On Nov 7, 2007, at 11:06 AM, Jeff Squyres wrote:
Gleb -- I finally talked with Galen and Don about this issue in depth. Our understanding is that the "request may get freed before recursion unwinds" issue is *only* a problem within the context of a single MPI call (e.g., MPI_SEND). Is that right?
I wonder how this happens ?
Specifically, if in an MPI_SEND, the BTL ends up buffering the message and setting early completion, but then recurses into opal_progress() and ends up sending the message and freeing the request during the recursion, then when the recursion unwinds, the original caller will have a stale request.
The same callback is called in both cases. In the case that you described, the callback is called just a little bit deeper into the recursion, when in the "normal case" it will get called from the first level of the recursion. Or maybe I miss something here ...
george.
This is *only* a problem for requests that are involved from the current top-level MPI call. Request from prior calls to MPI functions (e.g., a request from a prior call to MPI_ISEND) are ok because a) we've already done the Right Things to ensure the safety of that request, and b) that request is not on the recursive stack anywhere to become stale as the recursion unwinds. Right? If so, Galen proposes the following: 1. in conjunction with the NOT_ON_WIRE proposal...2. make a new PML request flag DONT_FREE_ME (or some better name :-) ).3. blocking MPI_SEND/MPI_RECV calls will set this flag (or, more specifically, the top of the PML calls for blocking send/receive) right when the request is allocated (i.e., before calling btl_send()). 4. when the PML is called for completion on this request, it will do all the stuff that it needs to effect completion -- but then it will see the DONT_FREE_ME flag and not actually free the request. Obviously, if DONT_FREE_ME is *not* set, then the PML does what it does today: it frees the request. 5. the top-level PML call will eventually complete: 5a. For blocking PML calls (e.g., corresponding to MPI_SEND and MPI_RECV), the request can be unconditionally freed. 5b. For non-blocking PML calls (e.g., corresponding to MPI_ISEND), only free the request if it was completed. Note that with this scheme, it becomes irrelevant as to whether the PML completion call is invoked on the first descent into the BTL or recursively via opal_progress. How does that sound? If that all works, it might be beneficial to put this back to the 1.2 branch because there are definitely apps that would benefit from it. On Oct 23, 2007, at 10:19 AM, Gleb Natapov wrote:So this problem goes WAY back.. The problem here is that the PML marks MPI completion just prior to calling btl_send and then returns to the user. This wouldn't be a problem if the BTL then did something, but in the case of OpenIB this fragment may not actually be on the wire (the joys of user level flow control). One solution that we proposed was to allow btl_send to return either OMPI_SUCCESS or OMPI_NOT_ON_WIRE. OMPI_NOT_ON_WIRE would allow the PML to not mark MPI completion of the fragment and then MPI_WAITALL and others will do there job properly.I even implemented this once, but there is a problem. Currently we mark request as completed on MPI level and then do btl_send(). Whenever IB completion will happen the request will be marked as complete on PML level and freed. The fix requires to change the order like this: Call btl_send(), check return value from BTL and mark request complete as necessary. The problem is that because we allow BTL to call opal_progress() internally the request may be already completed on MPI and MPL levels and freed before return from the call to btl_send().I did a code review to see how hard it will be to get rid of recursionin Open MPI and I think this is doable. We have to disallow calling progress() (or other functions that may call progress() internally) from BTL and from ULP callbacks that are called by BTL. There is no much places that break this law. The main offenders are calls to FREE_LIST_WAIT(), but those never actually call progress if they cangrow without limit and this is the most common use of FREE_LIST_WAIT()so they may be safely changed to FREE_LIST_GET(). After we will solve recursion problem the fix to the problem will be a couple of lines of code.- Galen On 10/11/07 11:26 AM, "Gleb Natapov" <gl...@voltaire.com> wrote:On Fri, Oct 05, 2007 at 09:43:44AM +0200, Jeff Squyres wrote:David -- Gleb and I just actively re-looked at this problem yesterday; we think it's related to https://svn.open-mpi.org/trac/ompi/ticket/ 1015. We previously thought this ticket was a different problem, butour analysis yesterday shows that it could be a real problem in theopenib BTL or ob1 PML (kinda think it's the openib btl because it doesn't seem to happen on other networks, but who knows...). Gleb is investigating.Here is the result of the investigation. The problem is different than#1015 ticket. What we have here is one rank calls isend() of a smallmessage and wait_all() in a loop and another one calls irecv(). The problem is that isend() usually doesn't call opal_progress() anywhere and wait_all() doesn't call progress if all requests are already completed so messages are never progressed. We may force opal_progress() to be called by setting btl_openib_free_list_max to 1000. Then wait_all() will call progress because not every request will be immediately completed by OB1. Or we can limit a number of uncompleted requests that OB1 can allocate by setting pml_ob1_free_list_max to 1000. Then opal_progress() will be called from a free_list_wait() when max will be reached. The second option works much faster for me.On Oct 5, 2007, at 12:59 AM, David Daniel wrote:Hi Folks, I have been seeing some nasty behaviour in collectives, particularly bcast and reduce. Attached is a reproducer (for bcast). The code will rapidly slow to a crawl (usually interpreted as a hang in real applications) and sometimes gets killed with sigbus or sigterm. I see this with openmpi-1.2.3 or openmpi-1.2.4 ofed 1.2 linux 2.6.19 + patches gcc (GCC) 3.4.5 20051201 (Red Hat 3.4.5-2) 4 socket, dual core opterons run as mpirun --mca btl self,openib --npernode 1 --np 4 bcast-hang To my now uneducated eye it looks as if the root process is rushing ahead and not progressing earlier bcasts. Anyone else seeing similar? Any ideas for workarounds? As a point of reference, mvapich2 0.9.8 works fine. Thanks, David <bcast-hang.c> _______________________________________________ devel mailing list de...@open-mpi.org http://www.open-mpi.org/mailman/listinfo.cgi/devel-- Jeff Squyres Cisco Systems _______________________________________________ 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_______________________________________________ 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-- Jeff Squyres Cisco Systems _______________________________________________ devel mailing list de...@open-mpi.org http://www.open-mpi.org/mailman/listinfo.cgi/devel
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