Re: C vs C++

[Charles Wilt <charles.wilt@xxxxxxxxx>]

Any chance your C++ programs are using ACTGRP(*NEW) and the C ones are not?

Charles

On Mon, Feb 16, 2015 at 9:56 AM, Jevgeni Astanovski <jevgeniast@xxxxxxxxx>
wrote:

> Hi all.
> Does anyone have any idea why C++ program is significantly slower than
> C program?
>
> Explain the situation.
> I have a number of API-s that are called via RPC, access some tables,
> make some calculations and return some structures to the caller.
> They've all been written on ILE/C.
> Recently I started to experiment with porting them to ILE/C++. There
> was a number of reasons why it would be nice.
> After I rewrote the first one, I started to measure performance. This
> is an issue for me as some of them are called hundreds of thousands
> times per day.
> Found out that it is approximately 2 times slower than C version.
> Shared a code to colleges - no one found anything suspicious. However
> one guy came out with a "brilliant" idea - he advised me to try to
> recompile my C program with CPP compiler and see the performance.
> Guess what? C program compiled with CPP compiler was the same 2 times
> slower than C program compiled with C compiler....
>
> And the only modifications that I did for compiling C with CPP compiler
> was:
> 1. Substituted "#pragma mapinc" table structue definitions with those
> generated by GENCSRC;
> 2. Changed char and unsigned char in a couple of places as C++ is more
> strict;
> 3. Made a substitution for "NNNND" constants (like 100D for example)
> as C++ does not support that.
>
> What I saw is that time increase looks like it has a proportional nature.
> I tested API in 3 cases (number of tests is over 100): simple request,
> medium request; complicated request and the timing is:
> Simple C: 1ms; Simple C++ : 2ms
> Medium C: 3ms; Simple C++ : 7ms
> Long C: 25ms; Long C++: 60ms.
>
> The difference is in number of records read from table and a number of
> calculations.
> That is a simple one retrieves less that 10 records; medium - more
> than 10 and long - more than 100. Program makes a lot of calculations
> with packed decimal.
>
> Any idea why it is like that and is there anything that can be done about
> it?
>
> Thanks in advance,
>
> Jevgeni.
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