JEP 248: Make G1 the Default Garbage Collector

Ben Evans ben at
Mon Jun 1 10:36:47 UTC 2015


I'm somewhat late to this, having missed the original discussion
whilst travelling.

Mark targeted this JEP to JDK 9 but has since put that on hold to
allow more discussion.

I made this comment to Mark on jdk9-dev:

"I have been working with G1 for ~5 years, ever since it was
experimental (& highly crash-prone in JDK 6).

In the intervening time, I have seen dozens (if not hundreds) of
installations, across a wide range of customers. I have participated
in, or been consulted on at least a dozen direct trials of GC

It is only in the last 18 months that I have seen *any* real-life
workload on G1 beat the alternatives, and only in the last 12 months
that I've had any customer prepared to go live with G1 in production.

>From my experience, I think that G1 is a fine collector, with a bright
future that should be pursued. However, I haven't seen anything that
would make a switch to it as default collector seem compelling in the
JDK 9 timeframe.

Obviously, my experience is not universal, so I'd like to ask you / Oracle:

1) Can you explain the survey methodology and customer testing that
you performed to arrive at the conclusion that G1 is ready to become

2) Can you share aggregate results of the surveying ("We worked with X
customers and ran Y tests of G1 vs alternatives, and in Z% of cases,
G1 worked better by W margin")?

3) Can you ask some of the customers you worked with to speak publicly
about the trials you ran with them?"

>From reading this thread, am I right to conclude that no formal study
of this issue has been done?

If that's the case, then are we really happy to make G1 default
without some more systematic efforts and attempts to obtain actual

The questions that I'd like to see answered are:

a) How short a pause time can G1 support being tuned to? 50ms? 20?
Personally, I haven't seen it getting close to CMS in terms of STW

b) What is the impact on throughput due to G1?

I do like G1 as a collector, but can we really organise enough field
tests in the pre-9 timeframe to justify such a large and potentially
breaking change? We managed to do some good community compatibility
testing for JDK 8, and we could think about a similar effort for "make
G1 default". However, with modules, HTTP/2 and JShell all happening
for 9, I question whether there is simply enough community bandwidth
to do a decent effort for G1 as well, whereas, if we were targeting
JDK 10 we'd have a lot more time to plan and to try to improve the
quality and range of the field data to hopefully de-risk a potential
large, high-profile failure.



