[ofa-general] [RFC] XRC -- make receiving XRC QP independent of any one user process
Tang, Changqing
changquing.tang at hp.com
Thu Jan 3 07:49:17 PST 2008
Thanks for the comment.
Another issue I have after thinking about the interface more.
Rank A is the sender, rank B and C are two ranks on a remote node. At first, B creates the
receiving QP and make connection to A and register the QP number for receiving. And A gets
the receiving QP nubmer from B. After some communication between A and B, B decides to close
the connection, and unregister the QP number. Then A and C want to talk, so A tell C the
receiving QP number, C tries to register the QP number.
I wonder at the time when C tries to register the QP number, the receiving QP has been
destroyed by the kernel, since when B unregister the QP number, the reference count becomes
zero, and kernel will cleanup it.
Am I right ?
--CQ
> -----Original Message-----
> From: Ishai Rabinovitz [mailto:ishai at mellanox.co.il]
> Sent: Thursday, January 03, 2008 2:59 AM
> To: panda at cse.ohio-state.edu; Tang, Changqing; Jack
> Morgenstein; Pavel Shamis
> Cc: Gleb Natapov; Roland Dreier; general at lists.openfabrics.org
> Subject: RE: [ofa-general] [RFC] XRC -- make receiving XRC QP
> independent of any one user process
>
> Please see my comments (prefix [Ishai])
>
> -----Original Message-----
> From: Tang, Changqing [mailto:changquing.tang at hp.com]
> Sent: ד 02 ינואר 2008 17:27
> To: Jack Morgenstein; Pavel Shamis
> Cc: Ishai Rabinovitz; Gleb Natapov; Roland Dreier;
> general at lists.openfabrics.org
> Subject: RE: [ofa-general] [RFC] XRC -- make receiving XRC QP
> independent of any one user process
>
>
> This interface is OK for me.
>
> Now, every rank on a node who wants to receive message from
> the same remote rank must know the same receiving QP number,
> and register for receiving using this QP number.
>
> If rank B does not register (receiving QP has been created by
> another rank A on the node), and sender know B's SRQ number,
> if sender sends a message to B, can B still receive this
> message ? (I hope, no register, no receive)
>
> [Ishai] I guess that from the MPI layer prospective, the
> sender can not know B's SRQ number until it ask B to give it
> to him. So B can register to this QP before sending the SRQ number.
>
> I hope to know the opinion from other MPI team, or other XRC user.
>
> [Ishai] We already discussed this issues with Open MPI IB
> group, and it looks fine to them. I'm sending this mail to
> Prof. Panda, so he can comment on it as well.
>
> --CQ
>
>
>
> > -----Original Message-----
> > From: Jack Morgenstein [mailto:jackm at dev.mellanox.co.il]
> > Sent: Monday, December 31, 2007 5:40 AM
> > To: pasha at mellanox.co.il
> > Cc: ishai at mellanox.co.il; Gleb Natapov; Roland Dreier; Tang,
> > Changqing; general at lists.openfabrics.org
> > Subject: Re: [ofa-general] [RFC] XRC -- make receiving XRC QP
> > independent of any one user process
> >
> > > Tang, Changqing wrote:
> > > > If I have a MPI server processes on a node, many
> > other MPI
> > > > client processes will dynamically connect/disconnect with the
> > > > server. The server use same XRC domain.
> > > >
> > > > Will this cause accumulating the "kernel" QP for such
> > > > application ? we want the server to run 365 days a year.
> > > >
> > > > I have some question about the scenario above. Did you
> > call for the
> > > > mpi disconnect on the both ends (server/client) before
> the client
> > > > exit (did we must to do it?)
> > >
> > > Yes, both ends will call disconnect. But for us,
> > MPI_Comm_disconnect()
> > > call is not a collective call, it is just a local operation.
> > >
> > > --CQ
> > >
> > Possible solution (internal review as yet):
> >
> > Each user process registers with the XRC QP:
> > a. each process registers ONCE. If it registers multiple times,
> > there is no reference increment --
> > rather the registration succeeds, but only one PID entry is
> > kept per QP.
> > b. Can have cleanup in the event of a process dying suddenly.
> > c. QP cannot be destroyed while there are any user
> processes still
> > registered with it.
> >
> > libibverbs API is as follows:
> >
> > ==============================================================
> > ========================
> > /**
> > * ibv_xrc_rcv_qp_alloc - creates an XRC QP for serving as a
> > receive-side only QP,
> > * and moves the created qp through the RESET->INIT and
> > INIT->RTR transitions.
> > * (The RTR->RTS transition is not needed, since this QP
> > does no sending).
> > * The sending XRC QP uses this QP as destination, while
> > specifying an XRC SRQ
> > * for actually receiving the transmissions and
> > generating all completions on the
> > * receiving side.
> > *
> > * This QP is created in kernel space, and persists
> > until the last process registered
> > * for the QP calls ibv_xrc_rcv_qp_unregister() (at
> > which time the QP is destroyed).
> > *
> > * @pd: protection domain to use. At lower layer, this provides
> > access to userspace obj
> > * @xrc_domain: xrc domain to use for the QP.
