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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
6f18be3
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
aam Oct 31, 2024
1766a28
Fix url links again
aam Oct 31, 2024
89162f2
Move text around
aam Nov 4, 2024
250255c
Merge branch 'main' into comment-on-limit-concurrency
aam Nov 4, 2024
beed89f
rewrite/restructure
MaryaBelanger Nov 7, 2024
c0c61bc
Merge branch 'main' into comment-on-limit-concurrency
MaryaBelanger Nov 7, 2024
a578081
fix typos, links
MaryaBelanger Nov 7, 2024
473bf61
aam reviews
MaryaBelanger Nov 7, 2024
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rewrite/restructure
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@MaryaBelanger
MaryaBelanger committed Nov 7, 2024
commit beed89f30c25fa41e5fb2aebfe7df582cccb54be
32 changes: 24 additions & 8 deletions src/content/language/concurrency.md
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Expand Up @@ -398,14 +398,6 @@ 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).

#### Message types

Messages sent via [`SendPort`][]
Expand Down Expand Up @@ -436,6 +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* communction 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 do synchronous communication between isolates in FFI calls
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 that does synchronous communication
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