Re: Variable-block memory managers bad in realtime

[dewar@merv.cs.nyu.edu (Robert Dewar)]

Joe says

<<Waste lots of memory, actually.  Sometimes we don't have the memory to
  waste, or we would rather waste it on something else.  I don't know that
  these algorithms have guaranteed absolute upper bounds on response time,
  and absolute protection against fragmentation, together with the requisite
  efficiency.

  Fixed-block allocators waste no space, and have absolutely constant
  service time.>>

There are most certainly algorithms that have guaranteed worst case
(I assume that is what you mean by absolute) upper bounds on response
time. Obviously absolute protection against fragmentation is impossible
for an arbitrary allocation pattern unless you do compacting (and there
are algorithms for doing compacting that meet the other requirements,
talk to Henry Baker for example!) As to requisite efficienty, well that
of course is undefined.

It is of course not true that fixed-block allocators waste no space. For
an arbitrary allocation pattern, they waste lots of space by allocating
more than space than is required.

If on the other time you only need fixed-blocks of a given size in a
particular application, it is trivial to write a general purpose
algorithm that behaves as you demand for this particular allocation
pattern.

<<See above.  The vast bulk of that large established literature solves
  problems we don't have in realtime, at the expense of problems we do
  have.  Perhaps you have been so fortunate as to find one of the few truly
  relevant articles.  I haven't.  If so, a pointer or two would be greatly
  appreciated.  I doubt that I will ever have the time to read a significant
  fraction of that literature.>>

There is a substantial literature that specifically addresses the issues
of dynamic allocation for non-critical real time systems (critical real
time systems usuall don't do dynamic allocation at all, which is why I
make the distinction -- except in very trivial cases, certification of
safety-critical requirements is very dificult if there is dynamic allocation
of any kind, which is why most protocols for SC programming forbid dynamic
allocation completely).

<<Well, we are trying to solve much different problems.  We are trying to do
  by comparison simple things, but do them at 64 Hz, controlling hardware
  and *never* overrunning.  I doubt that those scientific programmers would
  learn much useful from our projects, and vice versa.>>

Your may doubt, but my experience (from doing a great deal of real time
programming in very small [4K to 16K byte memories] real time systems is
that sophisticated algorithmic techniques, and particularly the use of
clever data compression techniques, is highly relevant to that domain.
(and needs people who know the current algorithms literature reasonably
well).

<<True, but our applications are happy with from three to six different
  block sizes; the specific spectrum of sizes varies with the specific
  application.  Remember, I don't have to solve as general a problem as does
  a compiler or runtime writer, and I am more interested in predictability
  and behaviour under overload and near overload than memory efficiency per
  se.  Nor do I agree that there is one "best" algorithm, for all
  applications.>>

No, of course not, and that is why Ada provides the storage_pool capability
so that you can write specialized allocators that are tailored to your
application.

However, one can do a lot better than you think in providing a general
algorithm. Despite your skepticism, it is quite possible to create general
purpose algorithms that give guaranteed worst case response times that are
highly efficient for simple allocation patterns of the type you mention here.

Robert