Need help in tuning Compute Statement - performance

vijikesavan · Posted: Wed Jul 11, 2007 8:05 pm

Hi,
We have a highly CPU intensive program and we executed strobe on it.
The report pointed to a COMPUTE statement as contributor to CPU.

The statement:

William Thompson · Posted: Wed Jul 11, 2007 9:25 pm

vijikesavan · Posted: Wed Jul 11, 2007 11:00 pm

Thanks William.
I will change the code accordinlgy and pass it to the group who has access to test.
Will post the results soon I get..
Eagerly waiting to see some improvements.

Thanks,
Viji

vijikesavan · Posted: Wed Jul 11, 2007 11:34 pm

Hi William,
I think the results won't match. I might be wrong.
for example,
A=10.5, B=12.3 and C=9.5
A*B*C = 10.5*12.3*9.5 = 1226.925

whereas,
(10*12*9)+(0.5*0.3*0.5) = 1080+0.075 = 1080.075

Am I missing something? pls help.
Thanks,
Viji

William Thompson · Posted: Wed Jul 11, 2007 11:39 pm

Yup, ya got me....I guess I didn't have my head screwed on tightly....

Let me think on it a little....

dick scherrer · Posted: Wed Jul 11, 2007 11:50 pm

Hello,

What kind of processing is happening in this program? If the majority of the code is a read, some moves, this calculation, and a write, the posted compute will surely be the bit of code the largest cpu use.

Is this necessarily a problem?

vijikesavan · Posted: Wed Jul 11, 2007 11:59 pm

Yes. The program reads 2 input files and DB2 database and based on few conitions the WS-Value is calculated.
This job runs for around 30 mins with 10 mins CPU. Though it might look ok, but this job shows growing trend for the past 1 year.
The volume of data it is processing didnot increase much, but the job used to take just 5 mins of CPU and it is now taking 10mins of CPU.

dick scherrer · Posted: Thu Jul 12, 2007 12:28 am

Hello,

Does the amount of cpu time grow with each run or have there been a few "jumps" in usage? How much intermediate cpu usage info is available? Are there enough "footprints" to tell what changes occurred when the cpu usage began to rise? Has the size of these fields that are used in the calculation been increased?

How are values put iinto these fields?

vijikesavan · Posted: Thu Jul 12, 2007 12:42 am

Thanks Dick.
I am changing the code making the fields to have odd numbers (15 or less).

Regarding the 9.9 field I agree with you I too feel the same, but I need to get consent from business as how many positions I can reduce.

The CPU usage is growing steadily and there are no jumps. No code changes recently.

Letme try this and get back to you.

Thanks for all your suggesstions.
Viji

William Thompson · Posted: Thu Jul 12, 2007 12:55 am

From What I understand, the optimizer can take duplicate portions of computes (from the left side or withing parens) and carry the intermediate results through multiple calculations.....

It might be interesting to see which works best.

- Important -
The optimizer must be used and 15 or fewer digits in the PICTURE specification to avoid using library routines for multiplication.

dick scherrer · Posted: Thu Jul 12, 2007 2:09 am

Hello,

TG Murphy · Posted: Thu Jul 12, 2007 2:42 am

Just a quick note: When rounding can occur we stay clear of the COMPUTE statement and instead use separate statements for each operation such as MULTIPLY and DIVIDE. This way you can be in complete control of how the intermediate variables get defined (ie number of digits after decimal point).

In your case, this advice may not be necessary as you do not do any division so perhaps COMPUTE works fine. Just beware.

In situation where you multiply and divide - all within the same COMPUTE
statement - you can easily get incorrect results due to rounding. The COMPUTE statement uses intermediate variables - the datatypes of these intermediate variables are decided by the compiler.