[SimGrid-user] Latency issues

Tristan Glatard tglatard at encs.concordia.ca
Wed Jul 5 15:24:51 CEST 2017


Perfect, thanks a lot for your answer!

     Tristan


On 07/05/2017 09:14 AM, Frédéric Suter wrote:
>
> Hi Tristan,
>
> This can easily be explained. The underlying network model makes a 
> correction of the latency declared in the platform file to reflect the 
> impact of TCP slow start on "large transfers". Such a correction is 
> motivated and explained in 
> http://hal.inria.fr/hal-00646896/PDF/rr-validity.pdf (Note that the 
> domain validity of this model is for transfers larger than 100kB). It 
> is also mentioned when adding the --help-models flag to your command 
> line.
>
> Long description of the network models accepted by this simulator:
>   LV08: Realistic network analytic model (slow-start modeled by 
> multiplying latency by 10.4, bandwidth by .92; bottleneck sharing uses 
> a payload of S=8775 for evaluating RTT).
>
> Unfortunately, these values are outdated. The latency correction 
> factor is now 13.01 
> (https://github.com/simgrid/simgrid/blob/master/src/surf/network_cm02.cpp#L46) 
> which explain the observed results. The description given by 
> --help-models has been corrected and the documentation on these hidden 
> parameters will be improved.
>
> Coming back to your experiments there are two situations:
>
> 1) if your workload comprises a majority of transfers larger than 
> 100kB, this factor is important to reflect a start-up cost of TCP.
>
> 2) if the transfers are smaller than 100kB, you may want to either 
> override this default parameter of the model with 
> --cfg=network/latency-factor:1.0 (to disable it completely or switch 
> to the former default model that didn't apply corrections with 
> --cfg=network/model:CM02
>
> Cheers
>
> Fred
>
>
>
> Le 05/07/2017 à 03:52, Tristan Glatard a écrit :
>> Hi,
>>
>> We're trying to simulate a latency-bound application with Simgrid 
>> 3.16 using MSG, and we're a bit puzzled by simulation times.
>>
>> In example "app-pingpong" the ping time between hosts Tremblay and 
>> Jupiter, i.e., the time for Jupiter to receive a 1-bit task from 
>> Tremblay, is 0.019014s while the latency of the link between Tremblay 
>> and Jupiter (id 9 in small_platform.xml) is only 1.461517ms (factor 
>> 13 difference). And if we edit the platform to contain only the bare 
>> minimal (2 hosts and 1 link with 1ms latency, see attached), we still 
>> get a ping time of 13ms while we would expect 1ms:
>>
>>> [glatard at sapajou app-pingpong]$ ./app-pingpong 
>>> ./smaller_platform.xml ./app-pingpong_d.xml
>>> [Tremblay:pinger:(1) 0.000000] [mag_app_pingpong/INFO] Ping -> Jupiter
>>> [Jupiter:ponger:(2) 0.000000] [mag_app_pingpong/INFO] Pong -> Tremblay
>>> [Jupiter:ponger:(2) 0.013010] [mag_app_pingpong/INFO] Task received 
>>> : small communication (latency bound)
>>> [Jupiter:ponger:(2) 0.013010] [mag_app_pingpong/INFO]  Ping time 
>>> (latency bound) 0.013010
>>> [Jupiter:ponger:(2) 0.013010] [mag_app_pingpong/INFO] task_bw->data 
>>> = 0.013
>>> [Tremblay:pinger:(1) 150.166348] [mag_app_pingpong/INFO] Task 
>>> received : large communication (bandwidth bound)
>>> [Tremblay:pinger:(1) 150.166348] [mag_app_pingpong/INFO] Pong time 
>>> (bandwidth bound): 150.153
>>> [150.166348] [mag_app_pingpong/INFO] Total simulation time: 150.166
>> The same factor-13 difference is still observed on a different but 
>> similar application.
>>
>> Do you have any idea what's going on?
>>
>> Thanks!
>> Tristan
>>
>>
>>
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>> Simgrid-user at lists.gforge.inria.fr
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>
> -- 
> One should, for example, be able to see that things are hopeless
> and yet be determined to make them otherwise.
>                                            -- F. Scott Fitzgerald

-- 

Tristan Glatard

Assistant Professor, Dept of Computer Science and Software Engineering, 
Concordia University

Office: EV 6.225

Phone: +1 514 848 2424, ext 5888
Cell: +1 514 660 9179

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