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| 2025-01-31 21:52:42 +0100 | <dminuoso> | mauke: Yes. Its really just a kind of mutual exclusion lock for thunks. |
| 2025-01-31 21:52:24 +0100 | <dminuoso> | You can think of it as some kind of mutual exclusion lock, but with special logic to detect if the entry code recursed into itself. |
| 2025-01-31 21:51:58 +0100 | <euouae> | oh mauke's example relates to black holes? I'll read the whole convo then |
| 2025-01-31 21:51:48 +0100 | <dminuoso> | If not, it will set that mark. |
| 2025-01-31 21:51:39 +0100 | <dminuoso> | Now that entry code both checks for a particular mark BLACKHOLE to be set, if its set, you get a <<loop>> assuming this happened from within the same haskell thread. |
| 2025-01-31 21:51:15 +0100 | <euouae> | interestingly `let x = head [x] in x` just hangs in ghci |
| 2025-01-31 21:50:41 +0100 | <dminuoso> | euouae: and you demand that value by just jmp'ing into that memory region. |
| 2025-01-31 21:50:25 +0100 | <dminuoso> | euouae: So roughly, if you have `let x = <expensive> in ..` then we can think of x being represented in memory as some memory region with a bunch of code |
| 2025-01-31 21:49:53 +0100 | <euouae> | sorry I'm reading from the top so I'm trying to catch up on what was said |
| 2025-01-31 21:49:44 +0100 | <euouae> | mauke, is <<loop>> possible because of sharing? |
| 2025-01-31 21:49:12 +0100 | <dminuoso> | Even in single threaded RTS you will have concurrency. |
| 2025-01-31 21:49:09 +0100 | <mauke> | ah, right |
| 2025-01-31 21:48:34 +0100 | <dminuoso> | While the other use of threads if about haskell threads. |
| 2025-01-31 21:48:24 +0100 | <dminuoso> | Note that "threaded RTS" talks about OS threads |
| 2025-01-31 21:48:08 +0100 | <dminuoso> | mauke: Im not sure how the threaded RTS changes, but blackholing should be needed for single threaded RTS too. |
| 2025-01-31 21:46:05 +0100 | <dminuoso> | So consider it a bonus *if* it triggers. |
| 2025-01-31 21:46:00 +0100 | <mauke> | i.e. in multi-thread mode a thread could re-enter the thunk and end up waiting for itself (deadlock) |
| 2025-01-31 21:45:54 +0100 | <dminuoso> | (And it does not work reliably either for a bunch of reasons) |
| 2025-01-31 21:45:44 +0100 | dsrt^ | (~dsrt@108.192.66.114) (Ping timeout: 252 seconds) |
| 2025-01-31 21:45:23 +0100 | <dminuoso> | The <<loop>> is just some opportunistic debugging helper, its not the core feature. |
| 2025-01-31 21:45:18 +0100 | <mauke> | did they change that? I have a vague memory that <<loop>> detection didn't work in multi-thread mode |
| 2025-01-31 21:45:04 +0100 | <dminuoso> | So it gets woken up whenever the thunk finished. |
| 2025-01-31 21:44:48 +0100 | <dminuoso> | If another thread enters a blackhole, it gets put on a list to be woken up later. |
| 2025-01-31 21:44:33 +0100 | <dminuoso> | mauke: If the same thread enters a blackhole, that blackhole acts as loop detection., |
| 2025-01-31 21:44:25 +0100 | <dminuoso> | mauke: Okay, so there's two behaviors to blackhole. |
| 2025-01-31 21:44:08 +0100 | alexherbo2 | (~alexherbo@2a02-8440-3503-94e0-1866-04f2-f81a-c1ec.rev.sfr.net) (Remote host closed the connection) |
| 2025-01-31 21:44:07 +0100 | <mauke> | so if another thread tries to evaluate the same thunk later, it will simply wait until the first thread is done computing a value |
| 2025-01-31 21:43:33 +0100 | <mauke> | in a multi-threaded environment, the first thread to reach a given thunk instead switches out the code pointer to an "enter waiting queue" subroutine |
| 2025-01-31 21:42:48 +0100 | <mauke> | at least in single-threaded mode |
| 2025-01-31 21:42:30 +0100 | <mauke> | this is implemented by temporarily switching the code pointer of a thunk to a subroutine that throws an exception |
| 2025-01-31 21:41:58 +0100 | euouae | furiously types some notes of the previous discussion, needs more time to read the latest stuff being said |
| 2025-01-31 21:41:55 +0100 | <mauke> | anyway, the <<loop>> exception happens when evaluation of a thunk tries to re-enter the same thunk (i.e. you have a value that depends on itself) |
| 2025-01-31 21:41:06 +0100 | <mauke> | huh, interesting |
| 2025-01-31 21:40:03 +0100 | <dminuoso> | See https://simonmar.github.io/bib/papers/multiproc.pdf |
| 2025-01-31 21:39:37 +0100 | <mauke> | I may be wrong on my terminology, but I think blackholing only happens in single-threaded mode |
| 2025-01-31 21:39:25 +0100 | <dminuoso> | And that all alone pretty much stops memory issues. |
| 2025-01-31 21:38:58 +0100 | <dminuoso> | euouae: That is, there is automatic protection that no two threads attempt to evaluate the same expression concurrently. |
| 2025-01-31 21:38:38 +0100 | <lambdabot> | *Exception: <<loop>> |
| 2025-01-31 21:38:37 +0100 | <dminuoso> | euouae: First off, when switchinig between threads, entered thunks are blackholed. |
| 2025-01-31 21:38:36 +0100 | <mauke> | > let x = head [x] in x |
| 2025-01-31 21:38:18 +0100 | <euouae> | No |
| 2025-01-31 21:38:12 +0100 | <mauke> | on a related topic, have you ever seen the <<loop>> exception? |
| 2025-01-31 21:38:02 +0100 | <dminuoso> | No, its all build to handle that. |
| 2025-01-31 21:38:02 +0100 | <euouae> | I can see why memory can blow up then |
| 2025-01-31 21:37:49 +0100 | <euouae> | Oh it does? That can be bad |
| 2025-01-31 21:37:40 +0100 | sprotte24 | (~sprotte24@p200300d16f06b9001d5c2b08794be0ce.dip0.t-ipconnect.de) |
| 2025-01-31 21:37:34 +0100 | <mauke> | euouae: yes |
| 2025-01-31 21:37:15 +0100 | <mauke> | my point is that just as main() is a regular C function (has an address, can be called from inside the program, etc), so Haskell main is a regular IO () value and can be used as such |
| 2025-01-31 21:37:11 +0100 | <dminuoso> | Oh you're really diving deep now. |
| 2025-01-31 21:36:58 +0100 | <euouae> | so what about threads? is sharing happening across threads? |