linux-kernel - Re: [RFC PATCH 9/9] iov_iter: Add benchmarking kunit tests for UBUF/IOVEC

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Message-ID: <3370515.1694772627@warthog.procyon.org.uk>
Date:   Fri, 15 Sep 2023 11:10:27 +0100
From:   David Howells <dhowells@...hat.com>
To:     David Laight <David.Laight@...LAB.COM>
Cc:     dhowells@...hat.com, Al Viro <viro@...iv.linux.org.uk>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Jens Axboe <axboe@...nel.dk>, Christoph Hellwig <hch@....de>,
        "Christian Brauner" <christian@...uner.io>,
        Matthew Wilcox <willy@...radead.org>,
        "Brendan Higgins" <brendanhiggins@...gle.com>,
        David Gow <davidgow@...gle.com>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kselftest@...r.kernel.org" <linux-kselftest@...r.kernel.org>,
        "kunit-dev@...glegroups.com" <kunit-dev@...glegroups.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Christian Brauner <brauner@...nel.org>,
        David Hildenbrand <david@...hat.com>,
        John Hubbard <jhubbard@...dia.com>
Subject: Re: [RFC PATCH 9/9] iov_iter: Add benchmarking kunit tests for UBUF/IOVEC

David Laight <David.Laight@...LAB.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