Comparison of actual and expected Delay length on small durations

Hi

At some point I got a feeling that actual Delay time is longer than expected. It is especially visible on small delays less than 100ms (otherwise difference is < 1%).

Documentation says that Delay waits approximately for specified amount of time. However, according to experimental data an actual difference is always positive with the value around 1.5-2 seconds. It is actually strange where such a constant difference, independent from delay length, comes from.

To conclude, if you want to wait more precisely you need to create delays with duration 1.5 seconds less than expected. Also it would be interesting to test on different hardwares to make sure that difference stays constant (around 1.5-2s)

I used the following script to gather data and save in .csv file

http://ws.stfx.eu/DSCLG2ON0VBI

Tested on:

Pharo: #50636

Spur-VM: #463

OS: Mac OS Yosemite

CPU: Intel Core i7 2.39 GHz

Cheers,

Alex

Le 12/03/2016 18:47, Aliaksei Syrel a écrit :
> Hi
>
> At some point I got a feeling that actual Delay time is longer than
> expected. It is especially visible on small delays less than 100ms
> (otherwise difference is < 1%).
>
> Inline image 1
> Documentation says that _Delay waits approximately_ for specified amount
> of time. However, according to experimental data an actual difference is
> always positive with the value around 1.5-2 seconds. It is actually
> strange where such a constant difference, independent from delay length,
> comes from.
> Inline image 2
> To conclude, if you want to wait more precisely you need to create
> delays with duration 1.5 seconds less than expected. Also it would be
> interesting to test on different hardwares to make sure that difference
> stays constant (around 1.5-2s)
>
> I used the following script to gather data and save in .csv file
> http://ws.stfx.eu/DSCLG2ON0VBI
>
> Tested on:
> Pharo: #50636
> Spur-VM: #463
> OS: Mac OS Yosemite
> CPU: Intel Core i7 2.39 GHz
>
> Cheers,
> Alex
>

If you wish to compare I join a csv. This is my setup:

Image
-----
C:\Users\JeCisC\AppData\Roaming\Pharo\images\Common tools-Pharo 5.0
(beta)-26\Common tools-Pharo 5.0 (beta)-26.image
Pharo5.0
Latest update: #50636
Unnamed

Virtual Machine
---------------
C:\Users\JeCisC\Desktop\PharoVMSpur\Pharo.exe
CoInterpreter * VMMaker.oscog-rmacnak.1586 uuid:
8e44b2a7-68ee-4fc9-8513-a04520267053 Dec 13 2015
StackToRegisterMappingCogit * VMMaker.oscog-rmacnak.1586 uuid:
8e44b2a7-68ee-4fc9-8513-a04520267053 Dec 13 2015
https://github.com/estebanlm/pharo-vm.git Commit:
6dc730123e8f2b0327839b235d16e0604a92288e Date: 2015-12-13 00:23:41 +0100
By: Esteban Lorenzano <[hidden email]> Jenkins build #456

Win32 built on Dec 13 2015 00:58:42 Compiler: 4.6.2
VMMaker versionString https://github.com/estebanlm/pharo-vm.git Commit:
6dc730123e8f2b0327839b235d16e0604a92288e Date: 2015-12-13 00:23:41 +0100
By: Esteban Lorenzano <[hidden email]> Jenkins build #456
CoInterpreter * VMMaker.oscog-rmacnak.1586 uuid:
8e44b2a7-68ee-4fc9-8513-a04520267053 Dec 13 2015
StackToRegisterMappingCogit * VMMaker.oscog-rmacnak.1586 uuid:
8e44b2a7-68ee-4fc9-8513-a04520267053 Dec 13 2015

Operating System/Hardware
-------------------------
Win32 6.1 IX86

Operating System Details
------------------------
Operating System: Windows 7 Professional N (Build 7601 Service Pack 1)
        Registered Owner: JeCisC
        Registered Company:
        SP major version: 1
        SP minor version: 0
        Suite mask: 100
        Product type: 1



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Cyril Ferlicot

http://www.synectique.eu

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Lille 59000 France

Hi

At some point I got a feeling that actual Delay time is longer than expected. It is especially visible on small delays less than 100ms (otherwise difference is < 1%).

Documentation says that Delay waits approximately for specified amount of time. However, according to experimental data an actual difference is always positive with the value around 1.5-2 seconds. It is actually strange where such a constant difference, independent from delay length, comes from.

To conclude, if you want to wait more precisely you need to create delays with duration 1.5 seconds less than expected. Also it would be interesting to test on different hardwares to make sure that difference stays constant (around 1.5-2s)

I used the following script to gather data and save in .csv file

http://ws.stfx.eu/DSCLG2ON0VBI

Tested on:

Pharo: #50636

Spur-VM: #463

OS: Mac OS Yosemite

CPU: Intel Core i7 2.39 GHz

On Sun, Mar 13, 2016 at 1:47 AM, Aliaksei Syrel <[hidden email]> wrote:

Hi

At some point I got a feeling that actual Delay time is longer than expected. It is especially visible on small delays less than 100ms (otherwise difference is < 1%).

Documentation says that Delay waits approximately for specified amount of time. However, according to experimental data an actual difference is always positive with the value around 1.5-2 seconds. It is actually strange where such a constant difference, independent from delay length, comes from.

To conclude, if you want to wait more precisely you need to create delays with duration 1.5 seconds less than expected. Also it would be interesting to test on different hardwares to make sure that difference stays constant (around 1.5-2s)

I used the following script to gather data and save in .csv file

http://ws.stfx.eu/DSCLG2ON0VBI

Tested on:

Pharo: #50636

Spur-VM: #463

OS: Mac OS Yosemite

CPU: Intel Core i7 2.39 GHz

Nice analysis.  I've previously wondered about such an effect from resumptionTime being calculated by the high priority thread in #handleTimerEvent: and thus delayed by switching threads, rather than being calculated as-soon-as-possible by the low priority process when #schedule: is called.   I can't remember my line of reasoning, but doing it the low priority process seemed to open the potential for nasty race conditions, so I left it as I found it.  Perhaps we should take a time snapshot in the #schedule: and pass that through to #handleTimerEvent: similar to scheduledDelay.  Would you like to try that?

btw, I have some cleanup of DelayScheduler almost complete that I had meant to get into Pharo 5 earlier, but now I think best to wait for early Pharo 6.

cheers -ben