Re: MAXACTION exceeded error while using systemtap

[Marcelo Tosatti <marcelo dot tosatti at cyclades dot com>]

On Thu, Dec 08, 2005 at 08:57:46AM -0500, Frank Ch. Eigler wrote:
> Hi -
> 
> jamesd.wi wrote:
> 
> > [...]  perhaps you two, should look at how your neighbor DTrace
> > deals with this issue. It seems to work pretty well.
> 
> Their situation is a little simpler because the language forbids
> looping and recursion, so the individual handler run times are already
> statically bounded.
> 
> 
> > [...]  btw MAXACTION really can't work reliably what happens when
> > the box is underload, say with 4 gigabit nics all being flooded with
> > data, and you are probing a function in the fast path?
> 
> Yes, this is a good point.  A system-level watchdog would be useful.
> 
> 
> > P.S. I would recomend the members of this list subscribe to the
> > dtrace-discuss list on www.opensolaris.org, its a low traffic list,
> > but they do discuss issues you will face as you proceed. I can
> > assure that the dtrace programers monitor your mailing list.
> 
> Good point.  Welcome all, and feel free to delurk.

Hi folks,

I think that SystemTap needs some method to periodically (or when memory
pressure arrives) dump the parts of the array to userspace via
relayfs.

relayfs already supports such mechanism with its "sub-buffer" structure,
where the userspace reader is allowed to read full buffers (thus freeing
them afterwards) while the hooks continue to push data to empty or
partially filled buffers.

James mentions that

"btw MAXACTION really can't work reliably what happens when the box is
underload, say with 4 gigabit nics all being flooded with data, and
you are probing a function in the fast path?

Not everyone has a box that can sum 4 billion numbers in less than a minute."

relayfs works in two modes once its memory pool is full:

- panic's 
- drops new entries

I imagine that by using relayfs to send data to userspace one should
automatically get those very important features.

Hope to be on the same page!