linux-kernel - Re: [patch 02/12] Immediate Values

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Message-ID: <20090928220114.GB27947@Krystal>
Date:	Mon, 28 Sep 2009 18:01:14 -0400
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Andi Kleen <andi@...stfloor.org>, mingo@...e.hu,
	linux-kernel@...r.kernel.org, jbaron@...hat.com,
	rusty@...tcorp.com.au, bunk@...sta.de, hch@...radead.org,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [patch 02/12] Immediate Values - Architecture Independent Code

* Andrew Morton (akpm@...ux-foundation.org) wrote:
> On Mon, 28 Sep 2009 22:11:08 +0200
> Andi Kleen <andi@...stfloor.org> wrote:
> 
> > > For example, how do we know it's safe to use immediate-values for
> > > anything which can be modified from userspace, such as a sysfs-accessed
> > > tunable?  How do we know this won't take someone's odd-but-legitimate
> > > workload and shoot it in the head?
> > 
> > You're arguing we should tune for sysctl performance? That doesn't make
> > sense to me.
> 
> We're talking about a tiny tiny performance gain (one which thus far
> appears to be unobserveable) on the read-side traded off against a
> tremendous slowdown on the write-side.
> 
> That's OK for people whose workloads use the expected read-vs-write
> ratio.  But there's always someone out there who does something
> peculiar.  There will be people who simply cannot accept large
> slowdowns in writes to particular tunables.  Who these people are and
> which tunables they care about we do not know.
> 
> No, I'm not saying we should "tune for sysctl performance".  I'm saying
> we should tune for not making Linux utterly uselessly slow for people
> for whom it previously worked OK.
> 
> It means we'd have to look very carefully at each tunable and decide
> whether there's any conceivable situation in which someone would want
> to alter it frequently.  If so, we need to leave it alone.
> 
> How many tunables will that leave behind, and how much use was it to
> speed that remainder up by a teensy amount?  Who knows.
> 

BTW, when/if we get the OK from Intel to use a breakpoint/IPI-based
scheme to perform the updates rather than using the heavyweight
stop_machine(), this update performance question will be much less of a
concern.

hpa is currently looking into this.

Thanks,

Mathieu

-- 
Mathieu Desnoyers
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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