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From: Arvind Sankar <nivedita@alum.mit.edu>
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
Date: Wed, 11 Mar 2020 16:32:47 -0400	[thread overview]
Message-ID: <20200311203246.GA3971914@rani.riverdale.lan> (raw)
In-Reply-To: <20200311183240.GA3880414@rani.riverdale.lan>

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

  reply	other threads:[~2020-03-11 20:32 UTC|newest]

Thread overview: 23+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-03-07  1:03 Cannon Matthews
2020-03-07 22:06 ` Andrew Morton
2020-03-09  0:08 ` Kirill A. Shutemov
2020-03-09  9:06   ` Michal Hocko
2020-03-09  9:35     ` Kirill A. Shutemov
2020-03-09 11:36     ` Kirill A. Shutemov
2020-03-09 12:26       ` Michal Hocko
2020-03-09 18:01         ` Mike Kravetz
2020-03-09 15:38     ` Andi Kleen
2020-03-09 18:37       ` Matthew Wilcox
2020-03-11  0:21         ` Cannon Matthews
2020-03-11  0:54           ` Kirill A. Shutemov
2020-03-11  3:35             ` Arvind Sankar
2020-03-11  8:16               ` Kirill A. Shutemov
2020-03-11 18:32                 ` Arvind Sankar
2020-03-11 20:32                   ` Arvind Sankar [this message]
2020-03-12  0:52                     ` Kirill A. Shutemov
2020-03-31  0:40                   ` Elliott, Robert (Servers)
2020-03-16 10:18             ` Michal Hocko
2020-03-16 12:19               ` Kirill A. Shutemov
2020-03-26 19:46                 ` Matthew Wilcox
2020-03-11 15:07       ` David Laight
2020-03-09 15:33   ` Andi Kleen

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