[Cuis-dev] Jitter source during benchmarks

Fri May 15 09:11:52 PDT 2026

Hi folks,

I've tried the following without success. The same jitter is still present
in these scenarios:

   - Running the experiment using the -headless VM flag
   - Running the experiment using the -noheartbeat VM flag
   - Running the experiment inside a block with valueUnpreemtively
   - Configuring Smalltalk vmParameterAt: 26 put: 10 before running the
   experiment
   - Replace idle process: 50ms relinquish instead of 1ms -> this required
   making the change and re-starting the image to run the experiment.

I'll try to provide exact steps to reproduce, but for now, I have no clue
about where this is coming from.

Is there anything else I could be missing?

Thanks in advance,
Nico PM

On Wed, May 13, 2026 at 11:00 AM Phil B via Cuis-dev <cuis-dev at lists.cuis.st>
wrote:

> I have not looked recently, but Morphic used to have an idle throttle
> where (IIRC) if there weren't keyboard/mouse inputs for a period of time,
> Morphic updates would occur at a decreased rate.  It occurred after some
> number of seconds without input.  Assuming that functionality is still in
> place, you might be seeing the other side of it (i.e. when the Morphic
> update rate is higher your code is less performant, when it is lower your
> code is more performant.  While the *drawing* may be on another thread, the
> Morphic Smalltalk overhead would still be sharing/competing for a single
> core with the rest of the image.)  I don't recall if it was hard-coded or a
> preference but I'm sure Juan would be able to speak to the current state of
> that functionality if it still exists.  If this information is out of date,
> please disregard...
>
> On Tue, May 12, 2026 at 3:18 PM Nicolás Papagna Maldonado via Cuis-dev <
> cuis-dev at lists.cuis.st> wrote:
>
>> Hi folks,
>>
>> I'm in the process of doing performance benchmarks of my Code Coverage
>> tool for my thesis, and I've encountered something interesting while
>> measuring the test execution times of the Aconcagua package.
>>
>> *Observation:*
>> When running the Aconcagua test suite 150 times in a clean image, the
>> execution times are not normally distributed. Instead, they fall into two
>> distinct "modes":
>>
>> [image: image.png]
>> A Base Mode (~6.28 ms): Representing roughly 72% of the iterations.
>> A Spike Mode (~7.0 ms): Representing roughly 28% of the iterations.
>>
>> As the next plot shows, the spikes are not random; they seem to be
>> kind-of deterministic (roughly every 4th iteration).
>> I think these perturbations are visible because I'm taking very small
>> measurements (~6.28 ms).
>> [image: image.png]
>>
>> *Question:*
>> I am trying to isolate whether this "heartbeat" is originating from
>> within the Cuis image or the VM/OS layer (I'm thinking of VM timer/events,
>> maybe UI timers?)
>> Does anyone know what could be causing this?
>> Are there any known events that run at this particular frequency?
>>
>> I repeated the experiment a couple of times and the perturbations are
>> consistent (in terms of ms and frequency).
>>
>> *Methodology:*
>>
>>    - Measurements were taken in batches of 50 test runs.
>>    - Each batch was run in a fresh image, waiting 2 minutes for the VM
>>    to stabilize Cuis code.
>>    - 10 warmup runs were done and discarded before measuring test run
>>    times.
>>    - Measurements were taken using Time primHighResClock and converted
>>    to ms using Time highResTimerTicksPerMillisecond.
>>    - VM stats were collected before and after measurements. A full GC
>>    was run before taking measurements.
>>    - No GC activity was observed while measuring the tests run (VM stats
>>    report no full nor incremental GCs).
>>    - The test suite was created and run from a workspace
>>
>> *Cuis environment:*
>>
>>    - Cuis Smalltalk commit:
>>    https://github.com/Cuis-Smalltalk/Cuis-Smalltalk-Dev/commit/b221722348cc46ebb9f5d9f3a057a3f361aa3d55
>>    - Code Coverage commit:
>>    https://github.com/npapagna/cuis-code-coverage/commit/db19de02250a3fd50dfc4e0b4d0893b656e31e9b
>>    - Aconcagua commit:
>>    https://github.com/Cuis-Smalltalk/Measures/commit/05aa02aada64f7b64898a5e1452ae8fb76f79427
>>
>> *Machine environment:*
>>
>>    - Model Name: MacBook Pro
>>    - Model Identifier: MacBookPro18,3
>>    - Model Number: MKGR3LL/A
>>    - Chip: Apple M1 Pro
>>    - Total Number of Cores: 8 (6 Performance and 2 Efficiency)
>>    - Memory: 16 GB
>>    - System Firmware Version: 13822.81.10
>>    - OS Loader Version: 13822.81.102
>>    - APPLE SSD AP0512R (NVMe, avg read 5671 MB/s, avg write 5670 MB/s)
>>    - macOs 26.3 (25D125)
>>    - A dedicated fresh profile was created for the benchmarks (indexing,
>>    time machine, screen lock/saver, display shutdown, bluetooth and wifi
>>    disabled).
>>
>> Thanks in advance,
>> Nico PM
>>
>> --
>> Cuis-dev mailing list
>> Cuis-dev at lists.cuis.st
>> https://lists.cuis.st/mailman/listinfo/cuis-dev
>>
> --
> Cuis-dev mailing list
> Cuis-dev at lists.cuis.st
> https://lists.cuis.st/mailman/listinfo/cuis-dev
>

-- 

Nicolás Papagna
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