Calculating item: chose the max value

[Dimenty]

Hi, i need some help with calculating items.
I got 2 items (i get values via snmp at the same time) and i need 3rd item that will compare these 2 values and chose max value. How can i create this item?

[Dimenty]

up up up up up

[glebs.ivanovskis]

Maybe this will help you.

[Dimenty]

Thanks! That really what i need. Why there is no information about that trick in documentation?

[glebs.ivanovskis]

I think our documentation person has been too busy documenting 3.0 features recently.

[Dimenty]

I got a strange issue with "<=" sign in formula. When i use "<=" in formula, it returns 0 even if the left value less then the right. When i use ">=" sign - is ok.

So for max item its better to use this
(item1 >= item2) * item1 +
(item1 < item2) * item2
or this
(item1 > item2) * item1 +
(item1 < item2) * item2 +
(item1 = item2) * item1

This formula
(item1 > item2) * item1 + (item1 <= item2) * item2
returns 0 because of <= sign

I have no idea what's wrong with it

[glebs.ivanovskis]

Can you give me actual numbers?

[Dimenty]

Do you mean real formula?

(last("avgIncomingIn10Mins[{#SNMPVALUE}]") > last("avgOutcomingIn10Mins[{#SNMPVALUE}]")) * last("avgIncomingIn10Mins[{#SNMPVALUE}]") + 
(last("avgIncomingIn10Mins[{#SNMPVALUE}]") <= last("avgOutcomingIn10Mins[{#SNMPVALUE}]")) * last("avgOutcomingIn10Mins[{#SNMPVALUE}]")

If you want real values of my items (avgIncomingIn10Mins and avgOutcomingIn10Mins), i could tell you that they are not 0, because they store actual speed on working router port.


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And suddenly i discovered that >= is also doesnt work and returns 0.
So the best way to compare 2 items is that formula.

(item1 > item2) * item1 +
(item1 < item2) * item2 +
(item1 = item2) * item1

[glebs.ivanovskis]

I was thinking of the actual numbers because comparison operators work in a very tricky way and some round-off magic can take place.

Actually, a new value can be obtained in the middle of expression evaluation. In this case your third expression will return 0 like two previous.

[Dimenty]

Numbers that i compare are huge (9-11 digits) so i think there is no rounding problems.

Actually, a new value can be obtained in the middle of expression evaluation. In this case your third expression will return 0 like two previous.

Sorry, but i can't catch what are you talking about.

[glebs.ivanovskis]

Sorry, but i can't catch what are you talking about.

How Zabbix calculates trigger expressions? Let's evaluate this one:

(item1 >= item2) * item1 +
(item1 < item2) * item2

Let's assume last value of item1 is 1 and last value of item2 is 1. (1 >= 2) evaluates to 0 (false) and 0 * 1 evaluates to 0. Now a new value for item1 comes in, let's say 3. (3 < 2) evaluates to 0 and 0 * 2 evaluates to 0. 0 + 0 is 0.

[Dimenty]

I still don't understand how can your values of item1 and item2 can be different at the same time. They are equal on every iteration.

1st iteration.
i1=5, i2=3.
(5>3)*5 + (5<=3)*3 = 1*5+0*3 = 5.

On the next iteration there will be another values.
i1=2, i2=6.
(2>6)*2 + (2<=6)*6 = 0*2+1*6 = 6.

But when i use >= or <= signs in formula it returns 0.
So i need a 3rd part of my formula with = sign:
i1=5, i2=3.
(5>3)*5 + (5<3)*3 + (5=3)*5 = 1*5+0*3+0*5 = 5.

[glebs.ivanovskis]

Can you then explain how did you get 0 with <= or >= signs?

[Dimenty]

I have no idea why i get 0. Thats why i refused to use <= or >= in formula