Subject: Re: Better pagecache statistics ?
From: Badari Pulavarty <pbadari@us.ibm.com>
To: Marcelo Tosatti <marcelo.tosatti@cyclades.com>
Cc: Arjan van de Ven <arjan@infradead.org>, linux-mm <linux-mm@kvack.org>,
       lkml <linux-kernel@vger.kernel.org>
In-Reply-To: <20051201175711.GA17169@dmt.cnet>
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Date: Thu, 01 Dec 2005 10:20:12 -0800
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On Thu, 2005-12-01 at 15:57 -0200, Marcelo Tosatti wrote:
> On Thu, Dec 01, 2005 at 06:21:39PM +0100, Arjan van de Ven wrote:
> > On Thu, 2005-12-01 at 09:15 -0800, Badari Pulavarty wrote:
> > > > Most of the issues you mention are null if you move the stats
> > > > maintenance burden to userspace. 
> > > > 
> > > > The performance impact is also minimized since the hooks 
> > > > (read: overhead) can be loaded on-demand as needed.
> > > > 
> > > 
> > > The overhead is - going through each mapping/inode in the system
> > > and dumping out "nrpages" - to get per-file statistics. This is
> > > going to be expensive, need locking and there is no single list 
> > > we can traverse to get it. I am not sure how to do this.
> 
> Can't you add hooks to add_to_page_cache/remove_from_page_cache 
> to record pagecache activity ?

In theory, yes. We already maintain info in "mapping->nrpages".
Trick would be to collect all of them, send them to user space.

Thanks,
Badari

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