The __TEXT Segment: Read Only

The __TEXT segment is a read-only area containing executable code and constant data. By convention, the compiler tools create every executable file with at least one read-only __TEXT segment. Because the segment is read-only, the kernel can map the __TEXT segment directly from the executable into memory just once. When the segment is mapped into memory, it can be shared among all processes interested in its contents. (This is primarily the case with frameworks and other shared libraries.) The read-only attribute also means that the pages that make up the __TEXT segment never have to be saved to backing store. If the kernel needs to free up physical memory, it can discard one or more __TEXT pages and re-read them from disk when they are needed.

Table 1 lists some of the more important sections that can appear in the __TEXT segment. For a complete list of segments, see Mach-O Runtime Architecture.

The __DATA Segment: Read/Write

The __DATA segment contains the non-constant data for an executable. This segment is both readable and writable. Because it is writable, the __DATA segment of a framework or other shared library is logically copied for each process linking with the library. When memory pages are readable and writable, the kernel marks them copy-on-write. This technique defers copying the page until one of the processes sharing that page attempts to write to it. When that happens, the kernel creates a private copy of the page for that process.

The __DATA segment has a number of sections, some of which are used only by the dynamic linker. Table 2 lists some of the more important sections that can appear in the __DATA segment. For a complete list of segments, see Mach-O Runtime Architecture.

Mach-O Performance Implications

The composition of the __TEXT and __DATA segments of a Mach-O executable file has a direct bearing on performance. The techniques and goals for optimizing these segments are different. However, they have as a common goal: greater efficiency in the use of memory.

Most of a typical Mach-O file consists of executable code, which occupies the __TEXT, __text section. As noted in The __TEXT Segment: Read Only, the __TEXT segment is read-only and is mapped directly to the executable file. Thus, if the kernel needs to reclaim the physical memory occupied by some __text pages, it does not have to save the pages to backing store and page them in later. It only needs to free up the memory and, when the code is later referenced, read it back in from disk. Although this is cheaper than swapping—because it involves one disk access instead of two—it can still be expensive, especially if many pages have to be recreated from disk.

One way to improve this situation is through improving your code’s locality of reference through procedure reordering, as described in Improving Locality of Reference. This technique groups methods and functions together based on the order in which they are executed, how often they are called, and the frequency with which they call one another. If pages in the __text section group functions logically in this way, it is less likely they have to be freed and read back in multiple times. For example, if you put all of your launch-time initialization functions on one or two pages, the pages do not have to be recreated after those initializations have occurred.

Unlike the __TEXT segment, the __DATA segment can be written to and thus the pages in the __DATA segment are not shareable. The non-constant global variables in frameworks can have an impact on performance because each process that links with the framework gets its own copy of these variables. The main solution to this problem is to move as many of the non-constant global variables as possible to the __TEXT,__const section by declaring them const. Reducing Shared Memory Pages describes this and related techniques. This is not usually a problem for applications because the __DATA section in an application is not shared with other applications.

The compiler stores different types of nonconstant global data in different sections of the __DATA segment. These types of data are uninitialized static data and symbols consistent with the ANSI C notion of “tentative definition” that aren’t declared extern. Uninitialized static data is in the __bss section of the __DATA segment. Tentative-definition symbols are in the __common section of the __DATA segment.

The ANSI C and C++ standards specify that the system must set uninitialized static variables to zero. (Other types of uninitialized data are left uninitialized.) Because uninitialized static variables and tentative-definition symbols are stored in separate sections, the system needs to treat them differently. But when variables are in different sections, they are more likely to end up on different memory pages and thus can be swapped in and out separately, making your code run slower. The solution to these problems, as described in Reducing Shared Memory Pages, is to consolidate the non-constant global data in one section of the __DATA segment.