> > * @attr: modify-qp attributes needed to bring the QP to RTR.
> > * @attr_mask: bitmap indicating which attributes are
> provided in the
> > attr struct.
> > * used for validity checking.
> > * @xrc_rcv_qpn: qp_num of created QP (if success). To be passed to
> > the remote node (sender).
> > * The remote node will use xrc_rcv_qpn in
> > ibv_post_send when sending to
> > * XRC SRQ's on this host in the same xrc domain.
> > *
> > * RETURNS: success (0), or a (negative) error value.
> > *
> > * NOTE: this verb also registers the calling user-process
> with the QP
> > at its creation time
> > * (implicit call to ibv_xrc_rcv_qp_register), to avoid
> > race conditions.
> > * The creating process will need to call
> > ibv_xrc_qp_unregister() for the QP to release it from
> > * this process.
> > */
> >
> > int ibv_xrc_rcv_qp_alloc(struct ibv_pd *pd,
> > struct ibv_xrc_domain *xrc_domain,
> > struct ibv_qp_attr *attr,
> > enum ibv_qp_attr_mask attr_mask,
> > uint32_t *xrc_rcv_qpn);
> >
> >
> =====================================================================
> >
> > /**
> > * ibv_xrc_rcv_qp_register: registers a user process with an XRC QP
> > which serves as
> > * a receive-side only QP.
> > *
> > * @xrc_domain: xrc domain the QP belongs to (for verification).
> > * @xrc_qp_num: The (24 bit) number of the XRC QP.
> > *
> > * RETURNS: success (0),
> > * or error (-EINVAL), if:
> > * 1. There is no such QP_num allocated.
> > * 2. The QP is allocated, but is not an receive XRC QP
> > * 3. The XRC QP does not belong to the given domain.
> > */
> > int ibv_xrc_rcv_qp_register(struct ibv_xrc_domain *xrc_domain,
> > uint32_t xrc_qp_num);
> >
> >
> =====================================================================
> > /**
> > * ibv_xrc_rcv_qp_unregister: detaches a user process from
> an XRC QP
> > serving as
> > * a receive-side only QP. If as a result, there are
> > no remaining userspace processes
> > * registered for this XRC QP, it is destroyed.
> > *
> > * @xrc_domain: xrc domain the QP belongs to (for verification).
> > * @xrc_qp_num: The (24 bit) number of the XRC QP.
> > *
> > * RETURNS: success (0),
> > * or error (-EINVAL), if:
> > * 1. There is no such QP_num allocated.
> > * 2. The QP is allocated, but is not an XRC QP
> > * 3. The XRC QP does not belong to the given domain.
> > * NOTE: I don't see any reason to return a special code if
> the QP is
> > destroyed -- the unregister simply
> > * succeeds.
> > */
> > int ibv_xrc_rcv_qp_unregister(struct ibv_xrc_domain *xrc_domain,
> > uint32_t xrc_qp_num);
> > ==============================================================
> > ===============================
> >
> > Usage:
> >
> > 1. Sender creates an XRC QP (sending QP) 2. Sender sends some
> > receiving process on a remote node (say R1) a request to provide an
> > XRC QP and XRC SRQ for
> > receiving messages (the request includes the sending QP number).
> > 3. R1 calls ibv_xrc_rcv_qp_alloc() to create a receiving XRC QP in
> > kernel space, and move
> > that QP up to RTR state. This function also registers process R1
> > with the XRC QP.
> > 4. R1 calls ibv_create_xrc_srq() to create an SRQ for
> receive messages
> > via the just created XRC QP.
> > 5. R1 responds to request, providing the XRC qp number, and XRC SRQ
> > number to be used in communication.
> > 6. Sender then may wish to communicate with another
> receiving process
> > on the remote host (say R2).
> > it sends a request to R2 containing the remote XRC QP number
> > (obtained from R1)
> > which it will use to send messages.
> > 7. R2 creates an XRC SRQ (if one does not already exist for the
> > domain), and also
> > calls ibv_xrc_rcv_qp_register() to register the process
> R2 with the
> > XRC QP created by R1.
> > 8. If R1 no longer needs to communicate with the sender, it calls
> > ibv_xrc_rcv_qp_unregister() for the QP.
> > The QP will not yet be destroyed, since R2 is still
> registered with
> > it.
> > 9. If R2 no longer needs to communicate with the sender, it calls
> > ibv_xrc_rcv_qp_unregister() for the QP.
> > At this point, the QP is destroyed, since no processes remain
> > registered with it.
> >
> > NOTES:
> > 1. The problem of the QP being destroyed and quickly
> re-allocated does
> > not exist -- the upper bits of the
> > QP number are incremented at each allocation (except for the MSB
> > which is always 1 for XRC QPs). Thus,
> > even if the same QP is re-allocated, its QP number
> (stored in the
> > QP object) will be different than
> > expected (unless it is re-destroyed/re-allocated several hundred
> > times).
> >
> > 2. With this model, we do not need a heartbeat: if a
> receiving process
> > dies, all XRC QPs it has registered for will
> > be unregistered as part of process cleanup in kernel space.
> >
> > - Jack
> >
> >
>
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