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Add note regarding synchronous communication between too many isolates #6182

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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
335c807
Fix url links
aam Oct 31, 2024
8a902ad
Use site variable
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Fix url links again
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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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fix typos, links
MaryaBelanger Nov 7, 2024
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aam reviews
MaryaBelanger Nov 7, 2024
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24 changes: 24 additions & 0 deletions src/content/language/concurrency.md
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Expand Up @@ -428,6 +428,30 @@ objects, so they're subject to the same limitations.
[`Pointer`]: {{site.dart-api}}/{{site.sdkInfo.channel}}/dart-ffi/Pointer-class.html
[`UserTag`]: {{site.dart-api}}/{{site.sdkInfo.channel}}/dart-developer/UserTag-class.html

#### Synchronous blocking communication between isolates

There is a limit to the number of isolates that can run in parallel, or *synchronously*.
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This limit doesn't affect the standard *asynchronous* communication between isolates
via messages in Dart. You can have hundreds of isolates running concurrently
and making progress. The isolates are scheduled on the CPU in round-robin fashion,
and yield to each other often.

Isolates can only communicate *synchronously* outside of pure Dart,
using C code via [FFI] to do so.
Attempts to perform synchronous communication between isolates in FFI calls
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over the limit may result in deadlock unless special care is taken.
The limit is not hardcoded to a particular number,
it's calculated based on the Dart VM heap size available to the Dart application.

To avoid this situation, the C code performing synchronous communication
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needs to leave the current isolate before performing the blocking operation
and re-enter it before returning to Dart from the FFI call.
Read about [`Dart_EnterIsolate`] and [`Dart_ExitIsolate`] to learn more.

[FFI]: /interop/c-interop
[`Dart_EnterIsolate`]: {{site.repo.dart.sdk}}/blob/c9a8bbd8d6024e419b5e5f26b5131285eb19cc93/runtime/include/dart_api.h#L1254
[`Dart_ExitIsolate`]: {{site.repo.dart.sdk}}/blob/c9a8bbd8d6024e419b5e5f26b5131285eb19cc93/runtime/include/dart_api.h#L1455

<a id="web"></a>
## Concurrency on the web

Expand Down