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From:   David Laight <David.Laight@ACULAB.COM>
To:     'David Howells' <dhowells@redhat.com>
CC:     Al Viro <viro@zeniv.linux.org.uk>,
        Linus Torvalds <torvalds@linux-foundation.org>,
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        Christian Brauner <christian@brauner.io>,
        "Matthew Wilcox" <willy@infradead.org>,
        Brendan Higgins <brendanhiggins@google.com>,
        David Gow <davidgow@google.com>,
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        "David Hildenbrand" <david@redhat.com>,
        John Hubbard <jhubbard@nvidia.com>
Subject: RE: [RFC PATCH 9/9] iov_iter: Add benchmarking kunit tests for
 UBUF/IOVEC
Thread-Topic: [RFC PATCH 9/9] iov_iter: Add benchmarking kunit tests for
 UBUF/IOVEC
Thread-Index: AQHZ51kQ26Nrcgq1TkqWWiI5nPv6dLAbdkPggAAj+YCAABqPYA==
Date:   Fri, 15 Sep 2023 10:51:36 +0000
Message-ID: <5017b9fa177f4deaa5d481a5d8914ab4@AcuMS.aculab.com>
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 <20230914221526.3153402-1-dhowells@redhat.com>
 <20230914221526.3153402-10-dhowells@redhat.com>
 <3370515.1694772627@warthog.procyon.org.uk>
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From: David Howells
> Sent: 15 September 2023 11:10
> 
> 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.

You could also just not do the copy!
Although you need (say) asm volatile("\n",:::"memory") to
stop it all being completely optimised away.
That might show up a difference in the 'out_of_line' test
where 15% on top on the data copies is massive - it may be
that the data cache behaviour is very different for the
two cases.

...
> > 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.

Using /dev/null does exercise the 'copy iov from user' code.
Last time I looked at that the 32bit compat code was faster than
the 64bit code on x86!

	David

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