HP 3000 Manuals HP 3000 Manuals
MPE/iX System Performance
MPE/iX provides efficient performance through use of the mapped file
technique and concurrent directories described in the following
subsections.
Mapped Files
MPE/iX employs the mapped file technique for performing file access. It
is an improved version of the disc caching-capability of MPE V/E. File
access efficiency is improved when code and data portions of files
required for processing reside in memory. Accessing memory is faster
than performing physical disc I/O operations. The mapped file technique
eliminates file system buffering and optimizes global system memory
management.
File mapping is based on MPE/iX demand paged virtual memory, which uses
to advantage the large amount of virtual memory on the system. When a
file is opened, it is logically mapped into virtual memory. An open file
and its contents are referenced by virtual addresses. Each byte of each
opened file has a unique virtual address.
File mapping improves I/O performance without imposing additional CPU
overhead or sacrificing data integrity and protection. Traditional disc
caching schemes for increasing I/O performance impose a CPU overhead
penalty. The 900 Series hardware and system architecture allow MPE/iX to
perform file mapping without incurring this penalty. System hardware
performs the virtual to physical address translations for locating
portions of the mapped files, thus eliminating CPU overhead for this
function.
If the required pages are not in memory, the MPE/iX Memory Manager
fetches them directly from disc and places them in the user's area in
memory. This eliminates File System buffering. Pages are "prefetched"
to reduce the amount of physical disc I/O. Prefetching means that the
page specified for fetching and the group of pages surrounding it are a
fetched all at once. This improves efficiency because the processor is
likely to require pages that are located near each other. Two benefits
of this are:
* Eliminating unnecessary data movement in memory improves system
performance.
* Memory space usage is optimized.
MPE/iX file system access intrinsics are built on the mapped file
technique. Programs using file access methods supported by MPE file
types and intrinsics obtain the benefits of file mapping without
requiring changes.
You can directly access mapped files when programming in languages with
pointers. For example, you can obtain the advantage of File System
naming and data protection for accessing array type structures and
developing specialized access methods.
You can write programs that address files through virtual memory, instead
of calling File System intrinsics for disc reading and writing. The file
interface provides opening and closing of user mapped files with normal
naming and security, but with improved LOAD and STORE speed on file
references.
Directory Entries
On operating systems that have the system directory centralized on one
disc, access to directory services for files on any disc on the system
requires serial access to the system directory. In other words, they
must occur one at a time. At peak usage times, this creates a bottleneck
due to physical contention for one disc and logical contention for one
directory user.
The space for the directory structure is spread across the volume set,
not necessarily located on one volume. The locking mechanism allows
multiple readers and ensures that the proper locks for specific changes
in the directory structure, such as file name insertions and deletions.
MPE/iX uses directory entries that are spread across all members of the
system volume set to speed up file access and eliminate the physical or
logical serialization imposed by a centralized directory.
With MPE/iX, each disc in a system volume set has a directory of files
located on it. Thus, user requests automatically go through the
directories and arrive at the disc containing the requested files without
going through a centralized directory. Multiple users can simultaneously
access a system directory.
On nonsystem volume sets, the directory is restricted to the MASTER
volume of the set, so that it is not necessary to mount the entire set at
one time.