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POinfiniband rdma poor transfer bw
 

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  1. POinfiniband rdma poor transfer bw
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    <p>In my application I use an infiniband infrastructure to send a stream of data from a server to another one. I have used to easy the development ip over infiniband because I'm more familiar with socket programming. Until now the performance (max bw) was good enough for me (I knew I wasn't getting the maximum bandwith achievable), now I need to get out from that infiniband connection more bandwidth.</p> <p>ib_write_bw claims that my max achievable bandwidth is around 1500 MB/s (I'm not getting 3000MB/s because my card is installed in a PCI 2.0 8x).</p> <p>So far so good. I coded my communication channel using ibverbs and rdma but I'm getting far less than the bandwith I can get, I'm even getting a bit less bandwidth than using socket but at least my application doesn't use any CPU power:</p> <p>ib_write_bw: 1500 MB/s</p> <p>sockets: 700 MB/s &lt;= One core of my system is at 100% during this test</p> <p>ibvers+rdma: 600 MB/s &lt;= No CPU is used at all during this test</p> <p>It seems that the bottleneck is here:</p> <pre><code>ibv_sge sge; sge.addr = (uintptr_t)memory_to_transfer; sge.length = memory_to_transfer_size; sge.lkey = memory_to_transfer_mr-&gt;lkey; ibv_send_wr wr; memset(&amp;wr, 0, sizeof(wr)); wr.wr_id = 0; wr.opcode = IBV_WR_RDMA_WRITE; wr.sg_list = &amp;sge; wr.num_sge = 1; wr.send_flags = IBV_SEND_SIGNALED; wr.wr.rdma.remote_addr = (uintptr_t)thePeerMemoryRegion.addr; wr.wr.rdma.rkey = thePeerMemoryRegion.rkey; ibv_send_wr *bad_wr = NULL; if (ibv_post_send(theCommunicationIdentifier-&gt;qp, &amp;wr, &amp;bad_wr) != 0) { notifyError("Unable to ibv post receive"); } </code></pre> <p>at this point the next code waiting for completation that is:</p> <pre><code>//Wait for completation ibv_cq *cq; void* cq_context; if (ibv_get_cq_event(theCompletionEventChannel, &amp;cq, &amp;cq_context) != 0) { notifyError("Unable to get a ibv cq event"); } ibv_ack_cq_events(cq, 1); if (ibv_req_notify_cq(cq, 0) != 0) { notifyError("Unable to get a req notify"); } ibv_wc wc; int myRet = ibv_poll_cq(cq, 1, &amp;wc); if (myRet &gt; 1) { LOG(WARNING) &lt;&lt; "Got more than a single ibv_wc, expecting one"; } </code></pre> <p>The time from my ibv_post_send and when ibv_get_cq_event returns an event is 13.3ms when transfering chuncks of 8 MB achieving then around 600 MB/s.</p> <p>To specify more (in pseudocode what I do globally):</p> <p>Active Side:</p> <pre><code>post a message receive rdma connection wait for rdma connection event &lt;&lt;at this point transfer tx flow starts&gt;&gt; start: register memory containing bytes to transfer wait remote memory region addr/key ( I wait for a ibv_wc) send data with ibv_post_send post a message receive wait for ibv_post_send event ( I wait for a ibv_wc) (this lasts 13.3 ms) send message "DONE" unregister memory goto start </code></pre> <p>Passive Side:</p> <pre><code>post a message receive rdma accept wait for rdma connection event &lt;&lt;at this point transfer rx flow starts&gt;&gt; start: register memory that has to receive the bytes send addr/key of memory registered wait "DONE" message unregister memory post a message receive goto start </code></pre> <p>Does anyone knows what I'm doing wrong? Or what I can improve? I'm not affected by "Not Invented Here" syndrome so I'm even open to throw away what I have done until now and adopting something else. I only need a point to point contiguous transfer.</p>
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