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Using Shared Memory  



Application programs access shared memory by mapping Galaxy-wide shared sections. The programming model is the same as for standard OpenVMS global sections; that is, you create, map, unmap, and delete them on each instance where you want to use them. Some shared memory global section characteristics are:

From a programmer's point of view, shared memory global sections are similar to memory resident sections. You use the same system services to create Galaxy-wide shared sections that you would use to create memory resident sections. Setting the flag SEC$M_SHMGS lets the service operate on a shared memory global section.

In contrast to memory resident sections, the Reserved Memory Registry is not used to allocate space for Galaxy-wide sections. The SYSMAN RESERVE commands affect only node-private memory. Shared memory is not used for normal OpenVMS paging operations and does not need to be reserved.

There is also no user interface to specify whether shared page tables should be created for Galaxy-wide sections. Instead, the creation of shared page tables for Galaxy-wide sections is tied to the section size. As of OpenVMS Version 7.2, shared page tables are created for sections of 128 pages (1 MB) or more. Galaxy-wide shared page tables are shared between all Galaxy instances.


 
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