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Date: Fri, 20 Jul 2007 20:11:43 +0800
From: Fengguang Wu <wfg@mail.ustc.edu.cn>
To: Andi Kleen <andi@firstfloor.org>
Cc: Andrew Morton <akpm@osdl.org>,
       Linus Torvalds <torvalds@linux-foundation.org>,
       linux-kernel <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 1/6] compacting file_ra_state
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	Andrew Morton <akpm@osdl.org>,
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On Fri, Jul 20, 2007 at 01:34:42PM +0200, Andi Kleen wrote:
> Fengguang Wu <wfg@mail.ustc.edu.cn> writes:
> 
> > Use 'unsigned int' instead of 'unsigned long' for the readahead indexes/sizes.
> > 
> > This helps reduce memory consumption on 64bit CPU when
> > a lot of files are opened.
> > 
> > Note that the (smaller) 32bit index can support up to 16PB file.
                                                          ~~~~
                                                sorry, it's 16TB ;)

> > Which should be sufficient large at least for now.
> 
> This would add a new limit to 64bit architectures.  Surely keeping
> start at pgoff_t will not be a big issue? The other fields can be
> 32bit.

Yeah, it counts for about 4MB memory for 1M opened files.

But, the filp size is now 296 on x86_64, so slab-objects-per-page = 13.
Adding another 4bytes, it remains in 13;
Taking 4bytes more, it increases to 14.
So pgoff_t consumes no more memory actually.