Async Post Load Test Fixes #36596

dustin12 · 2025-10-22T19:44:49Z

Fixes two problems found in async during load testing:

We should not do any blocking operations under a lock in commit_finished_items. Add items outside the lock.
Timers should be skewed so they don't all fire at once.

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Fixes three problems found in load testing: 1) We should not do any blocking operations under a lock in commit_finished_items. Add items outside the lock. 2) Timers should be skewed so they don't all fire at once.

gemini-code-assist · 2025-10-22T19:45:07Z

Summary of Changes

Hello @dustin12, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses critical performance and stability issues identified during load testing of asynchronous processing within the system. The primary focus is on optimizing resource utilization and preventing contention by ensuring that blocking operations do not occur under locks and by distributing the load of timer-triggered events. These changes collectively aim to enhance the robustness and efficiency of the asynchronous DoFn, particularly under high-load scenarios.

Highlights

Non-blocking operations under lock: Refactored the commit_finished_items method to ensure that potentially blocking schedule_item calls are performed outside of critical sections protected by locks, improving concurrency and preventing deadlocks or performance bottlenecks.
Timer Skewing: Implemented a mechanism to introduce randomness into timer firing times, preventing all timers from triggering simultaneously. This 'skewing' helps distribute load and avoid 'thundering herd' issues during peak activity.
Improved State Coder: Updated the TO_PROCESS state specification to use FastPrimitivesCoder instead of StrUtf8Coder, allowing for more flexible and efficient serialization of primitive types.
Reduced Scheduling Sleep Time: Decreased the sleep duration in the schedule_item method from 1 second to 0.01 seconds, making the item scheduling process more responsive and reducing perceived latency.
Configurable Timer Frequency: Made the _timer_frequency parameter configurable during the initialization of AsyncWrapper, allowing for more dynamic control over timer behavior.

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github-actions · 2025-10-22T22:06:21Z

Checks are failing. Will not request review until checks are succeeding. If you'd like to override that behavior, comment assign set of reviewers

…e function does not define timeout.

codecov · 2025-10-23T22:24:48Z

Codecov Report

❌ Patch coverage is 14.28571% with 12 lines in your changes missing coverage. Please review.
✅ Project coverage is 40.21%. Comparing base (db92a3a) to head (b6c566e).
⚠️ Report is 89 commits behind head on master.

Files with missing lines	Patch %	Lines
sdks/python/apache_beam/transforms/async_dofn.py	14.28%	12 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##             master   #36596       +/-   ##
=============================================
- Coverage     56.93%   40.21%   -16.72%     
  Complexity     3394     3394               
=============================================
  Files          1222     1222               
  Lines        186787   187021      +234     
  Branches       3545     3545               
=============================================
- Hits         106348    75214    -31134     
- Misses        77067   108435    +31368     
  Partials       3372     3372

Flag	Coverage Δ
python	`40.51% <14.28%> (-40.50%)`	⬇️

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github-actions · 2025-10-24T21:35:11Z

Assigning reviewers:

R: @jrmccluskey for label python.

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…e unit test

dustin12 · 2025-10-27T19:27:49Z

Run precommit

jrmccluskey · 2025-10-29T13:51:23Z

sdks/python/apache_beam/transforms/async_dofn.py

-      coders.TupleCoder([coders.StrUtf8Coder(), coders.StrUtf8Coder()]),
-  )
-  _timer_frequency = 20
+      coders.TupleCoder(


This is a backwards incompatible change, since you're swapping to a different coder

There shouldn't be any usage of this yet so I'm OK with that.

jrmccluskey · 2025-10-29T13:57:50Z

sdks/python/apache_beam/transforms/async_dofn.py

    self._parallelism = parallelism
    self._max_wait_time = max_wait_time
-    self._timer_frequency = 20
+    self._timer_frequency = callback_frequency


As best I can tell, self._timer_frequency is used but self.timer_frequency_ is not. Is there any reason to have both? Same goes for all of these duped fields

jrmccluskey · 2025-10-29T14:01:04Z

sdks/python/apache_beam/transforms/async_dofn.py

+    sleep_time = 0.01
    total_sleep = 0
-    while not done:
+    timeout = 1


the timeout duration could be configurable in __init__()

jrmccluskey · 2025-10-29T14:07:03Z

sdks/python/apache_beam/transforms/async_dofn.py

+  def next_time_to_fire(self, key):
+    random.seed(key)
    return (
        floor((time() + self._timer_frequency) / self._timer_frequency) *
-        self._timer_frequency)
+        self._timer_frequency) + (
+            random.random() * self._timer_frequency)


I feel like doing all of the work to find a round increment of _timer_frequency is wasted compute once you add the extra fuzziness of random.random() * self._timer_frequency since you're no longer on a round increment afterwards

I started just having keys setting a timer now + 10s. That doesn't work because as new work arrives the timer firing time keeps getting pushed out. ie. an element arrives at t=1, we want to check back on it at t=11 so we set the timer, but then an element arrives at t=9 and overwrites the timer to t=19.

Next setup was having this round increment firing time. so any message that arrives between t=0 and t=10 sets the timer for 0:10. That way the element at t=9 doesn't override the timer to t=19 but keeps it at t=10.

That works but means we see a spike of timers at t=10, t=20, t=30 etc. There isn't any reason the timers all need to fire at these round increments so this is attempting to add fuzzing per key (since timers are per key). Ideally this means that any 1 key has buckets 10s apart so the overwriting problem is fixed but also means that across multiple keys the buckets don't all fire at the same time. I believe this is what the random.seed(key) on line 260 is doing but correct me if I'm wrong.

Also, let me know if you know an easier way to obtain this pattern.

dustin12 added 2 commits October 21, 2025 16:59

Update async_dofn.py

2364455

Fixes three problems found in load testing: 1) We should not do any blocking operations under a lock in commit_finished_items. Add items outside the lock. 2) Timers should be skewed so they don't all fire at once.

Formatting fixes

9e3fd6d

github-actions bot added the python label Oct 22, 2025

dustin12 added 2 commits October 22, 2025 13:07

extra tab

0a491a5

import random and make randomness deterministic per key.

cb533af

Fix unit test failure caused by passing timeout to a function when th…

4192812

…e function does not define timeout.

dustin12 added 2 commits October 24, 2025 11:17

Fix unit tests now that we don't block

badeb17

formatting

fe24a7f

github-actions bot added the Next Action: Reviewers label Oct 24, 2025

re add sleep so message submission doesn't race with processing in th…

d9eff31

…e unit test

jrmccluskey reviewed Oct 29, 2025

View reviewed changes

dustin12 added 2 commits October 30, 2025 23:22

Address Comments

b6c566e

fix formatting and typo

9fedd53

Provide feedback

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Async Post Load Test Fixes #36596

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