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

Eliot Miranda eliot.miranda at gmail.com
Fri Mar 3 16:22:07 UTC 2017

Hi Clément,

On Fri, Mar 3, 2017 at 4:54 AM, Clément Bera <bera.clement at gmail.com> wrote:

> On Fri, Mar 3, 2017 at 12:12 PM, Cyril Ferlicot D. <
> cyril.ferlicot at gmail.com> 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
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