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From: Gleb Natapov (glebn_at_[hidden])
Date: 2007-09-11 10:34:37


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 same
> communicator twice is later in the code (same file in the function
> ompi_comm_register_cid line 326). Once the function ompi_comm_register_cid
I 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 to
> the smallest com_id, which is what happens in the code you removed.
The code I removed was doing it wrongly. I.e the algorithm sometimes is executed
for different communicators simultaneously by different threads. Think
about the case where the function is running for cid 1 and then another
thread 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.

>
> Without 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, simply
> because they will not order the creation based on the com_id.
If the algorithm is really prone to deadlock in case it is concurrently
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=2
in section 5.3 works without it and we can do something similar.

>
> george.
>
>
>
> On Sep 11, 2007, at 9:23 AM, gleb_at_[hidden] 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
>> communicator
>> simultaneously, but is doing it incorrectly. If the function is running
>> already
>> 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 by
>> two threads simultaneously. There is nothing in the algorithm that prevent
>> it
>> from been running simultaneously for different communicators as far as I
>> can see,
>> but ompi_comm_unregister_cid() assumes that it is always called for a
>> communicator
>> with the lowest cid and this is not always the case. This patch removes
>> bogus
>> lowest cid check and fix ompi_comm_register_cid() to properly remove cid
>> from
>> 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, 11
>> Sep 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 and
>> try 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;
>> }
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>

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--
			Gleb.