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Subject: Re: [RFC PATCH 9/9] iov_iter: Add benchmarking kunit tests for UBUF/IOVEC
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Date:   Fri, 15 Sep 2023 11:10:27 +0100
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David Laight <David.Laight@ACULAB.COM> wrote:

> > Add kunit tests to benchmark 256MiB copies to a UBUF iterator and an IOVEC
> > iterator.  This attaches a userspace VM with a mapped file in it
> > temporarily to the test thread.
> 
> Isn't that going to be completely dominated by the cache fills
> from memory?

Yes...  but it should be consistent in the amount of time that consumes since
no device drivers are involved.  I can try adding the same folio to the
anon_file multiple times - it might work especially if I don't put the pages
on the LRU (if that's even possible) - but I wanted separate pages for the
extraction test.

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

That would actually be a separate benchmark case which I should try also.

> Some measurements can be made using readv() and writev()
> on /dev/zero and /dev/null.

Forget /dev/null; that doesn't actually engage any iteration code.  The same
for writing to /dev/zero.  Reading from /dev/zero does its own iteration thing
rather than using iterate_and_advance(), presumably because it checks for
signals and resched.

David