34 AM, Gleb Natapov wrote:
On Tue, Sep 11, 2007 at 10:14:30AM -0400, George Bosilca wrote:Gleb,This patch is not correct. The code preventing the registration of the samecommunicator twice is later in the code (same file in the functionompi_comm_register_cid line 326). Once the function ompi_comm_register_cidI saw this code and the comment. The problem is not with the same communicator but with different communicators.is called, we know that each communicator only handle one "communicator creation" function at the same time. Therefore, we want to give priority toThe code I removed was doing it wrongly. I.e the algorithm sometimes is executedthe smallest com_id, which is what happens in the code you removed.for different communicators simultaneously by different threads. Thinkabout the case where the function is running for cid 1 and then anotherthread runs it for cid 0. cid 0 will proceed although the function is executed on another CPU. And this is not something theoretical, that is happening with sun's thread test suit mpi_coll test case.If the algorithm is really prone to deadlock in case it is concurrentlyWithout the condition in the ompi_comm_register_cid (each communicator only get registered once) your comment make sense. However, with the condition your patch allow a dead end situation, while 2 processes try to create communicators in multiple threads, and they will never succeed, simplybecause they will not order the creation based on the com_id.executed for several different communicators (I haven't check this), then we may want to fix original code to really prevent two threads to enter the function, but then I don't see the reason for all those complications with ompi_comm_register_cid()/ompi_comm_unregister_cid() The algorithm described here:http://209.85.129.104/search?q=cache:5PV5MMRkBWkJ:ftp:// info.mcs.anl.gov/pub/tech_reports/reports/P1382.pdf+MPI+communicator +dup+algorithm&hl=en&ct=clnk&cd=2in section 5.3 works without it and we can do something similar.george. On Sep 11, 2007, at 9:23 AM, g...@osl.iu.edu wrote:Author: gleb Date: 2007-09-11 09:23:46 EDT (Tue, 11 Sep 2007) New Revision: 16088 URL: https://svn.open-mpi.org/trac/ompi/changeset/16088 Log: The code tries to prevent itself from running for more then one communicatorsimultaneously, but is doing it incorrectly. If the function is runningalready for one communicator and it is called from another thread for other communicator with lower cid the check comm->c_contextid != ompi_comm_lowest_cid() will fail and the function will be executed for two different communicators bytwo threads simultaneously. There is nothing in the algorithm that preventitfrom been running simultaneously for different communicators as far as Ican see,but ompi_comm_unregister_cid() assumes that it is always called for acommunicatorwith the lowest cid and this is not always the case. This patch removesboguslowest cid check and fix ompi_comm_register_cid() to properly remove cidfrom the list. Text files modified:trunk/ompi/communicator/comm_cid.c | 24 +++++++++++ +------------1 files changed, 12 insertions(+), 12 deletions(-) Modified: trunk/ompi/communicator/comm_cid.c==================================================================== ==========--- trunk/ompi/communicator/comm_cid.c (original)+++ trunk/ompi/communicator/comm_cid.c 2007-09-11 09:23:46 EDT (Tue, 11Sep 2007) @@ -11,6 +11,7 @@ * All rights reserved.* Copyright (c) 2006-2007 University of Houston. All rights reserved.* Copyright (c) 2007 Cisco, Inc. All rights reserved. + * Copyright (c) 2007 Voltaire All rights reserved. * $COPYRIGHT$ * * Additional copyrights may follow @@ -170,15 +171,6 @@ * This is the real algorithm described in the doc */ - OPAL_THREAD_LOCK(&ompi_cid_lock); - if (comm->c_contextid != ompi_comm_lowest_cid() ) {- /* if not lowest cid, we do not continue, but sleep andtry again */ - OPAL_THREAD_UNLOCK(&ompi_cid_lock); - continue; - } - OPAL_THREAD_UNLOCK(&ompi_cid_lock); - - for (i=start; i < mca_pml.pml_max_contextid ; i++) { flag=ompi_pointer_array_test_and_set_item(&ompi_mpi_communicators, i, comm); @@ -365,10 +357,18 @@ static int ompi_comm_unregister_cid (uint32_t cid) { - ompi_comm_reg_t *regcom=NULL; - opal_list_item_t *item=opal_list_remove_first(&ompi_registered_comms); + ompi_comm_reg_t *regcom; + opal_list_item_t *item; - regcom = (ompi_comm_reg_t *) item; + for (item = opal_list_get_first(&ompi_registered_comms); + item != opal_list_get_end(&ompi_registered_comms); + item = opal_list_get_next(item)) { + regcom = (ompi_comm_reg_t *)item; + if(regcom->cid == cid) { + opal_list_remove_item(&ompi_registered_comms, item); + break; + } + } OBJ_RELEASE(regcom); return OMPI_SUCCESS; } _______________________________________________ svn-full mailing list svn-f...@open-mpi.org http://www.open-mpi.org/mailman/listinfo.cgi/svn-full_______________________________________________ devel mailing list de...@open-mpi.org http://www.open-mpi.org/mailman/listinfo.cgi/devel-- Gleb. _______________________________________________ devel mailing list de...@open-mpi.org http://www.open-mpi.org/mailman/listinfo.cgi/devel
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