linux-kernel - Re: Interacting with coherent memory on external devices

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Message-ID: <20150424141359.GX5561@linux.vnet.ibm.com>
Date:	Fri, 24 Apr 2015 07:13:59 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Christoph Lameter <cl@...ux.com>
Cc:	Jerome Glisse <j.glisse@...il.com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, jglisse@...hat.com, mgorman@...e.de,
	aarcange@...hat.com, riel@...hat.com, airlied@...hat.com,
	benh@...nel.crashing.org, aneesh.kumar@...ux.vnet.ibm.com,
	Cameron Buschardt <cabuschardt@...dia.com>,
	Mark Hairgrove <mhairgrove@...dia.com>,
	Geoffrey Gerfin <ggerfin@...dia.com>,
	John McKenna <jmckenna@...dia.com>, akpm@...ux-foundation.org
Subject: Re: Interacting with coherent memory on external devices

On Fri, Apr 24, 2015 at 09:01:47AM -0500, Christoph Lameter wrote:
> On Thu, 23 Apr 2015, Paul E. McKenney wrote:
> 
> > > As far as I know Jerome is talkeing about HPC loads and high performance
> > > GPU processing. This is the same use case.
> >
> > The difference is sensitivity to latency.  You have latency-sensitive
> > HPC workloads, and Jerome is talking about HPC workloads that need
> > high throughput, but are insensitive to latency.
> 
> Those are correlated.

In some cases, yes.  But are you -really- claiming that -all- HPC
workloads are highly sensitive to latency?  That would be quite a claim!

> > > What you are proposing for High Performacne Computing is reducing the
> > > performance these guys trying to get. You cannot sell someone a Volkswagen
> > > if he needs the Ferrari.
> >
> > You do need the low-latency Ferrari.  But others are best served by a
> > high-throughput freight train.
> 
> The problem is that they want to run 2000 trains at the same time
> and they all must arrive at the destination before they can be send on
> their next trip. 1999 trains will be sitting idle because they need
> to wait of the one train that was delayed. This reduces the troughput.
> People really would like all 2000 trains to arrive on schedule so that
> they get more performance.

Yes, there is some portion of the market that needs both high throughput
and highly predictable latencies.  You are claiming that the -entire- HPC
market has this sort of requirement?  Again, this would be quite a claim!

							Thanx, Paul

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