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svcrdma: Avoid DMA mapping small RPC Replies

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On some platforms, DMA mapping part of a page is more costly than
copying bytes. Indeed, not involving the I/O MMU can help the
RPC/RDMA transport scale better for tiny I/Os across more RDMA
devices. This is because interaction with the I/O MMU is eliminated
for each of these small I/Os. Without the explicit unmapping, the
NIC no longer needs to do a costly internal TLB shoot down for
buffers that are just a handful of bytes.

Since pull-up is now a more a frequent operation, I've introduced a
trace point in the pull-up path. It can be used for debugging or
user-space tools that count pull-up frequency.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
This commit is contained in:
Chuck Lever 2020-03-03 13:28:14 -05:00
parent aee4b74a3f
commit 0dabe948f2
3 changed files with 31 additions and 1 deletions
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1
include/linux/sunrpc/svc_rdma.h
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@ -52,6 +52,7 @@
/* Default and maximum inline threshold sizes */
enum {
RPCRDMA_PULLUP_THRESH = RPCRDMA_V1_DEF_INLINE_SIZE >> 1,
RPCRDMA_DEF_INLINE_THRESH = 4096,
RPCRDMA_MAX_INLINE_THRESH = 65536
};