Open MPI logo

Open MPI Development Mailing List Archives

  |   Home   |   Support   |   FAQ   |   all Development mailing list

From: George Bosilca (bosilca_at_[hidden])
Date: 2007-09-11 10:54:25


We don't want to prevent two thread from entering the code is same
time. The algorithm you cited support this case. There is only one
moment that is critical. The local selection of the next available
cid. And this is what we try to protect there. If after the first
run, the collective call do not manage to figure out the correct
next_cid then we will execute the while loop again. And then this
condition make sense, as only the thread running on the smallest
communicator cid will continue. This insure that it will pickup the
smallest next available cid, and then it's reduce operation will
succeed. The other threads will wait until the selection of the next
available cid is unlocked.

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 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;
>>> }
>>> _______________________________________________
>>> svn-full mailing list
>>> svn-full_at_[hidden]
>>> http://www.open-mpi.org/mailman/listinfo.cgi/svn-full
>>
>
>
>
>> _______________________________________________
>> devel mailing list
>> devel_at_[hidden]
>> http://www.open-mpi.org/mailman/listinfo.cgi/devel
>
> --
> Gleb.
> _______________________________________________
> devel mailing list
> devel_at_[hidden]
> http://www.open-mpi.org/mailman/listinfo.cgi/devel



  • application/pkcs7-signature attachment: smime.p7s