Improving performance

This section contains some hints on using DLLs efficiently. Effective use of DLLs may improve the performance of your application. Following are some suggestions that may improve performance:

If you are using a particular DLL frequently across multiple address spaces, the DLL can be installed in the LPA or ELPA. When the DLL resides in a PDSE, the dynamic LPA services should be used (this will always be the case for XPLINK applications). Installing in the LPA/ELPA may give you the performance benefits of a single rather than multiple load of the DLL
When writing XPLINK applications, avoid frequent calls from XPLINK to non-XPLINK DLLs, and vice-versa. These transitions are expensive, so you should build as much of the application as possible as either XPLINK or non-XPLINK. When there is a relatively large amount of function calls compared to the rest of the code, the performance of an XPLINK application can be significantly better than non-XPLINK. It is acceptable to make calls between XPLINK and non-XPLINK, when a relatively large amount of processing will be done once the call is made.
Be sure to specify the RENT option when you bind your code. Otherwise, each load of a DLL results in a separately loaded DLL with its own writable static. Besides the performance implications of this, you are likely to get incorrect results if the DLL exports variables (data).
Group external variables into one external structure.
When using z/OS® UNIX avoid unnecessary load attempts.
z/OS Language Environment® supports loading a DLL residing in the UNIX file system or a data set. However, the location from which it tries to load the DLL first varies depending whether your application runs with the runtime option POSIX(ON) or POSIX(OFF).
If your application runs with POSIX(ON), z/OS Language Environment tries to load the DLL from the UNIX file system first. If your DLL is a data set member, you can avoid searching the UNIX file system directories. To direct a DLL search to a data set, prefix the DLL name with two slashes (//), as shown in the following example.
```
//MYDLL
```
If your application runs with POSIX(OFF), z/OS Language Environment tries to load your DLL from a data set. If your DLL is a UNIX file system file, you can avoid searching a data set. To direct a DLL search to the UNIX file system, prefix the DLL name with a period and slash (./), as shown in the following example.
```
./mydll
```
Note: DLL names are case sensitive in the UNIX file system. If you specify the wrong case for your DLL that resides in the UNIX file system, it will not be found in the UNIX file system.
For IPA, you should only export subprograms (functions and C++ methods) or variables that you need for the interface to the final DLL. If you export subprograms or variables unnecessarily (for example, by using the EXPORTALL option), you severely limit IPA optimization. In this case, global variable coalescing and pruning of unreachable or 100% inlined code does not occur. To be processed by IPA, DLLs must contain at least one subprogram. Attempts to process a data-only DLL will result in a compilation error.
The suboption NOCALLBACKANY of the compiler option DLL is more efficient than the CALLBACKANY suboption. The CALLBACKANY option calls z/OS Language Environment at run time. This runtime service enables direct function calls. Direct function calls are function calls through function pointers that point to actual function entry points rather than function descriptors. The use of CALLBACKANY will result in extra overhead at every occurrence of a call through a function pointer. This is unnecessary if the calls are not direct function calls.