From 8aec63fddde1b4d27f533990924cf992a0c7749b Mon Sep 17 00:00:00 2001
From: Vitaliy Filippov <vitalif@yourcmc.ru>
Date: Fri, 24 Mar 2023 01:47:32 +0300
Subject: [PATCH] Implement simple "flow control" for RDMA (to check hypothesis
 about slowdowns)

---
 src/msgr_rdma.cpp | 25 +++++++++++++++++++++++--
 src/msgr_rdma.h   |  3 ++-
 2 files changed, 25 insertions(+), 3 deletions(-)

diff --git a/src/msgr_rdma.cpp b/src/msgr_rdma.cpp
index 8e63195f..20b48f13 100644
--- a/src/msgr_rdma.cpp
+++ b/src/msgr_rdma.cpp
@@ -353,8 +353,10 @@ static void try_send_rdma_wr(osd_client_t *cl, ibv_sge *sge, int op_sge)
         .wr_id = (uint64_t)(cl->peer_fd*2+1),
         .sg_list = sge,
         .num_sge = op_sge,
-        .opcode = IBV_WR_SEND,
+        .opcode = cl->rdma_conn->avail_recv > 0 ? IBV_WR_SEND_WITH_IMM : IBV_WR_SEND,
         .send_flags = IBV_SEND_SIGNALED,
+        // Notify peer about our available incoming buffers
+        .imm_data = (uint32_t)cl->rdma_conn->avail_recv,
     };
     int err = ibv_post_send(cl->rdma_conn->qp, &wr, &bad_wr);
     if (err || bad_wr)
@@ -363,15 +365,27 @@ static void try_send_rdma_wr(osd_client_t *cl, ibv_sge *sge, int op_sge)
         exit(1);
     }
     cl->rdma_conn->cur_send++;
+    cl->rdma_conn->avail_send--;
+    cl->rdma_conn->avail_recv = 0;
 }
 
 bool osd_messenger_t::try_send_rdma(osd_client_t *cl)
 {
     auto rc = cl->rdma_conn;
-    if (!cl->send_list.size() || rc->cur_send >= rc->max_send)
+    if (rc->cur_send >= rc->max_send)
     {
         return true;
     }
+    if (!cl->send_list.size() || rc->use_flow_control && rc->avail_send <= 0)
+    {
+        if (rc->avail_recv)
+        {
+            // Only notify about available buffers so 2 peers don't lock each other
+            rc->send_sizes.push_back(0);
+            try_send_rdma_wr(cl, NULL, 0);
+        }
+        return true;
+    }
     uint64_t op_size = 0, op_sge = 0;
     ibv_sge sge[rc->max_sge];
     while (rc->send_pos < cl->send_list.size())
@@ -431,6 +445,7 @@ static void try_recv_rdma_wr(osd_client_t *cl, void *buf)
         exit(1);
     }
     cl->rdma_conn->cur_recv++;
+    cl->rdma_conn->avail_recv++;
 }
 
 bool osd_messenger_t::try_recv_rdma(osd_client_t *cl)
@@ -492,6 +507,11 @@ void osd_messenger_t::handle_rdma_events()
             if (!is_send)
             {
                 rc->cur_recv--;
+                if ((wc[i].wc_flags & IBV_WC_WITH_IMM) && wc[i].imm_data > 0)
+                {
+                    rc->avail_send += wc[i].imm_data;
+                    rc->use_flow_control = true;
+                }
                 if (!handle_read_buffer(cl, rc->recv_buffers[rc->next_recv_buf], wc[i].byte_len))
                 {
                     // handle_read_buffer may stop the client
@@ -499,6 +519,7 @@ void osd_messenger_t::handle_rdma_events()
                 }
                 try_recv_rdma_wr(cl, rc->recv_buffers[rc->next_recv_buf]);
                 rc->next_recv_buf = (rc->next_recv_buf+1) % rc->recv_buffers.size();
+                try_send_rdma(cl);
             }
             else
             {
diff --git a/src/msgr_rdma.h b/src/msgr_rdma.h
index 7c56f4a1..659fbd56 100644
--- a/src/msgr_rdma.h
+++ b/src/msgr_rdma.h
@@ -45,7 +45,8 @@ struct msgr_rdma_connection_t
     ibv_qp *qp = NULL;
     msgr_rdma_address_t addr;
     int max_send = 0, max_recv = 0, max_sge = 0;
-    int cur_send = 0, cur_recv = 0;
+    int cur_send = 0, cur_recv = 0, avail_recv = 0, avail_send = 0;
+    bool use_flow_control = false;
     uint64_t max_msg = 0;
 
     int send_pos = 0, send_buf_pos = 0;