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

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Message-ID: <3545840.1694780385@warthog.procyon.org.uk>
Date:   Fri, 15 Sep 2023 13:19:45 +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:

> 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