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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
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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@MaryaBelanger
MaryaBelanger committed Nov 7, 2024
commit 473bf616f2e3c7d770a28de2a13ff61ccd6e6194
10 changes: 6 additions & 4 deletions src/content/language/concurrency.md
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Expand Up @@ -430,20 +430,22 @@ objects, so they're subject to the same limitations.

#### Synchronous blocking communication between isolates

There is a limit to the number of isolates that can run in parallel, or *synchronously*.
There is a limit to the number of isolates that can run in parallel.
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
over the limit may result in deadlock unless special care is taken.
Attempting synchronous communication between isolates
by synchronous blocking in FFI calls
may result in deadlock if the number of isolates is over the limit,
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
To avoid this situation, the C code performing synchronous blocking
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.
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