linux-kernel - Re: [PATCH v2 18/26] selftests/resctrl: Read in less obvious order to defeat prefetch optimizations

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Message-ID: <03e9c565-ddc3-4f2d-beda-5095d3f3aa37@intel.com>
Date:   Tue, 28 Nov 2023 14:17:18 -0800
From:   Reinette Chatre <reinette.chatre@...el.com>
To:     Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>,
        <linux-kselftest@...r.kernel.org>, Shuah Khan <shuah@...nel.org>,
        "Shaopeng Tan" <tan.shaopeng@...fujitsu.com>,
        Maciej Wieczór-Retman 
        <maciej.wieczor-retman@...el.com>,
        Fenghua Yu <fenghua.yu@...el.com>
CC:     <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 18/26] selftests/resctrl: Read in less obvious order to
 defeat prefetch optimizations

Hi Ilpo,

On 11/20/2023 3:13 AM, Ilpo Järvinen wrote:
> When reading memory in order, HW prefetching optimizations will
> interfere with measuring how caches and memory are being accessed. This
> adds noise into the results.
> 
> Change the fill_buf reading loop to not use an obvious in-order access
> using multiply by a prime and modulo.
> 
> Using a prime multiplier with modulo ensures the entire buffer is
> eventually read. 23 is small enough that the reads are spread out but
> wrapping does not occur very frequently (wrapping too often can trigger
> L2 hits more frequently which causes noise to the test because getting
> the data from LLC is not required).
> 
> It was discovered that not all primes work equally well and some can
> cause wildly unstable results (e.g., in an earlier version of this
> patch, the reads were done in reversed order and 59 was used as the
> prime resulting in unacceptably high and unstable results in MBA and
> MBM test on some architectures).
> 
> Link: https://lore.kernel.org/linux-kselftest/TYAPR01MB6330025B5E6537F94DA49ACB8B499@TYAPR01MB6330.jpnprd01.prod.outlook.com/
> Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
> ---

I am not very comfortable with all the uncertainty involved in this
patch. A consolation is that this is surely an improvement.

Reviewed-by: Reinette Chatre <reinette.chatre@...el.com>

Reinette