On Thu, Apr 30, 2015 at 2:55 PM, Monica Beckwith
<monica at> wrote:
> I am also FOR the change in the default GC. Charlie and Mattis bring up
> great points. It's about time G1 gets put out there (as the default GC)
> since most of the development work is going into G1. As for documentation,
> we not only need to document the change in the default collector but also
> the defaults for the collector; that are enabled as soon as G1 is employed -
> e.g. MaxGCPauseMillis, IHOP, etc.
> With more and more input coming in, G1 is only going to get better and
> hopefully more adaptive :)
> And as for Charlie's question - I don't remember the last time that I didn't
> see an explicit GC mentioned on the command line (even if it was the default
> GC).
> These are just my two cents.
> -Monica
> On 4/30/15 8:17 AM, charlie hunt wrote:
>> Fwiw, we should not forget that anyone who is currently specifying an
>> explicit GC to use in his or her JVM command line args will not experience
>> any difference in behavior. They will still get the collector they specify
>> to use. The (potential) impact will be on those who do not specify a GC to
>> use.
>> What I would like to hear from Kirk and others who frequently work with
>> customers on GC, what’s the percentage of Java applications they have worked
>> with that do not explicitly specify a GC?  And, of those, what percentage of
>> those apps fall into the categories of small heap and desire low latency, or
>> desire high throughput even at the cost of frequent full GCs?
>> thanks,
>> charlie
>>> On Apr 30, 2015, at 7:27 AM, Mattis Castegren
>>> <mattis.castegren at> wrote:
>>> Hi.
>>> I also work with customers but I would like to give an argument FOR
>>> changing the default.
>>> I don't think we will ever come to a point where G1 is better for ALL
>>> users. Even with a near perfect G1 implementation there may be cases where
>>> the parallel collector gives better throughput.
>>> Right now, I think G1 will be better for most users. There are probably
>>> also corner cases where G1 COULD be better, but where small issues reduces
>>> performance. By changing the default to G1, we will be able to easier find
>>> these as we will expose more users to G1.
>>> Finally, there will be a set of users who only care about throughput, and
>>> who will see a performance regression. In those cases, they can go back to
>>> using parallel. But hopefully, there will be far fewer users who need to
>>> tune their application to run with parallel GC than there are users who have
>>> to (or should) tune their application to run with G1.
>>> In the case of huge, business critical, applications, we will always
>>> introduce a risk by changing default collectors. This is true if we change
>>> to G1 in JDK 9, 10 or 11. I prefer to just rip the band aid off. We know
>>> that the collector we will focus on going forward is G1, so we should let as
>>> many people use it as possible.
>>> Of course we should document this a lot, so that users who go up to JDK 9
>>> and see performance regressions can at least try to run with Parallel to see
>>> if it is due to the GC.
>>> Kind Regards
>>> /Mattis
>>> -----Original Message-----
>>> From: Kirk Pepperdine [mailto:kirk at]
>>> Sent: den 30 april 2015 13:18
>>> To: Stefan Johansson
>>> Cc: hotspot-dev at Source Developers
>>> Subject: Re: JEP 248: Make G1 the Default Garbage Collector
>>> Hi Stefan,
>>> Indeed, the improvements have been amazing. I have been getting many
>>> clients to bench with it and although the results have been mixed, overall
>>> many have been able to move forward. However I still would not recommend G1
>>> to anyone who can't move to 1.8.0_40. Of course this change will obviously
>>> come post _40 but still, the recent emergence of the G1 as a viable
>>> production ready collector suggests that making it a default maybe a wee bit
>>> optimistic.
>>> The change is based on the assumption that limiting latency is often more
>>> important than maximizing throughput. If this assumption is incorrect then
>>> this change might need to be reconsidered.
>>> I would agree with this assumption. In most cases latency is more
>>> important. However G1 doesn't always provide lowest latency especially in
>>> smaller heaps.
>>> G1 is seen as a robust and well-tested collector. It is not expected to
>>> have stability problems, but becoming the default collector will increase
>>> its visibility and may reveal previously-unknown issues.
>>> I not sure it's prudent to treat the entire Java eco-system as guinea
>>> pigs. I believe it's more prudent to have the willing take that first step
>>> rather than have it unwittingly dropped on everyone
>>> At the end of the day, I don't have any say in any of this (as it should
>>> be). All I can do is let you know what I'm seeing through my straw with the
>>> hope that you'll find the information useful. From what I see, there is not
>>> nearly enough experience in the tuning the G1 in that is especially true in
>>> the general population to make this type of change at this point in time.
>>> I'm also not sure that we have all the tuning options we need to ensure
>>> "happy apps" in the wild. For example, I think the incremental accumulated
>>> waste in tenured regions is a problem that I'm not sure we have the tools to
>>> solve. I'm not even sure if it's a recognized problem. In fact I'm not even
>>> sure it's a real problem as at the moment it's only a theory based on
>>> observations I'm making by looking at numbers of GC logs produced by
>>> applications using recent releases of the G1.
>>> I would suggest that for Tiered the default config for 8 is was also a
>>> bit premature. I've had to have a number of clients have to roll back off of
>>> it.
>>> - Kirk
>>> On Apr 29, 2015, at 3:03 PM, Stefan Johansson
>>> <stefan.johansson at> wrote:
>>>> Hi Kirk,
>>>> A lot of effort is put into G1, it has been continuously improving over
>>>> the last couple of years and we now believe that G1 is ready to become the
>>>> default. G1 will not improve all use case, but the same is true for the
>>>> other collectors. For users where throughput is the main concern, Parallel
>>>> GC can still be used by specifying -XX:+UseParallelGC on the command-line.
>>>> Regards,
>>>> Stefan
>>>> On 2015-04-29 09:10, Kirk Pepperdine wrote:
>>>>> Hi all,
>>>>> Is the G1 ready for this? I see many people moving to G1 but also I'm
>>>>> not sure that we've got the tunable correct. I've been sorting through a
>>>>> number of recent tuning engagements and my  conclusion is that I would like
>>>>> the collector to be aggressive about collecting tenured regions at the
>>>>> beginning of a JVM's life time but then become less aggressive over time.
>>>>> The reason is the residual waste that I see left behind because certain
>>>>> regions never hit the threshold needed to be included in the CSET. But, on
>>>>> aggregate, the number of regions in this state does start to retain a
>>>>> significant about of dead data. The only way to see the effects is to run
>>>>> regular Full GCs.. which of course you don't really want to do. However, the
>>>>> problem seems to settle down a wee bit over time which is why I was thinking
>>>>> that being aggressive about what is collected in the early stages of a JVMs
>>>>> life should lead to better packing and hence less waste.
>>>>> Note, I don't really care about the memory waste, only it's effect on
>>>>> cycle frequencies and pause times.
>>>>> Sorry but I don't have anything formal about this as I (and I believe
>>>>> many others) are still sorting out what to make of the G1 in prod. Generally
>>>>> the overall results are good but sometimes it's not that way up front and
>>>>> how to improve things is sometimes challenging.
>>>>> On a side note, the move to Tiered in 8 has also caused a bit of grief.
>>>>> Metaspace has caused a bit of grief and even parallelStream, which works,
>>>>> has come with some interesting side effect. Everyone has been so enamored
>>>>> with Lambdas (rightfully so) that the other stuff has been completely
>>>>> forgotten and some of it has surprised people. I guess I'll be submitting a
>>>>> talk for J1 on some of the field experience I've had with the other stuff.
>>>>> Regards,
>>>>> Kirk
>>>>> On Apr 28, 2015, at 11:02 PM, mark.reinhold at wrote:
>>>>>> New JEP Candidate:
>>>>>> - Mark

Ben Evans, Co-founder jClarity @jclarity

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