linux-kernel - Re: vmevent: question?

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Message-ID: <alpine.LFD.2.02.1205031019410.3686@tux.localdomain>
Date:	Thu, 3 May 2012 10:24:42 +0300 (EEST)
From:	Pekka Enberg <penberg@...nel.org>
To:	Minchan Kim <minchan@...nel.org>
cc:	linux-mm@...ck.org, LKML <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>,
	Anton Vorontsov <anton.vorontsov@...aro.org>,
	Leonid Moiseichuk <leonid.moiseichuk@...ia.com>
Subject: Re: vmevent: question?

On Mon, 30 Apr 2012, Minchan Kim wrote:
> > What kind of consistency guarantees do you mean? The data sent to
> > userspace is always a snapshot of the state and therefore can be stale
> > by the time it reaches userspace.
> 
> Consistency between component of snapshot.
> let's assume following as
> 
> 1. User expect some events's value would be minus when event he expect happen.
>    A : -3, B : -4, C : -5, D : -6
> 2. Logically, it's not possible to mix plus and minus values for the events.
>    A : -3, B : -4, C : -5, D : -6 ( O )
>    A : -3, B : -4, C : 1, D : 2   ( X )
>    
> But in current implementation, some of those could be minus and some of those could be plus.
> Which event could user believe?
> At least, we need a _captured_ value when event triggered so that user can ignore other values.

Sorry, I still don't quite understand the problem.

The current implementation provides the same kind of snapshot consistency 
as reading from /proc/vmstat does (modulo the fact that we read them 
twice) for the values we support.

			Pekka
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