Cacti - ERROR: RPN final stack size != 1 with 4 Graphs

I use Cacti and rrdxport to get the sum of 2 Graphs and it is working fine.

rrdtool xport --start now-5min --end now-5min DEF:out1=sbc1_fs_call_five_min_do_137.rrd:fs_call_five_min_do:LAST DEF:out2=sbc2_fs_call_five_min_do_147.rrd:fs_call_five_min_do:LAST CDEF:sum=out1,out2,+ XPORT:sum:"output sum"

Output:

<xport> 
<meta>
<start>1524226500</start>
<step>300</step>
<end>1524226500</end>
<rows>1</rows>
<columns>1</columns>
<legend>
<entry>output sum</entry>
</legend>
</meta>  
<data>
<row><t>1524226500</t><v>7.1630000000e+02</v></row> 
</data>
</xport>

Now I want to add 4 graphs but I always get the error RPN final stack size != 1.

rrdtool xport --start now-5min --end now-5min DEF:out1=sbc1_fs_call_five_min_do_137.rrd:fs_call_five_min_do:LAST DEF:out2=sbc1_berlin_fs_call_five_min_do_1176.rrd:fs_call_five_min_do:LAST DEF:out3=sbc2_fs_call_five_min_do_147.rrd:fs_call_five_min_do:LAST DEF:out4=sbc2_berlin_fs_call_five_min_do_1187.rrd:fs_call_five_min_do:LAST CDEF:sum=out1,out2,out3,out4,+ XPORT:sum:"output sum" ERROR: RPN final stack size != 1

Why does it work with 2 graphs but not with 4?
thanks for help!

Solution

You are incorrectly specifying your RPN function, and have more than one item left in the stack.

This function works as expected:

CDEF:sum=out1,out2,+

This is because the sequence of RPN commands says:

Put out1 on the stack,
Put out2 on the stack,
Remove the top two items and add them, and push the result onto the stack

This results in the stack holding a single item, with a value (out1+out2).

However, your second attempt does this:

CDEF:sum=out1,out2,out3,out4,+

This means:

Put out1 on the stack,
Put out2 on the stack,
Put out3 on the stack,
Put out4 on the stack,
Remove the top two items and add them, and push the result onto the stack

As a result, the stack now holds three things -- out1, out2, and (out3+out4). RRDTool checks the stack size to catch any RPN errors, and errors because it sees more than one item left in the stack.

What you should do is to add two more addition operations to your definition:

CDEF:sum=out1,out2,out3,out4,+,+,+

This adds two more steps to the RPN, to add in the remaining two items on the stack and obtain the result you want.

I suggest you work through the RPN tutorial to gain a better understanding of how to specify RPN.