Version 1.2 of ai05s/ai05-0190-1.txt
!standard D.7(8) 09-11-19 AI05-0190-1/02
!class amendment 09-11-03
!status work item 09-11-03
!status received 09-11-03
!priority Low
!difficulty Easy
!subject Global storage pool controls
!summary
Provide a means to force the use of user-defined pools, and a means to specify a
particular pool to be used by default.
!problem
Some applications need to keep tight control over heap allocation. For example,
it is common for object-oriented applications to have many access-to-class-wide
types, but few used for allocation. Another example is an embedded system for
which is is inappropriate to rely on the implementation-provided pools.
This can be done by appplying "Storage_Size use 0" to all types that should not
have allocators, and explicitly specifying a Storage_Pool for the few others.
But this is error prone; one might forget the "Storage_Size use 0".
Another problem is that it is technically erroneous to deallocate from the
"wrong" pool. But it is implementation defined which pool is used for each
access type! Many Ada programmers don't know about this rule, and most Ada
implementations use a single global heap by default, so it is common to write
code that does "new" for one type, converts to another type, and does
Unchecked_Deallocation. This common coding practise could be erroneous on some
implementations. It is uncomfortable for erroneousness to be implementation
defined in this way.
!proposal
(see summary)
!wording
Syntax
The form of a pragma No_Default_Storage_Pool or Default_Storage_Pool is as
follows:
pragma No_Default_Storage_Pool;
pragma Default_Storage_Pool(storage_pool_name);
A pragma No_Default_Storage_Pool is a configuration pragma.
A pragma Default_Storage_Pool is allowed at the place where a declarative_item
is allowed.
Name Resolution Rules
The storage_pool_name is expected to be of type Root_Storage_Pool'Class.
Legality Rules
The storage_pool_name shall denote a variable.
A Default_Storage_Pool shall not be within the immediate scope of another
Default_Storage_Pool. [???Or should the inner/later one override the other?
Sounds complicated, wording-wise and implementation-wise.]
Static Semantics
Within the immediate scope of a pragma Default_Storage_Pool, the Storage_Pool
attribute of any access type to which no Storage_Pool nor Storage_Size clause
applies is defined to be the pool denoted by the storage_pool_name.
If No_Default_Storage_Pool applies to a given compilation_unit, then any access
type within that compilation_unit that has neither a specified Storage_Pool nor
Storage_Size is defined by the language to have a Storage_Size of zero.
[Therefore, an allocator for such a type is illegal.]
AARM: This implies that Default_Storage_Pool trumps
No_Default_Storage_Pool.
!discussion
Expected usage scenarios are:
- No_Default_Storage_Pool as a configuration pragma applying to the whole
program.
To use an allocator for an access type, you have to apply a Storage_Pool
or Storage_Size pragma to that type, or else put a Default_Storage_Pool
pragma such that the type is within the pragma's immediate scope.
- Default_Storage_Pool in the spec of the root package of a program or
subsystem.
This default pool is used within the package and its children,
but you can override it with a Storage_Pool or Storage_Size clause.
!example
!ACATS test
ACATS B and C tests are needed.
!appendix
From: Bob Duff
Sent: Thursday, November 19, 2009 12:52 PM
New version of AI05-0190-1. [This is version /02 of this AI. - ED]
The previous version used a Restriction to specify "no allocators allowed by
default". I added pragma Default_Storage_Pool to specify a pool to be used by
default. Having done that, it seemed better to use a pragma
No_Default_Storage_Pool instead of that Restriction.
I changed the !subject accordingly.
****************************************************************
From: Bob Duff
Sent: Sunday, November 22, 2009 1:16 PM
I wrote:
> Provide a means to force the use of user-defined pools, and a means to
> specify a particular pool to be used by default.
One interesting question, which comes from a private conversation I had with
Laurent Guerby, is:
What happens if pragma Default_Storage_Spool applies to an instantiation?
Does it apply to access types in the instance?
If so, does that provide a way to control the storage pool used for instances of
Ada.Constainers.Vectors and friends?
****************************************************************
From: Randy Brukardt
Sent: Tuesday, November 24, 2009 12:28 AM
It's clear that No_Default_Storage_Pool had better apply to instances, so that
any allocators in the generic units are detected. So I suppose from that it
implies that a similar capability exists in the other direction.
But such a capability would be next to useless. There is not (and better not be,
IMHO) any requirements on how the containers packages use the (default) storage
pool (except of course for the bounded forms, where using a pool at all is
disallowed). In particular, a container can request storage from the pool with
any size and any alignment, and as part of any operation. I suppose a program
tied to a particular implementation of the containers might be able to gain some
advantage from a custom pool, but such code is unlikely to be portable.
Note that this is true of any generic that allocates from the default pool.
Unless you are willing to "break privacy" by looking into the body of the
generic, you can't assume anything about how the pool will be called.
But almost all interesting user-defined storage pools make some assumptions
about the alignment and size that will be requested. Such pools would not work
on instances. It would of course be possible to create a monitoring pool that
passes the actual allocation requests to the default pool, but that is not going
to be very helpful in managing storage use.
****************************************************************
From: Randy Brukardt
Sent: Tuesday, November 24, 2009 12:52 AM
> !wording
Minor issue - you didn't recommend some place to put this new wording. It can't
be floating in space!!
The old positioning of D.7(8) related specifically to it being a restriction, so
that doesn't work. So some other suggestion is needed.
****************************************************************
From: Bob Duff
Sent: Tuesday, November 24, 2009 7:01 AM
Right. I intended to suggest someplace in chap 13, but I forgot.
So I'd say it should be 13-dot-<rolldice>.
