Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
From:   Arvind Sankar <nivedita@alum.mit.edu>
Date:   Wed, 11 Mar 2020 16:32:47 -0400
To:     Arvind Sankar <nivedita@alum.mit.edu>
Cc:     "Kirill A. Shutemov" <kirill@shutemov.name>,
        Cannon Matthews <cannonmatthews@google.com>,
        Matthew Wilcox <willy@infradead.org>,
        Andi Kleen <ak@linux.intel.com>,
        Michal Hocko <mhocko@kernel.org>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        David Rientjes <rientjes@google.com>,
        Greg Thelen <gthelen@google.com>,
        Salman Qazi <sqazi@google.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, x86@kernel.org
Subject: Re: [PATCH] mm: clear 1G pages with streaming stores on x86
Message-ID: <20200311203246.GA3971914@rani.riverdale.lan>
References: <20200307010353.172991-1-cannonmatthews@google.com>
 <20200309000820.f37opzmppm67g6et@box>
 <20200309090630.GC8447@dhcp22.suse.cz>
 <20200309153831.GK1454533@tassilo.jf.intel.com>
 <20200309183704.GA1573@bombadil.infradead.org>
 <CAJfu=UfPKZwqjGR5AdhFRo_je7X5q2=zpBSBQkrbh2KhYrOJiA@mail.gmail.com>
 <20200311005447.jkpsaghrpk3c4rwu@box>
 <20200311033552.GA3657254@rani.riverdale.lan>
 <20200311081607.3ahlk4msosj4qjsj@box>
 <20200311183240.GA3880414@rani.riverdale.lan>
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Disposition: inline
In-Reply-To: <20200311183240.GA3880414@rani.riverdale.lan>
User-Agent: Mutt/1.10.1 (2018-07-13)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Wed, Mar 11, 2020 at 02:32:41PM -0400, Arvind Sankar wrote:
> On Wed, Mar 11, 2020 at 11:16:07AM +0300, Kirill A. Shutemov wrote:
> > On Tue, Mar 10, 2020 at 11:35:54PM -0400, Arvind Sankar wrote:
> > > 
> > > The rationale for MOVNTI instruction is supposed to be that it avoids
> > > cache pollution. Aside from the bench that shows MOVNTI to be faster for
> > > the move itself, shouldn't it have an additional benefit in not trashing
> > > the CPU caches?
> > > 
> > > As string instructions improve, why wouldn't the same improvements be
> > > applied to MOVNTI?
> > 
> > String instructions inherently more flexible. Implementation can choose
> > caching strategy depending on the operation size (cx) and other factors.
> > Like if operation is large enough and cache is full of dirty cache lines
> > that expensive to free up, it can choose to bypass cache. MOVNTI is more
> > strict on semantics and more opaque to CPU.
> 
> But with today's processors, wouldn't writing 1G via the string
> operations empty out almost the whole cache? Or are there already
> optimizations to prevent one thread from hogging the L3?

Also, currently the stringop is only done 4k at a time, so it would
likely not trigger any future cache-bypassing optimizations in any case.

> 
> If we do want to just use the string operations, it seems like the
> clear_page routines should just call memset instead of duplicating it.
> 
> > 
> > And more importantly string instructions, unlike MOVNTI, is something that
> > generated often by compiler and used in standard libraries a lot. It is
> > and will be focus of optimization of CPU architects.
> > 
> > -- 
> >  Kirill A. Shutemov