linux-kernel - Re: [RFC][PATCH 1/7][memcg] virtually indexed array library.

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Date:	Tue, 3 Aug 2010 08:45:00 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	balbir@...ux.vnet.ibm.com
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"nishimura@....nes.nec.co.jp" <nishimura@....nes.nec.co.jp>,
	gthelen@...gle.com, m-ikeda@...jp.nec.com,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC][PATCH 1/7][memcg] virtually indexed array library.

On Mon, 2 Aug 2010 23:30:51 +0530
Balbir Singh <balbir@...ux.vnet.ibm.com> wrote:

> * KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com> [2010-07-29 13:27:03]:
> 
> > On Thu, 29 Jul 2010 09:32:26 +0900
> > KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com> wrote:
> > 
> > > On Wed, 28 Jul 2010 12:45:13 -0700
> > > Andrew Morton <akpm@...ux-foundation.org> wrote:
> > 
> > > > My gut reaction to this sort of thing is "run away in terror".  It
> > > > encourages kernel developers to operate like lackadaisical userspace
> > > > developers and to assume that underlying code can perform heroic and
> > > > immortal feats.  But it can't.  This is the kernel and the kernel is a
> > > > tough and hostile place and callers should be careful and defensive and
> > > > take great efforts to minimise the strain they put upon other systems.
> > > > 
> > > > IOW, can we avoid doing this?
> > > > 
> > > 
> > 
> > I'll use pre-allocated pointer array in the next version. It's simple even
> > if a bit slow.
> > 
> > ==
> > struct mem_cgroup *mem_cgroups[CONFIG_MAX_MEM_CGROUPS] __read_mostly;
> > #define id_to_memcg(id)		mem_cgroups[id];
> > ==
> 
> Hmm.. I thought we were going to reuse css_id() and use that to get to
> the cgroup. May be I am missing something.
> 
?
lookup_css_id() requires multi-level table lookup because of radix-tree.
And compiler can't generate an optimized code. linear table lookup is quick.

-Kame


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