Re: Unbounded strings (Was: Java vs Ada 95 (Was Re: Once again, Ada absent from DoD SBIR solicitation))

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

Geert says, replying to Kevin:

Kevin D. Heatwole (heatwole@his.com) wrote:
 ``Is it really that much harder to provide an efficient ref-counted
   tasking safe implementation of unbounded strings than it is to
   provide an efficient tasking safe allocator? ''

No, it isn't. The only problems relate to the priority requirements of
Annex D.  When you want to use ceiling priority locks, it is necessary
to change and/or check priority of tasks which requires an expensive
system call.



Robert replies

That's incorrect, it is easy to think of strategies for writing task
safe allocators that greatly reduce the requirement for this expensive
locking. In particular, an obvious strategy is to cache a chunk of
storage for a task doing dynamic allocation, and use this cache to
satisfy local requests, so that the expensive locking is only required
to get a new chunk. We don't do that by default in GNAT, but it is
quite easy, using the storage pools mechanism and task attributes,
to write a storage pool that works that way and use it.

On the other hand, if you are forced into using locks for reference
counts, then no such tricks are at hand, and you will need the
expensive system call for every unbounded string assignment, which
is pretty fierce.

Note also that in a typical use of unbounded strings, you may do many
more assignments of UBS's than allocations, so even if you are using
the standard allocator which takes a lock, you pay that only on a new
string -- taking a lock on every assignment may be much worse.