linux-kernel - Re: [PATCH v4 3/3] mm/free_pcppages_bulk: prefetch buddy while not holding lock

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Message-ID: <787e53b9-20d3-a2df-44fc-ea8c31a6ced3@suse.cz>
Date:   Fri, 2 Mar 2018 19:08:00 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Dave Hansen <dave.hansen@...el.com>,
        Michal Hocko <mhocko@...nel.org>, Aaron Lu <aaron.lu@...el.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Huang Ying <ying.huang@...el.com>,
        Kemi Wang <kemi.wang@...el.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Matthew Wilcox <willy@...radead.org>,
        David Rientjes <rientjes@...gle.com>
Subject: Re: [PATCH v4 3/3] mm/free_pcppages_bulk: prefetch buddy while not
 holding lock

On 03/02/2018 07:00 PM, Dave Hansen wrote:
> On 03/02/2018 09:55 AM, Vlastimil Babka wrote:
>> It's even stranger to me. Struct page is 64 bytes these days, exactly a
>> a cache line. Unless that changed, Intel CPUs prefetched a "buddy" cache
>> line (that forms an aligned 128 bytes block with the one we touch).
> 
> I believe that was a behavior that was specific to the Pentium 4
> "Netburst" era.  I don't think the 128-byte line behavior exists on
> modern Intel cpus.

I remember it on Core 2 something (Nehalem IIRC). And this page suggests
up to Broadwell, and it can be disabled. And it's an L2 prefetcher indeed.
https://software.intel.com/en-us/articles/disclosure-of-hw-prefetcher-control-on-some-intel-processors