[rrd-users] RPN limitation

Simon Hobson linux at thehobsons.co.uk
Sat Oct 12 13:31:15 CEST 2013

Munroe Sollog wrote:
>It looks like I'm running up against an RPN limitation.  I'm trying to add ~1600 values, but the
>rrdgraph function dies with:
>invalid rpn expression in: <spits out the first 4081 chars for the string>
>My full CDEF function is ~20379 chars long
>Is this a known bug?  Any suggestions on how to work around it?

Three things come to mind ...

1) Can you split the expression into smaller chunks ?

2) How are you doing the add ?
If you do a,b,c, ...,+,+,+ then that's going to need an awful lot of stack to execute. While a,b,+,c,+,... shouldn't. But I don't know how RRD processes the expression - logically it should just work left-right adding things to the stack or executing operators as it find them.

3) How are you passing the instructions ? Could you be hitting an OS limitation - eg shells often have a limit on the length of command+paramters that will fit in a buffer.

I suspect it's not a specific limit in RRD, just a limit in <something in teh system>. I have graphs with 510 graph elements and 1020 legend items - so RRD can certainly handle large definitions. What I don't have is any particualrly large RPN expressions.

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