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Add note regarding synchronous communication between too many isolates #6182

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merged 12 commits into from
Nov 8, 2024
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Add note regarding synchronous communication between too many isolates #6182

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a654eca
Add note regarding synchronous communication between too many isolates
aam Oct 30, 2024
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Clean up
aam Oct 30, 2024
434eeb4
Concurrent vs parallel, added 'unless...'
aam Oct 31, 2024
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Fix url links
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Use site variable
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Merge branch 'main' into comment-on-limit-concurrency
aam Nov 4, 2024
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rewrite/restructure
MaryaBelanger Nov 7, 2024
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Merge branch 'main' into comment-on-limit-concurrency
MaryaBelanger Nov 7, 2024
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MaryaBelanger Nov 7, 2024
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MaryaBelanger Nov 7, 2024
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8 changes: 8 additions & 0 deletions src/content/language/concurrency.md
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Expand Up @@ -398,6 +398,14 @@ isolate, it will remain untouched in the main isolate. This is how isolates are
meant to function, and it's important to keep in mind when you’re considering
using isolates.

#### Synchronous blocking communication between isolates

There is a limit to the number of isolates running in parallel:

- The limit is not hardcoded to a particular number, it is calculated based on the Dart VM heap size available to the Dart application, can be considered to be between 8 and 32 depending on the platform.
- This limit doesn't affect asynchronous communction between isolates via messages - you can have hundreds of isolates running and making progress. The isolates are scheduled on the CPU in round-robin fashion and yield to each other often.
- Attempts to do *synchronous* communication between isolates over the limit though may result in a deadlock unless special care is taken (the C code that does synchronous communication would need to leave the current isolate before it blocks and re-enter it before returning to dart, see [`Dart_EnterIsolate`]({{site.repo.dart.sdk}}/blob/c9a8bbd8d6024e419b5e5f26b5131285eb19cc93/runtime/include/dart_api.h#L1254) and [`Dart_ExitIsolate`]({{site.repo.dart.sdk}}/blob/c9a8bbd8d6024e419b5e5f26b5131285eb19cc93/runtime/include/dart_api.h#L1455).
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FWIW I would expand on this last bullet a bit. It's full of advanced concepts that aren't discussed in these docs at all, and they're concepts that many Dart developers probably have never thought about in their life.

Given the recent user survey that found folks want more advanced isolate samples, we might even want to add example code of attempting synchronous communication between isolates. That would probably constitute a new issue and separate PR. I'll leave that up to you, @MaryaBelanger, and let me know if you want to discuss.


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