2.5. Memory Management Each process has its own private address space. The address space is initially divided into three logical segments: text, data, and stack. The text segment is read-only and contains the machine instructions of a program. The data and stack segments are both readable and writable. The data segment contains the initialized and uninitialized data portions of a program, whereas the stack segment holds the application's run-time stack. The stack segment is extended automatically by the kernel as the process executes. A process can expand or contract its data segment by making a system call, whereas a process can change the size of its text segment only when the segment's contents are overlaid with data from the filesystem or when debugging takes place. The initial contents of the segments of a child process are duplicates of the segments of a parent process. The entire contents of a process address space do not need to be resident for a process to execute. If a process references a part of its address space that is not resident in main memory, the system pages the necessary information into memory. When system resources are scarce, the system uses a two-level approach to maintain available resources. If a modest amount of memory is available, the system will take memory resources away from processes if these resources have not been used recently. Should there be a severe resource shortage, the system will resort to swapping the entire context of a process to secondary storage. The demand paging and swapping done by the system are effectively transparent to processes. A process may, however, advise the system about expected future memory utilization as a performance aid. BSD Memory-Management Design Decisions The support of large sparse address spaces, mapped files, and shared memory was a requirement for 4.2BSD. An interface was specified, called mmap(), that allowed unrelated processes to request a shared mapping of a file into their address spaces. If multiple processes mapped the same file into their address spaces, changes to the file's portion of an address space by one process would be reflected in the area mapped by the other processes, as well as in the file itself. Ultimately, 4.2BSD was shipped without the mmap() interface, because of pressure to make other features, such as networking, available. Further development of the mmap() interface continued during the work on 4.3BSD. Over 40 companies and research groups participated in the discussions leading to the revised architecture that was described in the Berkeley Software Architecture Manual [McKusick et al., 1994]. The first UNIX implementation of the interface was done by Sun Microsystems [Gingell et al., 1987]. Once again, time pressure prevented 4.3BSD from providing an implementation of the interface. Although the latter could have been built into the existing 4.3BSD virtual-memory system, the developers decided not to put it in because that implementation was nearly 10 years old. Furthermore, the original virtual-memory design was based on the assumption that computer memories were small and expensive, whereas disks were locally connected, fast, large, and inexpensive. Thus, the virtual-memory system was designed to be frugal with its use of memory at the expense of generating extra disk traffic. In addition, the 4.3BSD implementation was riddled with VAX memory-management hardware dependencies that impeded its portability to other computer architectures. Finally, the virtual-memory system was not designed to support the tightly coupled multiprocessors that are becoming increasingly common and important today. Attempts to improve the old implementation incrementally seemed doomed to failure. A completely new design, on the other hand, could take advantage of large memories, conserve disk transfers, and have the potential to run on multiprocessors. Consequently, the virtual-memory system was completely replaced in 4.4BSD. The 4.4BSD virtual-memory system was based on the Mach 2.0 virtual-memory system [Tevanian, 1987], with updates from Mach 2.5 and Mach 3.0. The FreeBSD virtual-memory system is an extensively tuned version of the virtual-memory implementation in 4.4BSD. It features efficient support for sharing, a clean separation of machine-independent and machine-dependent features, as well as multiprocessor support. Processes can map files anywhere in their address space. They can share parts of their address space by doing a shared mapping of the same file. Changes made by one process are visible in the address space of the other process and also are written back to the file itself. Processes can also request private mappings of a file, which prevents any changes that they make from being visible to other processes mapping the file or being written back to the file itself. Another issue with the virtual-memory system is the way that information is passed into the kernel when a read or write system call is made. For these system calls, FreeBSD always copies data from the process address space into a buffer in the kernel. The copy is done for several reasons. Often, the user data are not page aligned and are not a multiple of the hardware page length. If the page is taken away from the process, it will no longer be able to reference that page. Some programs depend on the data remaining in the buffer even after those data have been written. If the process is allowed to keep a copy of the page (as it is in current FreeBSD semantics), the page must be made copy-on-write. A copy-on-write page is one that is protected against being written by being made read-only. If the process attempts to modify the page, the kernel gets a write fault. The kernel then makes a copy of the page that the process can modify. Unfortunately, the typical process will immediately try to write new data to its output buffer, forcing the data to be copied anyway. When pages are remapped to new virtual-memory addresses, most memory-management hardware requires that the hardware address-translation cache be purged selectively. The cache purges are often slow. The net effect is that remapping is slower than copying for blocks of data less than 4 to 8 Kbyte.
For read or write operations that are transferring large quantities of data, doing the copy can be time-consuming. An alternative to doing the copying is to remap the process memory into the kernel. The biggest incentives for memory mapping are the needs for accessing big files and for passing large quantities of data between processes. The mmap() interface provides a way for both of these tasks to be done without copying. The mmap system call is not supported between processes running on different machines. Such processes must communicate using sockets connected across the network. Thus, sending the contents of a file across the network is another common operation where it is desirable to avoid copying. Historically, the sending of a file was done by reading the file into an application buffer, and then writing that buffer to a socket. This approach required two copies of the data: first from the kernel to the application buffer, and then from the application buffer back into the kernel to send on the socket. FreeBSD has the sendfile system call that sends data from a file down a socket without doing any copying. Memory Management Inside the Kernel The kernel often does allocations of memory that are needed for only the duration of a single system call. In a user process, such short-term memory would be allocated on the run-time stack. Because the kernel has a limited run-time stack, it is not feasible to allocate even moderate-sized blocks of memory on it. Consequently, such memory must be allocated through a more dynamic mechanism. For example, when the system must translate a pathname, it must allocate a 1-Kbyte buffer to hold the name. Other blocks of memory must be more persistent than a single system call, and thus could not be allocated on the stack even if there was space. An example is protocol-control blocks that remain throughout the duration of a network connection. Demands for dynamic memory allocation in the kernel have increased as more services have been added. A generalized memory allocator reduces the complexity of writing code inside the kernel. Thus, the FreeBSD kernel has a general memory allocator that can be used by any part of the system. It has an interface similar to the C library routines malloc() and free() that provide memory allocation to application programs [McKusick & Karels, 1988]. Like the C library interface, the allocation routine takes a parameter specifying the size of memory that is needed. The range of sizes for memory requests is not constrained; however, physical memory is allocated and is not paged. The free routine takes a pointer to the storage being freed, but it does not require the size of the piece of memory being freed. Some large and persistent allocations, such as the structure that tracks information about a process during its lifetime, are not well handled by the general memory allocator. The kernel provides a zone allocator for these types of allocations. Each memory type is given its own zone from which all its allocations are made. Memory allocated in one zone cannot be used by any other zone or by the general memory allocator. The semantics of the interface are similar to the general-memory allocator; memory is allocated from a zone with the zalloc() routine and freed with the zfree() routine. |
