Re: [PATCH 2/5] IB/core: Add max_sge argument to rdma_rw_ctx_init()

On 06/28/2016 01:52 PM, Christoph Hellwig wrote:

On Tue, Jun 28, 2016 at 01:26:03PM +0200, Bart Van Assche wrote:

quoted

The SGE limit for a queue pair is typically lower than what is
defined by the HCA limits. Hence make max_sge an argument.

I don't think this is the right way to approach any limit.  I'd rather
have a current limit in the ib_qp structure than having to pass it
explicitly and growing driver specific policy once again.

Hello Christoph,

The memory needed for QP buffers strongly depends on max_send_sge and 
max_recv_sge. Using a large value for max_send_sge and/or max_recv_sge 
for all drivers would increase the memory consumption for drivers that 
only submit RDMA requests with a low number of SG elements. Using a 
small value for max_send_sge and/or max_recv_sge would result in a 
performance decrease for drivers that benefit from large max_send_sge 
and/or max_recv_sge values. This is why I think that drivers should have 
the freedom to chose the max_send_sge and max_recv_sge values.

Bart.


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