Re: [Pharo-dev] Growing large images: the case of Moose models

Hi Clément,

On Fri, Mar 3, 2017 at 4:54 AM, Clément Bera <[hidden email]> wrote:

On Fri, Mar 3, 2017 at 12:12 PM, Cyril Ferlicot D. <[hidden email]> wrote:

On 03/03/2017 11:56, Clément Bera wrote:
> Hello everyone,
>
> This morning I investigated with Vincent Blondeau a problem reported by
> the Moose community a while ago: loading Moose model is slower in Spur
> (Pharo 5+) than in pre-Spur (Pharo 4 and older). In general, this
> problem was present for anyone growing images to a significant size.
>
> To investigate the problem, we loaded a 200Mb[3] Moose model on a 250Mb
> image, growing the image to 450Mb. Loading such a model takes 2 minutes
> in Spur and 1m30s in pre-Spur VMs.
>
> Using the stable Pharo VM, the analysis results were the following:
> - total time spent to load the Model: 2 minutes
> - time spent in full GC: 1 minute (4 fullGCs)
> - time spent in scavenges[1]: 15 seconds
> On the 2 minutes spent, we have 50% of the time spent in full GCs, 12.5%
> in scavenges, 37.5% executing code.
>
> We then used the latest VM that features the new compactor (VM from
> beginning of March 2017 and over). The full GC execution time went down
> from 1 minute to 2 seconds.
>
> In addition, we increased the size of Eden[2] from 4Mb to 12Mb. Time
> spent in scavenges decreased from 15 seconds to 5 seconds.
>
> Overall, loading the model is now taking ~50 seconds instead of 2 minutes.
>
> To increase Eden size, one needs to run a script similar to:
>
> | currentEdenSize desiredEdenSize |
> currentEdenSize := Smalltalk vm parameterAt: 44.
> desiredEdenSize := currentEdenSize * 4.
> Smalltalk vm parameterAt: 45 put: desiredEdenSize.
>

> _*And then restart the image.*_
>
> I hope this report can be useful for some of you. I will try to make a
> blog post out of it, detailing other GC settings one can change from the
> image to improve performance.
> _*
> *_
> Best,
>
> Clement
>
> [1] A scavenge is basically the garbage collection of only young objects
> [2] Eden is basically the space where objects are initially allocated.
> [3] All numbers in the report are order of magnitudes and not precise
> numbers
>
>
>

Hi,

This is great! We will probably try it soon on our models.

Guillaume had a question also, what is the counterparty if we let the
EdenSize at this size when we are not loading/exporting a MSE?

There are 2 main counterparts:

- You waste a bit of memory. If you increase from 4Mb to 12Mb, you waste 8Mb. 

- The user-pauses for scavenges may be more significant.

There are customers using 64Mb Eden in production. It improves their performance, they do not care about wasting 60Mb on machine with 16Gb RAM and their application does heavy computation and does not need to be that responsive.

It is important to realize that scavenge time depends on the amount of objects that survive, not on the size of new space.  So increasing the size of new space will only cause longer pause times when an application is growing the heap, which is the case when Moose reads models.  But if an application is following the standard pattern of creating many objects, most of which are collected, then a large eden should not cause noticeably longer pause times.  This is because a scavenge copies the surviving objects from eden and past space into future space, overflowing into old space, tracing reachable objects only.  So only if lots of objects survive does scavenging touch lots of data.  If only a few objects survive the scavenger touches (copies) only those objects.

The VM collects times so you could do some experiments and measure the average scavenge time for the Moose application during its growth phase and then during its normal phase.  I think you'll find that the large new space is not an issue for normal usage.

There is another variable that can affect pause times and that is the number of stack pages.  The roots of a scavenging collection are the remembered table and the stack zone.  So the larger the stack zone, the more time is spent scanning the stack looking for references to objects in new space.  This is a difficult trade off.  If one has lots of Smalltalk processes in one's application with lots of context switchers between them (this is the threading benchmark) then one wants lots of stack pages, because otherwise a process switch may involve evicting some contexts from a stack page in order to make room for the top context of a newly runnable process. But the more stack pages one uses the slower scavenging becomes.

Cog's default used to be very high (160 pages IIRC) which was determined at Qwaq, whose Teatime application uses lots of futures.  I've reduced the default to 50 but it is an important variable to play with.

However for UI applications (typically the IDE), the scavenge pauses may become significant enough to be noticed by the programmer. Maybe not at 12Mb, but certainly at 64Mb.

I doubt this very much (because of the argument above).  Remember that the scavenger is a garbage collector specifically designed to work well with systems like Smalltalk where lots of intermediate objects are created when computing results.  Scavenging doesn't touch objects that are reclaimed, only objects that survive.  So this works well.  I think you'll find that GUI applications fit this pattern very much and so a large new sad should not present a problem.

 

For your case:

- Do you care that your application use some more Mb of RAM ?

- Do you care if your application takes a couple extra ms to answer a request ? (In any case, the full GC takes much more time and also delays the answer right now)

If you don't care, you can use larger Eden. In any case, an Eden of 12 or 16 Mb should be safe.

 

There are other settings that can be useful in your case. I will try to write a post about it.

Because in our case we deploy a server that might need to read some MSE.
We cannot restart it with our current solution. In that case it would be
good to have more info to select the best EdenSize for the server.

Thank you!

--
Cyril Ferlicot

http://www.synectique.eu

2 rue Jacques Prévert 01,
59650 Villeneuve d'ascq France

--

_,,,^..^,,,_

best, Eliot