linux-kernel - Re: [PATCH v7 6/6] siphash: implement HalfSipHash1-3 for hash tables

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Date:   Wed, 21 Dec 2016 16:46:35 -0800
From:   Andi Kleen <ak@...ux.intel.com>
To:     "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     Netdev <netdev@...r.kernel.org>,
        kernel-hardening@...ts.openwall.com,
        LKML <linux-kernel@...r.kernel.org>,
        linux-crypto@...r.kernel.org,
        David Laight <David.Laight@...lab.com>,
        Ted Tso <tytso@....edu>,
        Hannes Frederic Sowa <hannes@...essinduktion.org>,
        edumazet@...gle.com,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Eric Biggers <ebiggers3@...il.com>,
        Tom Herbert <tom@...bertland.com>, davem@...emloft.net,
        luto@...capital.net,
        Jean-Philippe Aumasson <jeanphilippe.aumasson@...il.com>
Subject: Re: [PATCH v7 6/6] siphash: implement HalfSipHash1-3 for hash tables

> 64-bit x86_64:
> [    0.509409] test_siphash:     SipHash2-4 cycles: 4049181
> [    0.510650] test_siphash:     SipHash1-3 cycles: 2512884
> [    0.512205] test_siphash: HalfSipHash1-3 cycles: 3429920
> [    0.512904] test_siphash:    JenkinsHash cycles:  978267

I'm not sure what these numbers mean. Surely a single siphash2-4
does not take 4+ million cycles? 

If you run them in a loop please divide by the iterations.

But generally running small code in a loop is often an unrealistic
benchmark strategy because it hides cache misses, primes
predictors, changes frequencies and changes memory costs,
but also can overload pipelines and oversubscribe
resources.

[see also page 46+ in http://halobates.de/applicative-mental-models.pdf]

So the numbers you get there are at least somewhat
dubious. It would be good to have at least some test which
is not just a tiny micro benchmark to compare before making
conclusions.

-Andi