Date: Wed, 28 Jul 2010 09:08:54 +0900
From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
To: Jonathan Corbet <corbet@lwn.net>
Cc: "linux-mm@kvack.org" <linux-mm@kvack.org>,
        "nishimura@mxp.nes.nec.co.jp" <nishimura@mxp.nes.nec.co.jp>,
        "balbir@linux.vnet.ibm.com" <balbir@linux.vnet.ibm.com>,
        gthelen@google.com, m-ikeda@ds.jp.nec.com,
        "akpm@linux-foundation.org" <akpm@linux-foundation.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [RFC][PATCH 1/7][memcg] virtually indexed array library.
Message-Id: <20100728090854.9b255f7c.kamezawa.hiroyu@jp.fujitsu.com>
In-Reply-To: <20100727122949.3bfbfd0a@bike.lwn.net>
References: <20100727165155.8b458b7f.kamezawa.hiroyu@jp.fujitsu.com>
	<20100727165303.7d7d18e9.kamezawa.hiroyu@jp.fujitsu.com>
	<20100727122949.3bfbfd0a@bike.lwn.net>
Organization: FUJITSU Co. LTD.
Mime-Version: 1.0
Content-Type: text/plain; charset=US-ASCII
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 2174
Lines: 55

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

> On Tue, 27 Jul 2010 16:53:03 +0900
> KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.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
 

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/