Книга: Distributed operating systems

9.3.4. Kernel Calls for Memory Management

9.3.4. Kernel Calls for Memory Management

Memory management in Chorus supports 26 different system calls plus several upcalls from the kernel to the mappers. In this section we will briefly describe just the more important ones. The calls we will describe relate to region management (rgn prefix), segment management (sg prefix), and upcalls to the mappers (Mp prefix — not mp, which is used for miniport calls, described later). The calls not described here relate to managing local caches (lc prefix) and virtual memory (vm prefix).

A selection of the region management calls is given in Fig. 9-12. RgnAllocate specifies a region by giving the capability for a process, a starting address, a size, and various options. If there are no conflicts with other regions and no other problems, the region is created. The options include initializing the region to zero bytes, setting its read/write/executable bits, and wiring it down so it is not paged. RgnFree returns a previously allocated region so its portion of the address space is free for allocation to another region or regions.

Fig. 9-12. Selected calls supported by the Chorus kernel for managing regions.

RgnInit is similar to rgnAllocate except that after the region is allocated, it is also filled in from a segment whose capability is a parameter to the call. Several other calls that are similar to rgnInit are also present, filling the region in different ways.

The next three calls change the properties of an existing region in various ways. RgnSetInherit relates to the possibility that a region might later be copied, and specifies whether the copy is to get its own pages or to share the original region's pages. RgnSetPaging is used primarily by processes that are interested in providing real-time response and cannot tolerate page faults or being swapped out. It also tells what to do if the process is trying to allocate a nonswappable nonpageable region that is wired down, but there is insufficient memory available. RgnSetProtect changes the read/write/execute bits associated with a region and can also make a region accessible only to the kernel. Finally, rgnStat returns to the caller the size of a region and other information.

The segment I/O calls are shown in Fig. 9-13. sgRead and sgWrite are the basic I/O calls that allows a process to read and write a segment. The segment is specified by a descriptor. The offset and number of bytes are also provided, as is the buffer address to copy to or from. SgStat allows the caller, typically a mapper, to ask for statistical information about a page cache. SgFlush (and several related calls) allow mappers to ask the kernel to send them pages so they can be cached in the mapper's address space or written back to their segments on disk. This mechanism is needed, for example, to ensure that a group of mappers collectively supporting distributed shared memory can remove writable pages from all machines but one. Other calls in this group relate to locking and unlocking individual pages in memory and getting various sizes and other information about the paging system.

Fig. 9-13. Selected calls relating to segments.

The calls in Fig. 9-14 are calls to a mapper, either from the kernel or from an application program asking the mapper to do something for the caller. The first one, MpCreate is used when the kernel or a program wants to swap out a segment and needs to allocate disk space for it. The mapper responds by allocating a new segment on disk and returning a capability for it.

Fig. 9-14. Mapper calls.

The MpRelease call returns a segment created using MpCreate. MpPullIn is used by the kernel to acquire data from a newly created or existing segment. The mapper is required to respond to it by sending a message containing the pages needed. By using clever MMU programming, the pages need not be copied physically.

The MpPushOut call is for transfers the other way, from kernel to mapper, either in response to a sgFlush (or similar) call, or when the kernel wants to swap out a segment on its own. Although the list of calls described above is not complete, it does give a reasonable picture of how memory management works in Chorus.

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