Re: Better pagecache statistics ?

[Marcelo Tosatti <marcelo.tosatti@cyclades.com>]

> > I thought that it would be easy to use SystemTap for a such
> > a purpose?
> > 
> > The sys_read/sys_write example at 
> > http://www.redhat.com/magazine/011sep05/features/systemtap/ sounds
> > interesting.
> > 
> > What I'm I missing?
> 
> Well, Few things:
> 
> 1) We have to have those probes present in the system all the time
> collecting the information when read/write happens, maintaining it
> and spitting it out. Since its kernel probe, all this data will be
> in the kernel. 

Yeah, there is some overhead.

> 2) If we want to do this accounting (and you don't have those probes
> installed already) - we can't capture what happened earlier. 

I suppose that the vast majority of situations where such information is 
needed are special anyway? 

Why do you need it around all the time?

> 3) probing sys_read/sys_write() are going to tell you how much
> a data a process did read or wrote - but its not going to tell you
> how much is in the cache (now or 10 minutes later). 

Sure, that was just an example - need to insert probes
on the correct places.

> > > My final goal is to get stats like ..
> > > 
> > > 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.
> > > 
> > > Don't you think ?
> > 
> > Yep... /proc/<pid>/smaps provides that information on a per-process
> > basis already.
> 
> /proc/pid/smaps will give me information about text,data,shared libs,
> malloc etc. Not the filecache information about files process opened,
> pages read/wrote currently in the pagecache. Isn't it ?

Right. 

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