David Laight [off-list ref] wrote:

Isn't that going to be completely dominated by the cache fills
from memory?

I'd have thought you'd need to use something with a lot of
small fragments so that the iteration code dominates the copy.

Okay, if I switch it to using MAP_ANON for the big 256MiB buffer, switch all
the benchmarking tests to use copy_from_iter() rather than copy_to_iter() and
make the iovec benchmark use a separate iovec for each page, there's then a
single page replicated across the mapping.

Given that, without my macro-to-inline-func patches applied, I see:

	iov_kunit_benchmark_bvec: avg 3184 uS, stddev 16 uS
	iov_kunit_benchmark_bvec: avg 3189 uS, stddev 17 uS
	iov_kunit_benchmark_bvec: avg 3190 uS, stddev 16 uS
	iov_kunit_benchmark_bvec_outofline: avg 3731 uS, stddev 10 uS
	iov_kunit_benchmark_bvec_outofline: avg 3735 uS, stddev 10 uS
	iov_kunit_benchmark_bvec_outofline: avg 3738 uS, stddev 11 uS
	iov_kunit_benchmark_bvec_split: avg 3403 uS, stddev 10 uS
	iov_kunit_benchmark_bvec_split: avg 3405 uS, stddev 18 uS
	iov_kunit_benchmark_bvec_split: avg 3407 uS, stddev 29 uS
	iov_kunit_benchmark_iovec: avg 6616 uS, stddev 20 uS
	iov_kunit_benchmark_iovec: avg 6619 uS, stddev 22 uS
	iov_kunit_benchmark_iovec: avg 6621 uS, stddev 46 uS
	iov_kunit_benchmark_kvec: avg 2671 uS, stddev 12 uS
	iov_kunit_benchmark_kvec: avg 2671 uS, stddev 13 uS
	iov_kunit_benchmark_kvec: avg 2675 uS, stddev 12 uS
	iov_kunit_benchmark_ubuf: avg 6191 uS, stddev 1946 uS
	iov_kunit_benchmark_ubuf: avg 6418 uS, stddev 3263 uS
	iov_kunit_benchmark_ubuf: avg 6443 uS, stddev 3275 uS
	iov_kunit_benchmark_xarray: avg 3689 uS, stddev 5 uS
	iov_kunit_benchmark_xarray: avg 3689 uS, stddev 6 uS
	iov_kunit_benchmark_xarray: avg 3698 uS, stddev 22 uS
	iov_kunit_benchmark_xarray_outofline: avg 4202 uS, stddev 3 uS
	iov_kunit_benchmark_xarray_outofline: avg 4204 uS, stddev 9 uS
	iov_kunit_benchmark_xarray_outofline: avg 4210 uS, stddev 9 uS

and with, I get:

	iov_kunit_benchmark_bvec: avg 3241 uS, stddev 13 uS
	iov_kunit_benchmark_bvec: avg 3245 uS, stddev 16 uS
	iov_kunit_benchmark_bvec: avg 3248 uS, stddev 15 uS
	iov_kunit_benchmark_bvec_outofline: avg 3705 uS, stddev 12 uS
	iov_kunit_benchmark_bvec_outofline: avg 3706 uS, stddev 10 uS
	iov_kunit_benchmark_bvec_outofline: avg 3709 uS, stddev 9 uS
	iov_kunit_benchmark_bvec_split: avg 3446 uS, stddev 10 uS
	iov_kunit_benchmark_bvec_split: avg 3447 uS, stddev 12 uS
	iov_kunit_benchmark_bvec_split: avg 3448 uS, stddev 12 uS
	iov_kunit_benchmark_iovec: avg 6587 uS, stddev 22 uS
	iov_kunit_benchmark_iovec: avg 6587 uS, stddev 22 uS
	iov_kunit_benchmark_iovec: avg 6590 uS, stddev 27 uS
	iov_kunit_benchmark_kvec: avg 2671 uS, stddev 12 uS
	iov_kunit_benchmark_kvec: avg 2672 uS, stddev 12 uS
	iov_kunit_benchmark_kvec: avg 2676 uS, stddev 19 uS
	iov_kunit_benchmark_ubuf: avg 6241 uS, stddev 2199 uS
	iov_kunit_benchmark_ubuf: avg 6266 uS, stddev 2245 uS
	iov_kunit_benchmark_ubuf: avg 6513 uS, stddev 3899 uS
	iov_kunit_benchmark_xarray: avg 3695 uS, stddev 6 uS
	iov_kunit_benchmark_xarray: avg 3695 uS, stddev 7 uS
	iov_kunit_benchmark_xarray: avg 3703 uS, stddev 11 uS
	iov_kunit_benchmark_xarray_outofline: avg 4215 uS, stddev 16 uS
	iov_kunit_benchmark_xarray_outofline: avg 4217 uS, stddev 20 uS
	iov_kunit_benchmark_xarray_outofline: avg 4224 uS, stddev 10 uS

Interestingly, most of them are quite tight, but UBUF is all over the place.
That's with the test covering the entire 256M span with a single UBUF
iterator, so it would seem unlikely that the difference is due to the
iteration framework.

David