On Tue, Nov 20, 2018 at 09:35:07PM -0800, Sagi Grimberg wrote:

quoted

Given it is over TCP, I guess it should be doable for you to preallocate one
256-bvec table in one page for each request, then sets the max segment size as
(unsigned int)-1, and max segment number as 256, the preallocated table
should work anytime.

256 bvec table is really a lot to preallocate, especially when its not
needed, I can easily initialize the bvec_iter on the bio bvec. If this
involves preallocation of the worst-case than I don't consider this to
be an improvement.

Ok, I took a look at the nvme-tcp code and it seems you care about
bios because you want a contiguos bio chunk for sending it down
the networking code.  Yes, in that case we sort of need to iterate
over bios.  But you already have a special case for discard, so you
don't really need any of the magic in the bio_bvecs() helper either
can can just count bi_vcnt in the bio.