linux-kernel - Re: [RFC][PATCH 00/26] sched/numa

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Message-ID: <1332163594.18960.335.camel@twins>
Date:	Mon, 19 Mar 2012 14:26:34 +0100
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Andrea Arcangeli <aarcange@...hat.com>
Cc:	Avi Kivity <avi@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>, Paul Turner <pjt@...gle.com>,
	Suresh Siddha <suresh.b.siddha@...el.com>,
	Mike Galbraith <efault@....de>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Dan Smith <danms@...ibm.com>,
	Bharata B Rao <bharata.rao@...il.com>,
	Lee Schermerhorn <Lee.Schermerhorn@...com>,
	Rik van Riel <riel@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [RFC][PATCH 00/26] sched/numa

On Mon, 2012-03-19 at 14:04 +0100, Andrea Arcangeli wrote:

> About the cost of the actual pagetable scanner, you're not being
> rational about it. You should measure it for once, take khugepaged
> make it scan 1G of memory per millisecond and measure the cost.

Death by a thousand cuts.. 

> You keep complaining about the unaccountability of the pagetable
> scanners in terms of process load, and that's a red herring as far as
> I can tell. The irqs and ksoftirqd load in a busy server, is likely
> much higher than whatever happens at the pagetable scanner level (sure
> thing for khugepaged and by an huge order of magnitude so). 

Who says I agree with ksoftirqd? I would love to get rid of all things
softirq. And I also think workqueues are over-/ab-used.

> I don't
> think this is a relevant concern anyway because the pagetable scanners
> go over all memory in a equal amount so the cost would be evenly
> distributed for all processes over time (the same cannot be said about
> the irqs and ksoftrqid that will benefit only a few processes doing
> I/O). 

So what about the case where all I do is compile kernels and we already
have near perfect locality because everything is short running? You're
still scanning that memory, and I get no benefit.
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