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

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Message-Id: <20100728090854.9b255f7c.kamezawa.hiroyu@jp.fujitsu.com>
Date:	Wed, 28 Jul 2010 09:08:54 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Jonathan Corbet <corbet@....net>
Cc:	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"nishimura@....nes.nec.co.jp" <nishimura@....nes.nec.co.jp>,
	"balbir@...ux.vnet.ibm.com" <balbir@...ux.vnet.ibm.com>,
	gthelen@...gle.com, m-ikeda@...jp.nec.com,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC][PATCH 1/7][memcg] virtually indexed array library.

On Tue, 27 Jul 2010 12:29:49 -0600
Jonathan Corbet <corbet@....net> wrote:

> On Tue, 27 Jul 2010 16:53:03 +0900
> KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com> wrote:
> 
> > This virt-array allocates a virtally contiguous array via get_vm_area()
> > and allows object allocation per an element of array.
> 
> Quick question: this looks a lot like the "flexible array" mechanism
> which went in around a year ago, and which is documented in
> Documentation/flexible-arrays.txt.  I'm not sure we need two of
> these...  That said, it appears that there are still no users of
> flexible arrays.  If your virtually-indexed arrays provide something
> that flexible arrays don't, perhaps your implementation should replace
> flexible arrays?

Hmm. As Documentatin/flexible-arrays.txt says,

"The down sides are that the arrays cannot be indexed directly, individual object
 size cannot exceed the system page size, and putting data into a flexible array
 requires a copy operation. "

This virtually-indexed array is

 - the arrays can be indexed directly.
 - individual object size can be defined arbitrary.
 - puttind data into virt-array requires memory allocation via alloc_varray_item().

But, virtyally-indexed array has its own down side, too.

 - It uses vmalloc() area. This can be very limited in 32bit archs.
 - It cannot be used in !MMU archs.
 - It may consume much TLBs because vmalloc area tends not to be backed by hugepage.

Especially, I think !MMU case is much different. So, there are functional
difference. I implemented this to do quick direct access to objects by indexes.
Otherwise, flex-array may be able to provide more generic frameworks.

Then, I myself don't think virt-array is a replacemento for flex-array.

A discussion "flex-array should be dropped or not" is out of my scope, sorry.
I think you can ask to drop it just because it's almost dead without mentioning
virt-array.

Thanks,
-Kame
 


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