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void ibv_ack_cq_events(struct ibv_cq *cq, unsigned int nevents);


ibv_ack_cq_events() acknowledge Completion events.

In order to prevent races, all of the Completion events that were read using ibv_get_cq_event() must be acknowledged using ibv_ack_cq_events().

Calling ibv_ack_cq_events() may be relatively expensive in the data-path since it uses mutual exclusion object(s). Therefore, it may be better to amortize this cost by keeping a count of the number of events needing acknowledgement and acknowledging several Completion events in one call to ibv_ack_cq_events().


Name Direction Description
cq in CQ that was returned from ibv_create_cq()
nevents in Number of Completion events to acknowledge

Return Values

None (this function always succeeds).


Read a Completion event and acknowledge it:

struct ibv_context *context;
struct ibv_cq *cq;
void *ev_ctx = NULL; /* can be initialized with other values for the CQ context */
/* Create a CQ, which is associated with a Completion Event Channel */
cq = ibv_create_cq(ctx, 1, ev_ctx, channel, 0);
if (!cq) {
        fprintf(stderr, "Failed to create CQ\n");
        return -1;
/* Request notification before any completion can be created (to prevent races) */
ret = ibv_req_notify_cq(cq, 0);
if (ret) {
        fprintf(stderr, "Couldn't request CQ notification\n");
        return -1;
. /* Perform an operation that will eventually end with Work Completion */
/* The following code will be called each time you need to read a Work Completion */
struct ibv_cq *ev_cq;
void *ev_ctx;
int ret;
int ne;
/* Wait for the Completion event */
ret = ibv_get_cq_event(channel, &ev_cq, &ev_ctx);
if (ret) {
        fprintf(stderr, "Failed to get CQ event\n");
        return -1;
/* Request notification upon the next completion event */
ret = ibv_req_notify_cq(ev_cq, 0);
if (ret) {
        fprintf(stderr, "Couldn't request CQ notification\n");
        return -1;
/* Empty the CQ: poll all of the completions from the CQ (if any exist) */
do {
        ne = ibv_poll_cq(cq, 1, &wc);
        if (ne < 0) {
                fprintf(stderr, "Failed to poll completions from the CQ: ret = %d\n",
                return -1;
        /* there may be an extra event with no completion in the CQ */
        if (ne == 0)
        if (wc.status != IBV_WC_SUCCESS) {
                fprintf(stderr, "Completion with status 0x%x was found\n",
                return -1;
} while (ne);
/* Ack the event */
ibv_ack_cq_events(ev_cq, 1);


Why do I need to call ibv_ack_cq_events() anyway?

This verb is used in order to prevent internal races.

What will happen if I won't acknowledge all of Completion events?

If one won't acknowledge all of the Completion events that he reads using ibv_get_cq_event(), destroying the CQ, that got the events, will be blocked forever. This behavior is used in order to prevent an acknowledgment on a resource that has already been destroyed.

What will happen if I read a Completion event and my process will be terminated intentionally (for example, by calling exit()) or unintentionally (for example, by segmentation fault) before I could acknowledge the event?

Even if there is any unacknowledged Completion event, when the process will be terminated, no matter the reason is, all of the resources will be cleaned.

Should I acknowledge the Completion events one by one, or acknowledge several Completion events at once?

Acknowledging Completion events require the use of mutual exclusion object(s), so it is highly advised to acknowledged several Completion events at once (especially, in the data-path).

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Tell us what do you think.

  1. DjvuLee says: February 26, 2015

    "prevent internal races." means what? can you explain deeper, If we only one thread run the ibv_get_cq_event, how can we introduce the race?

    • Dotan Barak says: February 26, 2015


      Yes, but Libibverbs is thread safe, so it should be ready to work with thread.

      Furthermore, when reading Completion event from a channel - you get the CQ handler that got the event.
      Without a proper protection, you may get a handler to a CQ which was already destroyed;
      which may lead to inconsistency or segmentation fault.


      • DjvuLee says: March 4, 2015

        Thanks very much for your answer! I missed your wonderful answer in my Email box.

