netdev - RE: [RFC PATCH 9/9] iov_iter: Add benchmarking kunit tests for UBUF/IOVEC

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Message-ID: <5017b9fa177f4deaa5d481a5d8914ab4@AcuMS.aculab.com>
Date: Fri, 15 Sep 2023 10:51:36 +0000
From: David Laight <David.Laight@...LAB.COM>
To: 'David Howells' <dhowells@...hat.com>
CC: 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

From: David Howells
> Sent: 15 September 2023 11:10
> 
> 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.

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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