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xprtrdma: Scale receive batch size with credit window

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The fixed RPCRDMA_MAX_RECV_BATCH of 7 results in frequent
small ib_post_recv batches during high-rate workloads. With
a 128-slot credit window, receives are reposted every 7th
completion, each batch incurring atomic serialization and a
doorbell write.

Replace the fixed batch constant with a per-endpoint value
scaled to 25% of the negotiated credit window. For a typical
128-credit connection this raises the batch from 7 to 32,
reducing doorbell frequency by roughly 4x and amortizing the
per-batch atomic and MMIO costs over a larger group of
receive WRs.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
This commit is contained in:
Chuck Lever 2026-03-06 16:56:27 -05:00 committed by Trond Myklebust
parent 7a079ab57c
commit 93b4791adb
3 changed files with 4 additions and 2 deletions
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3
net/sunrpc/xprtrdma/frwr_ops.c
View File

@ -244,9 +244,10 @@ int frwr_query_device(struct rpcrdma_ep *ep, const struct ib_device *device)
}
ep->re_attr.cap.max_send_wr += RPCRDMA_BACKWARD_WRS;
ep->re_attr.cap.max_send_wr += 1; /* for ib_drain_sq */
ep->re_recv_batch = ep->re_max_requests >> 2;
ep->re_attr.cap.max_recv_wr = ep->re_max_requests;
ep->re_attr.cap.max_recv_wr += RPCRDMA_BACKWARD_WRS;
ep->re_attr.cap.max_recv_wr += RPCRDMA_MAX_RECV_BATCH;
ep->re_attr.cap.max_recv_wr += ep->re_recv_batch;
ep->re_attr.cap.max_recv_wr += 1; /* for ib_drain_rq */
ep->re_max_rdma_segs =

2
net/sunrpc/xprtrdma/verbs.c
View File

@ -1374,7 +1374,7 @@ void rpcrdma_post_recvs(struct rpcrdma_xprt *r_xprt, int needed)
if (likely(ep->re_receive_count > needed))
goto out;
needed -= ep->re_receive_count;
needed += RPCRDMA_MAX_RECV_BATCH;
needed += ep->re_recv_batch;
if (atomic_inc_return(&ep->re_receiving) > 1)
goto out_dec;

1
net/sunrpc/xprtrdma/xprt_rdma.h
View File

@ -96,6 +96,7 @@ struct rpcrdma_ep {
struct rpcrdma_notification re_rn;
int re_receive_count;
unsigned int re_max_requests; /* depends on device */
unsigned int re_recv_batch;
unsigned int re_inline_send; /* negotiated */
unsigned int re_inline_recv; /* negotiated */