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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
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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
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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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6 changes: 6 additions & 0 deletions src/content/language/isolates.md
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Expand Up @@ -39,6 +39,12 @@ but here are some more situations where they can be useful:
- Performing I/O, such as communicating with a database.
- Handling a large volume of network requests.

Also note a caveat due to a limit to the number of concurrently running isolates:
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I'd rephrase this as

"""
Synchronous blocking communication between isolates.

If isolates want to communicate synchronously they have to leave the safety of Dart and resort to using C code via FFI to do so. Special care must be taken in this circumstance due to the Dart VM imposing a limit on the number of isolates that can run in parallel. If the number of isolates that synchronously block in FFI calls exceeds that limit no other isolate can run. If those blocking isolates wait on another unscheduled isolate to act, then deadlock can occur. To avoid this situation (preventing other isolates from running & possibly deadlock scenarios) such blocking C code must take care of leaving the current isolate before performing the blocking operation and re-entering the isolate before returning from the FFI call (see ...)
"""

(especially concurrently -> in parallel - we can run thousands of isolates concurrently but not in parallel, i.e. at the same time)

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This text sounds good, but did you mean for it to replace my version or you thought about adding it?


- 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.
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... 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 ...)

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Thanks for the comment, I added this. PTAL.


[Flutter]: {{site.flutter-docs}}/perf/isolates

## Implementing a simple worker isolate
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