linux-kernel - Re: [PATCH v3 00/20] Speculative page faults

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Date:   Thu, 28 Sep 2017 13:38:50 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Laurent Dufour <ldufour@...ux.vnet.ibm.com>
Cc:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Paul McKenney <paulmck@...ux.vnet.ibm.com>,
        Peter Zijlstra <peterz@...radead.org>, kirill@...temov.name,
        Andi Kleen <ak@...ux.intel.com>,
        Michal Hocko <mhocko@...nel.org>, dave@...olabs.net,
        Jan Kara <jack@...e.cz>, Matthew Wilcox <willy@...radead.org>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Paul Mackerras <paulus@...ba.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Will Deacon <will.deacon@....com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>, haren@...ux.vnet.ibm.com,
        Anshuman Khandual <khandual@...ux.vnet.ibm.com>,
        npiggin@...il.com, bsingharora@...il.com,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        linuxppc-dev@...ts.ozlabs.org, "x86@...nel.org" <x86@...nel.org>
Subject: Re: [PATCH v3 00/20] Speculative page faults

On Thu, 28 Sep 2017 14:29:02 +0200 Laurent Dufour <ldufour@...ux.vnet.ibm.com> wrote:

> > Laurent's [0/n] provides some nice-looking performance benefits for
> > workloads which are chosen to show performance benefits(!) but, alas,
> > no quantitative testing results for workloads which we may suspect will
> > be harmed by the changes(?).  Even things as simple as impact upon
> > single-threaded pagefault-intensive workloads and its effect upon
> > CONFIG_SMP=n .text size?
> 
> I forgot to mention in my previous email the impact on the .text section.
> 
> Here are the metrics I got :
> 
> .text size	UP		SMP		Delta
> 4.13-mmotm	8444201		8964137		6.16%
> '' +spf		8452041		8971929		6.15%
> 	Delta	0.09%		0.09%	
> 
> No major impact as you could see.

8k text increase seems rather a lot actually.  That's a lot more
userspace cacheclines that get evicted during a fault...

Is the feature actually beneficial on uniprocessor?