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OK - I'll try and address that.
You're only looking at it in terms of a couple of simple requests. But
you also have to take into account requests for an increase in size
and also what happens when a number of the reservations are
relinquished. Over the life of a job (or even a program for that
matter) there can be a lot of these.
Other than simple "rounding" to its multiple size (typically a binary
related boundary like 16, 64 or whatever) the system will normally
respond to requests by supplying what is asked for. So (assuming no
rounding) a series of requests for 50, 100, 20, 60, 40, 80 - would be
met with the requested number. Suppose that the 20 allocation then
requests expansion to 40. The 20 is now unused and a new 40 allocation
is added at the end leaving a "gap" of 20 i.e. 50, 100, (20), 60, 40,
80, 40. Some time later the 60 reservation is released. The next
request received is for 70 bytes. At this point the system's algorithm
may decide to reuse the 20 and 60 blocks freed previously. The
algorithm itself will have some rule on what the minimum "gap" to be
left should be - suppose it is 20 in this case - so it will not leave
a a gap of 10 but rather allocate 80 bytes to the 70 byte request.
While it may be practical in the context of a single procedure to
consider that blocks of memory of 5 or 10 bytes are practical, from an
OS perspective units that small would be sure to cause problems in
terms of ability to allocate large amounts of contiguous memory.
I'm no OS expert, but my guess would be that the smallest allocation
unit would probably be a function of the type of process requesting it
(OS or USer for example). It may also be dynamically calculated based
on allocation requests, but I suspect that the overhead of doing so it
not worth the effort. Who cares if 200 bytes is allocated when 10 was
asked for? Most of the time it is irrelevant and if it means that less
work is required in terms of determining if a contiguous block is of a
particular size is available, then it is probably a good trade off.
I haven't tested this for a long time but I recall finding that if I
ran RPG code that allocated a chunk of memory and then allocated
another, the addresses returned demonstrated that the memory was not
contiguous but rather had been rounded out to some preferred
allocation size. Maybe one of these days I'll get round to testing it
with a number of different memory allocation methods and see what
happens. For example I suspect that little (if any) rounding would be
added to allocations for subprocedure variables since it is known that
they will all be released at the same time - so I would expect them to
be contiguous. But who knows.
Interesting academic exercise - but of no real practical use.
Jon Paris
www.Partner400.com
www.SystemiDeveloper.com
On Nov 21, 2010, at 5:50 AM, rpg400-l-request@xxxxxxxxxxxx wrote:
"Why is it better to given extra ram to a call to malloc, when you
know it
doesn't need it, when the next call might need it."
i.e. if call 1 asks for 90 bytes why give it 100 bytes when the next
call
might be for 10 bytes?
midrange.com
