Re: Low performance on varying length field

[Hans Boldt <boldt@xxxxxxxxxx>]

Titus Kruse wrote:

> Hello!
>
> I've written a procedure that returns a value of type varying length string.
> Because I want to let the maximum size of the field in the calling programm
> open, the size of the return field is defined with 65535 bytes.
>
> Independently of the number of bytes to be returned, the call of the
> procedure is very slow (time job running was 19 seconds). When I define the
> return field like the input field with 255 bytes the call is faster (runtime
> only 1 second).
>
> Why that? I would expect, that only characters needed and length information
> are copied to the calling procedure. Not the hole storage area.
>
> Here my coding example (V5R2):
>  * ------------------------------------------------------------------
> d #str1           s          65535    varying
> d #str2           s            255    varying
> d                                     inz('Let the sun shine!')
> d TSTPRC2         pr         65535    varying
> d                              255    varying
> c                   do        200000
> c                   eval      #str1 = TSTPRC2(#str2)
> c                   enddo
> c                   eval      *inlr = *on
>  * ------------------------------------------------------------------
> h nomain
> d TSTPRC2         pr         65535    varying
> d                              255    varying
> p TSTPRC2         b                   export
> d                 pi         65535    varying
> d $InpStr                      255    varying
> d #RtnStr         s          65535    varying
> c                   eval      #RtnStr = $InpStr
> c                   return    #RtnStr
> p                 e
>  * ------------------------------------------------------------------

I think Scott already addressed the issues, but I'd like to put a 
different perspective on this. It simply boils down to one thing: 
Varying length strings in RPG IV are handled by RPG, and not by the 
underlying W-Code. Operations handled by RPG can be done efficiently, 
dealing with only the number of bytes required. But operations that are 
left to W-Code are less speedy since W-Code doesn't understand the 
implementation of varying length variables, and so deal with the 
aggregate as a whole. When passing varying length string values as 
parameters or when returning varying length values, W-Code moves around 
the whole aggregate, not just what is specified by the current length.

Cheers! Hans