I'm presuming there was no deliberate intent to max this "optional annex" --
it's just where the Restrictions happen to live.
Actually, I guess it should be 13.11.3, "Default Storage Pools", and bump
"Pragma Controlled" up to 13.11.4.
What about the proposal overall? Do people like it?
It's radically different from the previous version.
****************************************************************
From: Tucker Taft
Sent: Tuesday, November 24, 2009 7:36 AM
> But such a capability would be next to useless.
I don't agree this would be next to useless for a container instance. Clearly
you would have to provide a general purpose storage pool if you are going to
apply it by default to your entire system. But often all that is wanted is an
ability to replace the system-defined default storage pool with a
project-specific default storage pool.
Unlike your experience, most of the storage pools I create are general purpose
in the sense that they handle any size or alignment, but are more specific in
when/if any automatic reclamation occurs.
****************************************************************
From: Randy Brukardt
Sent: Tuesday, November 24, 2009 5:45 PM
> I don't agree this would be next to useless for a container instance.
> Clearly you would have to provide a general purpose storage pool if
> you are going to apply it by default to your entire system. But often
> all that is wanted is an ability to replace the system-defined default
> storage pool with a project-specific default storage pool.
Not sure that makes much sense (unless your vendor's pool is particularly bad,
or if you need additional monitoring capabilities).
> Unlike your experience, most of the storage pools I create are general
> purpose in the sense that they handle any size or alignment, but are
> more specific in when/if any automatic reclamation occurs.
Fine, but those aren't very necessary with the containers, which already are
handling storage management (they're required not to leak, after all). So you
are essentially repeating the existing storage management. Moreover, the
containers (other than the bounded ones) are using controlled objects, and
freeing storage in that case makes your program erroneous (and might very well
cause it to crash hard, depending on how controlled objects are implemented).
The only time it is safe to free storage is if the instance has gone away, and
in that case, the "no leak" rule means that the storage has already been freed.
So while I could see that this capability might be useful with your own
generics, it wouldn't have any real value with the unbounded/indefinite
containers.
One could imagine trying to constrain implementation choices for the unbounded
containers so that pools could be useful (for instance, we'd have to ban reusing
nodes in a different object after they're deleted -- a technique I was planning
to use). But that comes uncomfortably close to requiring the containers to have
a particular body, something that was previously rejected.
Anyway, I still think this capability would necessarily have to be provided in
Bob's proposal (it doesn't make sense otherwise), but I don't think it should be
suggested that it has any particular utility for the language-defined packages.
****************************************************************
From: Tucker Taft
Sent: Tuesday, November 24, 2009 7:39 AM
I don't understand why No_Default_Storage_Pool is made into its own pragma,
rather than relying on a restriction. All you said was:
... it seemed better to use a pragma
No_Default_Storage_Pool instead of that Restriction.
Could you provide a bit more rationale? It smells and feels like a restriction,
so I don't see the advantage of not using the Restrictions pragma.
I also find the wording for No_Default_Storage_Pool a bit confusing, as it says
that an access type that
... has neither a specified Storage_Pool nor Storage_Size ...
ends up with Storage_Size 0. But it isn't clear from the pragma
Default_Storage_Pool that this has the effect of giving an access type a
*specified* Storage_Pool. I think from a language point of view it is still
unspecified, but it picks up the Default_Storage_Pool precisely because it is
unspecified. Hence I would recommend that you be more explicit and say that any
access type that is "not within the scope of a Default_Storage_Pool nor has a
specified Storage_Pool nor a specified Storage_Size..." ends up with a
Storage_Size of zero.
It is somewhat annoying that Default_Storage_Pool can't be a configuration
pragma, but I understand the problem. It would be nice if you could just
specify it once and have it apply "everywhere." I suppose if you organize your
library units into a small number of large subsystems, then you would only need
one per top-level library package. For example, in the RTS you would only need
it in the specs for Ada, Interfaces, and System. Of course creating a storage
pool object that can live in a Pure package would be a bit of a challenge!
Especially if it has to depend on System.Storage_Pools... ;-)
****************************************************************
From: Randy Brukardt
Sent: Tuesday, November 24, 2009 5:50 PM
> I don't understand why No_Default_Storage_Pool is made into its own
> pragma, rather than relying on a restriction.
Bob might have a better reason, but to me it seems that
"No_Default_Storage_Pool" is just another form of specifying the default storage
pool. Perhaps it would be better to just use one pragma:
pragma Default_Storage_Pool (null);
to mean no default pool, although that would require rather annoying resolution
and legality rules.
In that sense it isn't really a restriction, it is just the absence of a default
storage pool. (Every access type must have a pool before an allocator can be
used - Storage_Size = 0 could be thought of as the same as not having a pool. I
assume Bob worded it to the other way to minimize the wording changes needed.)
****************************************************************
From: Tucker Taft
Sent: Tuesday, November 24, 2009 7:17 PM
I don't think this accomplishes what Bob (and I want).
You want this to be a configuration pragma, and probably be one of those that
you give all by itself in a file, so it applies to the whole library. That's why
I think it makes the most sense as a restriction.
And yes, this is for cases where you want to monitor and/or control *all*
allocation, because you don't trust or you haven't certified the vendor-provided
default storage pool, or you simply have special requirements relating to
storage management in the given target environment.
****************************************************************
From: Randy Brukardt
Sent: Wednesday, November 25, 2009 12:38 PM
I was assuming that this would be a configuration pragma. I'm not sure why you
assumed otherwise. Of course, you couldn't give an actual pool in that case
because none would be visible. But there would be no problem with giving "null"
in that usage. So only one pragma is needed, and there is no need to worry about
what happens when they conflict.
****************************************************************
Questions? Ask the ACAA Technical Agent