QUESTION ON RS/6000 USING AIX 3.2.3 AND DISK
ITEM: RTA000027791
QUESTION:
Can you reclarify disk caching for me, for example how
is this set, what is the maximum disk cache I can set.
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A: The RISC SYSTEM/6000 does not use disk caching, but the similar
function is performed by the virtual memory manager. A memory-mapped
file is either fully or partly in real memory. The Real memory access
times are much faster than disk access files, so a memory mapped file
will provide higher performance than a file stored entirely on disk.
I will describe disk caching, then point out how AIX memory mapping of
files differs from this.
DISK CACHES
* A disk cache normally uses a fixed amount of memory. Some disk
caches use memory that comes on the disk itself, and the disk
or the disk's controller board have processors which manage this
cache. This frees the computer's CPU from having to manage the
cache. In contrast, some disk caches use the computer's memory,
and usually this kind of cache requires the computer's CPU to
manage it( this is software cache). They key issue is that if
you have X amount of total memory, then you have
X - SIZEOF(cache)
memory left for other things. Even if you are hardly using the
cache at a given period of time, the cache size remains fixed.
Even if for a period of time you need a bigger cache, the cache
size remains fixed.
AIX AUTOMATIC MEMORY MAPPING OF FILES
* AIX automatically memory maps files. It does this using
the same virtual memory manager that manages all other
RISC System/6000 memory. So when you access a file, the
area you access is read into real memory. If you access
that area of the file frequently, then that part of the
file will remain in real memory (just like with a cache).
On the other hand, if you do not access any files
frequently, then AIX will use real memory for other things
(an improvement over disk caches).
Furthermore, the AIX virtual memory manager classifies
memory into file memory and computational memory, and in
this way manages to give higher priority to computational
memory. Sometimes recently accessed memory must be paged
out to disk due to memory limitations. If some of this
memory must be paged back in (this is called a repage
fault), then AIX records this event and records whether the
memory is file or computational memory. If the frequency
of recent computational repage faults exceeds the frequency
of file repage faults, the virtual memory manger will only
page out file memory to disk. On the other hand, if file
repage faults are occurring more frequently, then the
virtual memory manager will page out whichever memory was
least recently accessed, regardless of whether it is file
or computational memory. In this way, AIX gives a sort of
higher priority to computational memory.
This differs from caching because a disk cache cannot
determine whether memory is computational or file. A
plain disk cache, in a sense, gives equal priority to
reading a file as it gives to running programs. Clearly
programs need to be able to run for any work to get done
on the system.
In summary:
* Memory mapped files offer performance improvement by storing
files in memory, which is much faster than disks.
* AIX memory mapping of files is just a normal function of its
virtual memory manager, thereby allowing memory to be
allocated more intelligently than with cache.
A good analogy is AIX makes all of the RISC's real memory a
disk cache, and it controls this cache intelligently, mapping
files, data, and code to memory as needed.
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This item was created from library item Q612899 BWQTK
Additional search words:
AIX ALTERNATE BWQTK CACHE CACHING DASD DISC DISKETTE INDEX IX JAN93
MAIN MAP MEASURE MEMORY OP PERFORMANCE QUESTION RISCPERF RISCSYSTEM
SOFTWARE STORAGE SYS TUNE 3.2.3
WWQA: ITEM: RTA000027791 ITEM: RTA000027791
Dated: 03/1996 Category: RISCPERF
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