linux-kernel - Re: Avoid allocating during interleave from almost full nodes

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Message-Id: <20061130235936.cd979ba5.akpm@osdl.org>
Date:	Thu, 30 Nov 2006 23:59:36 -0800
From:	Andrew Morton <akpm@...l.org>
To:	Paul Jackson <pj@....com>
Cc:	Christoph Lameter <clameter@....com>, linux-kernel@...r.kernel.org
Subject: Re: Avoid allocating during interleave from almost full nodes

On Thu, 30 Nov 2006 23:51:17 -0800
Paul Jackson <pj@....com> wrote:

> A month ago, Christoph replied to pj:
> >
> > On Sat, 4 Nov 2006, Paul Jackson wrote:
> > 
> > >   Do you know of any existing counters that we could use like this?
> > > 
> > > Adding a system wide count of pages allocated or scanned, just for
> > > these fullnode hint caches, bothers me.
> > 
> > There are already such counters. PGALLOC_* and PGSCAN_*. See 
> > include/linux/vmstat.h
> 
> These counters depend on CONFIG_VM_EVENT_COUNTERS.
> 
> The Kconfig comment for CONFIG_VM_EVENT_COUNTERS states:
> 
>           VM event counters are only needed to for event counts to be
>           shown. They have no function for the kernel itself. This
>           option allows the disabling of the VM event counters.
>           /proc/vmstat will only show page counts.
> 
> (By the way - note the "needed to for event" phrasing error.)
> 
> The header file, include/linux/vmstat.h, for these counters states:
> 
> 	/*
> 	 * Light weight per cpu counter implementation.
> 	 *
> 	 * Counters should only be incremented and no critical kernel component
> 	 * should rely on the counter values.
> 
> Both these clearly state that I should not use these counters for real
> kernel functions.
> 
> If that is so, I should find some other "time base" for the zonelist
> caching.
> 
> If that is not so, then these comments need updating.
> 
> Anybody have any idea which is the case?

You need to set EMBEDDED to disable VM_EVENT_COUNTERS.

Things like procps (vmstat, top, etc) now use /proc/vmstat and would likely
break.

I don't know how much space it saves, but I doubt if the world would end if
we removed CONFIG_VM_EVENT_COUNTERS.

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