Messages in this thread | | | Subject | Re: Better pagecache statistics ? | From | Badari Pulavarty <> | Date | Thu, 01 Dec 2005 10:25:59 -0800 |
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On Thu, 2005-12-01 at 16:15 -0200, Marcelo Tosatti wrote: > > > 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?
Otherwise, we need to insert hooks and ask the customer to reproduce the problem :(
> > 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.
Okay, I miss understood.
Thanks, Badari
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