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12:10:55
hayley
Perhaps this scheme is too slow in practise; I have no idea and would have to test. Speculative lock elision could be useful, if we get a working implementation in the rest of this century. And we can still decide to avoid local allocation or stomach a minor GC if the time overhead of replication is intolerable.
13:28:32
scymtym
beach: i have already written down some requirements for the environment library. i can try to make those available but i'm not sure whether i will manage to do it today
13:29:22
beach
Great! No rush. I decided Clostrum is good enough for me to continue working on bootstrapping, so I won't work on it for a while.
15:05:36
contrapunctus
scymtym: «I've been using `eclector.parse-result:read` to read forms in files; but I can't figure out how to get skipped input (specifically comments), or even its location. I've been trying out different things based on the manual (defining a `make-skipped-input-result` method, trying to access the second return value of `read`), but to no avail...»
15:20:39
scymtym
contrapunctus: https://techfak.de/~jmoringe/wad.lisp shows how to do that (the example was a test for second climacs originally). you should probably change the body of the MAKE-SKIPPED-INPUT-RESULT method to :reason reason :children '(). you can remove the code using utilities.print-tree or quickload that system, i think. the extra return value you mentioned is relevant for "top-level" skipped input since the corresponding results
19:32:01
moon-child
first atomically exchange the new value with the old one, in the global replica. Then cas it into the nursery replica
19:35:14
moon-child
if a fence has to move everything that's replicated, that's still a win over doing it eagerly; tackle n objects at a time instead of 1. And if you have some atomic-heavy code with a bunch of conses, a bunch of pointing to those new conses from the global heap, and a bunch of fences, the pre-tenurer should learn about that pretty quickly
19:40:38
moon-child
hmm. If you include a generation number, then you don't need a cas loop, just one cas. But you bloat your objects, and have to fight over the generation number, so maybe it's a wash
21:22:08
hayley
@[moon-child]: I don't think a fence has to fix anything, it just has to ensure that there are no replicated writes happening. If we are going lock-free we could use a counter of writes being performed, perhaps.
21:25:55
hayley
Remember that the write occurs to both replicas, so there is no need to fix anything eagerly. The fence just needs to ensure we can't "re-order" around the fence, which is handled by waiting for any writes to finish.
23:12:05
moon-child
hmm. It feels to me that it's better to shunt more work onto the 'owning' thread, since there can only be one of it. Also in the interest of fairness--if other threads are doing extra work only so that the owning thread can see a coherent state, but it never bothers to observe that state, then it's wasted work. But this argument may not hold water: if the overall amount of work done is still less,
23:12:52
moon-child
but something else to consider is that maintaining a counter creates 'false sharing'. If two different threads are diddling replicated objects that happen to come from the same nursery, there should be no issue, but they will have to play pingpong with that nursery's counter
23:14:30
moon-child
another thing: as there are not yet any cl concurrency semantics to adhere to, we don't necessarily have to support fences in the traditional sense. Eg (with-read-barriers ... (fence) ... (fence) ...)
23:18:03
moon-child
(& since the read barrier replaces pointers to the local replica with pointers to the global one, as I think it should, this has effects beyond the with-read-barriers block. That said, I'm not quite sure what a general version of that looks like, as it's rather tailored to this specific mechanism)
0:00:45
hayley
moon-child: If there is much contention on the counter, and thus many writes, it would be better to inform the thread whose nursery is being replicated to just do a minor GC, or pre-tenure harder.
0:01:54
hayley
That's an idea, if we track the probability that objects allocated at some site escape, we can adjust our threshold for directly allocating in the global heap.