        I have one more question.
        In a client-server scenario, if the server do not know the size of the request message will be send from the client, how can the server register the memory? bigger memory region can not totally deal with this problem?
        In TCP/IP, there is flow control inside the protocol, if the sender send too much message, the receiver can notify it to slow down, how can we deal with this in a RDMA protocol?

      • Dotan Barak says: March 4, 2015


        One can register big memory buffer (the maximum possible value) and be ready for this scenario;
        or the client side can split the message to the maximum buffer size, reported by the server.

        You can register big buffers without any problem (as long as you don't try to register most of the machine memory).

        The flow control that you described; do you mean in the application layer or in the protocol?

        Let's assume that you are talking about protocol:
        And what do you mean send too much? in RDMA Read/Writes there (almost) isn't any problem.
        If Receive Requests needs to be consumed in remote side, there is the RNR flow.

        Beside what I described, AFAIK, the protocol doesn't start slowing the traffic.


  2. DjvuLee says: March 4, 2015

    The answer is very clearly! Yes, I mean there is a receiver not ready in the Send/Recv way , I understand this now.

    But after the client received the RNR notify from the server, when the client can send message again? Does the server will send a message to notify the client to continue send message?

    Consider another question, If the client want to use the RDMA write operation to write some message to the server, how can the client get the rkey of the buffer in server, I think the server may send the buffer's rkey and location to the client, this may introduce some latency, because the client will wait the server send this info first, and send message next. If the client will write many times to the server, is there any way to avoid this or cut down the cost?

    • Dotan Barak says: March 4, 2015


      The 'min_rnr_timer' specify how much time the remote side should wait before resend the message in RNR flow.

      The answer to the second question: the client can't perform RDMA Write to the server memory unless its know the
      server's Memory Region attributes (size, key. address). This can only be done either by:
      1) When connecting (for example using rdmacm), send this info as private data
      2) Send RDMA messages (for example: using Send opcode)
      3) Using Out Of Band protocol/system

      Sorry, no free launches here ...


      • DjvuLee says: March 4, 2015

        Got it! thanks for your nice answer!

  3. neuralcn says: April 24, 2015

    But if set min_rnr_timer,when meet RNR,the transaction response time may change larger. If so, how to do with it?

    • Dotan Barak says: April 24, 2015


      I don't really understand the question here. Can you please rephrase it?


      • neuralcn says: April 27, 2015

        I mean: when set min_rnr_timer, the remote side should wait and retry, but the server side may be already post receive, so this will not real time.

      • Dotan Barak says: April 28, 2015

        The min_rnr_timer give enough time to the receiver until it will post more Receive Requests to its Receive Queue.
        Since it knows his priorities and algorithms, it specify the time that the sender should wait before resending the message.

        I must admit that i don't understand what "real time" time.

        There won't be any other message that the receiver will send to the receiver "hey, now I have Receive Requests; you can (re)send the message now).
        And the sender will wait until (at least) the min_rnr_timer will expire; even if just after the receiver send the RNR NACK, many Receive Requests were posted ...


  4. CQ Tang says: August 4, 2016

    About the race condition. Here is you statement:

    when reading Completion event from a channel - you get the CQ handler that got the event.
    Without a proper protection, you may get a handler to a CQ which was already destroyed;
    which may lead to inconsistency or segmentation fault.

    But after you call ibv_ack_cq_event(), the CQ can be destroyed immediately, then later as in your example code, ibv_req_notify_cq() and ibv_poll_cq() will use the destroyed CQ handle.

    Does that mean we should call ibv_ack_cq_event() last in your example code, instead of right after ibv_get_cq_event() call?

    • Dotan Barak says: August 11, 2016


      Any operation on a destroyed CQ may end with a segmentation fault.
      I understand what you mean, and I illustrated a code sample on how to use a CQ.

      If one wishes to be on the same side, he can move the ibv_ack_cq_event() after using the CQ.
      However to destroy the CQ, the associated QPs should be destroyed first;
      so this is part of the teardown flow...

      Nice comment though

      • Dotan Barak says: August 18, 2016

        After rethinking about, since I believe in protective programming,
        I agree.

        The ack on the event was moved to the end of the code sample.


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