From: ludovic.courtes@laas.fr (Ludovic Courtès)
Cc: guile-devel@gnu.org
Subject: Re: [PATCH] Marking weak alist vectors
Date: Wed, 09 Nov 2005 17:28:01 +0100 [thread overview]
Message-ID: <87y83xkcq6.fsf@laas.fr> (raw)
In-Reply-To: <4371CF46.4010708@xs4all.nl> (Han-Wen Nienhuys's message of "Wed, 09 Nov 2005 11:28:22 +0100")
Han-Wen Nienhuys <hanwen@xs4all.nl> writes:
> Does your patch solve the problem that cyclical structures (values that
> point back to keys) should also be GC-ed?
I guess you're talking about cyclical structures in doubly-weak alist
vectors. If so, it apparently does since if both WEAK_VALUES and
WEAK_KEYS are false in both functions, then neither the key nor the
value will ever be marked by those functions.
Actually, I've spent some time re-reviewing this patch and I'm having
starting to have a headache. But anyway, here are a few thoughts.
1. The tests in `weaks.test' are broken in several ways, not only
because "we have no way of knowing for certain that the object is
really dead" as stated there.
* First, they assume that
(begin
(hashq-set! h "a string" 123)
(hashq-ref h "a string))
returns true. This is wrong since `hashq-ref' uses `eq?' to
compare keys, and `(eq? "a string" "a string")' returns #f.
Instead, it could use `hash-map->list'.
* Second, it should perform a `(read-disable 'positions)' since
source properties rely on a weakly-keyed hash table where keys are
source expressions.
2. The C test I submitted, unlike `weaks.test', can *reliably*
determine whether an object was swept. However, it is clearly not
as complete as `weaks.test'.
3. Given the level of non-determinism I've been able to observe, I'm
afraid leaks are causing us difficulties. For instance, while
testing weakly-key alist vectors "by hand" in a REPL, it occurred to
me that the weak-key pair would reliably die, *unless* the hash
table was written (I mean using `write'):
guile> (define h (make-doubly-weak-alist-vector 12))
guile> (hashq-set! h "sdf" "paf")
guile> (hashq-set! h "hello" "world")
guile> (gc)
guile> h
#w(() () () () () () () () () () () ())
The same but print H before calling `gc':
guile> (hashq-set! h "sdf" "paf")
guile> (hashq-set! h "hello" "world")
guile> h
#w((("hello" . "world") ("sdf" . "paf")) () () () () () () () () () () ())
guile> (gc)
guile> (gc)
guile> (gc)
guile> h
#w((("hello" . "world") ("sdf" . "paf")) () () () () () () () () () () ())
4. Looking a Bruno Haible's paper[0] on this topic, it seems that getting
it right is, well, pretty hard. ;-)
> Why are you storing SCM references as properties? It's more efficient
> both in time and space to use a SCM member in my_object_t.
That's the whole point of the test: object properties are used because
they involve weak hash tables.
Thanks,
Ludovic.
[0] http://www.haible.de/bruno/papers/cs/weak/WeakDatastructures-writeup.html
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next prev parent reply other threads:[~2005-11-09 16:28 UTC|newest]
Thread overview: 35+ messages / expand[flat|nested] mbox.gz Atom feed top
2005-11-08 17:22 [PATCH] Marking weak alist vectors Ludovic Courtès
2005-11-08 23:51 ` Marius Vollmer
2005-11-09 9:03 ` Ludovic Courtès
2005-12-06 23:55 ` Marius Vollmer
2005-12-07 10:33 ` Ludovic Courtès
2005-12-13 23:45 ` Marius Vollmer
2005-12-14 9:30 ` Ludovic Courtès
2005-11-09 10:28 ` Han-Wen Nienhuys
2005-11-09 16:28 ` Ludovic Courtès [this message]
2005-11-09 18:36 ` Han-Wen Nienhuys
2005-11-09 21:11 ` Kevin Ryde
2005-11-09 22:45 ` Marius Vollmer
2005-11-10 12:11 ` Han-Wen Nienhuys
2005-11-10 9:47 ` [PATCH] Reference leak in `iprin1 ()' Ludovic Courtès
2005-11-12 9:23 ` Neil Jerram
2005-11-14 9:58 ` Ludovic Courtès
2005-11-16 21:18 ` Neil Jerram
2005-11-17 9:54 ` Ludovic Courtès
2005-11-17 18:52 ` Neil Jerram
2005-11-23 10:19 ` [PATCH] Marking weak alist vectors, #2 Ludovic Courtès
2005-11-24 0:59 ` Han-Wen Nienhuys
2005-11-24 9:01 ` Ludovic Courtès
2005-11-26 0:49 ` Kevin Ryde
2006-01-09 14:51 ` [PATCH] Marking weak alist vectors, epilogue Ludovic Courtès
2006-01-09 19:29 ` Neil Jerram
2006-01-10 8:21 ` Ludovic Courtès
2006-01-10 9:33 ` Neil Jerram
2006-01-10 15:43 ` Ludovic Courtès
2005-11-17 13:21 ` [PATCH] Fixing `gc-live-object-stats' Ludovic Courtès
2005-11-17 14:12 ` Han-Wen Nienhuys
2005-11-30 8:54 ` Ludovic Courtès
2005-11-30 23:45 ` Han-Wen Nienhuys
2005-12-03 19:31 ` Neil Jerram
2005-12-05 8:50 ` Ludovic Courtès
2005-12-06 19:14 ` Neil Jerram
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