Re: Better pagecache statistics ?

[Badari Pulavarty <pbadari@us.ibm.com>]

On Thu, 2005-12-01 at 15:08 -0200, Marcelo Tosatti wrote:
> On Thu, Dec 01, 2005 at 05:10:11PM +0100, Arjan van de Ven wrote:
> > > Out of "Cached" value - to get details like
> > > 
> > > 	<mmap> - xxx KB
> > > 	<shared mem> - xxx KB
> > > 	<text, data, bss, malloc, heap, stacks> - xxx KB
> > > 	<filecache pages total> -- xxx KB
> > > 		(filename1 or <dev>, <ino>) -- #of pages
> > > 		(filename2 or <dev>, <ino>) -- #of pages
> > > 		
> > > This would be really powerful on understanding system better.
> > 
> > to some extend it might be useful.
> > I have a few concerns though
> > 1) If we make these stats into an ABI then it becomes harder to change
> > the architecture of the VM radically since such concepts may not even
> > exist in the new architecture. As long as this is some sort of advisory,
> > humans-only file I think this isn't too much of a big deal though. 
> > 
> > 2) not all the concepts you mention really exist as far as the kernel is
> > concerned. I mean.. a mmap file is file cache is .. etc.
> > malloc/heap/stacks are also not differentiated too much and are mostly
> > userspace policy (especially thread stacks). 
> > 
> > A split in
> > * non-file backed
> >   - mapped once
> >   - mapped more than once
> > * file backed
> >   - mapped at least once
> >   - not mapped
> > I can see as being meaningful. Assigning meaning to it beyond this is
> > dangerous; that is more an interpretation of the policy userspace
> > happens to use for things and I think coding that into the kernel is a
> > mistake.
> > 
> > Knowing which files are in memory how much is, as debug feature,
> > potentially quite useful for VM hackers to see how well the various VM
> > algorithms work. I'm concerned about the performance impact (eg you can
> > do it only once a day or so, not every 10 seconds) and about how to get
> > this data out in a consistent way (after all, spewing this amount of
> > debug info will in itself impact the vm balances)
> 
> 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.

Thanks,
Badari 

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