RDMA/cma: Change RoCE packet life time from 18 to 16

The ack timeout retransmission time is affected by the following two
factors: one is packet life time, another is the HCA processing time.

Now the default packet lifetime(CMA_IBOE_PACKET_LIFETIME) is 18.

That means the minimum ack timeout is 2
seconds (2^(18+1)*4us=2.097seconds).  The packet lifetime means the
maximum transmission time of packets on the network, 2 seconds is too
long.

Assume the network is a clos topology with three layers, every packet will
pass through five hops of switches. Assume the buffer of every switch is
128MB and the port transmission rate is 25 Gbit/s, the maximum
transmission time of the packet is 200ms(128MB*5/25Gbit/s).  Add double
redundancy, it is less than 500ms.

So change the CMA_IBOE_PACKET_LIFETIME to 16, the maximum transmission
time of the packet will be about 500+ms, it is long enough.

Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Chao Leng <[email protected]>
Signed-off-by: Jason Gunthorpe <[email protected]>

0 files changed, 0 insertions, 0 deletions