[EXTERNAL] RFC: G1 as default collector (for real this time)

[Kirk Pepperdine]

Hi Monica,

Interesting results. Can you share GC logs for those runs? I’m asking because these results don’t correlate with what I’ve been seeing. That said, I don’t have enough data as of yet to draw any conclusions.

Kind regards,
Kirk


> On Mar 17, 2025, at 4:59 PM, Monica Beckwith <Monica.Beckwith at microsoft.com> wrote:
> 
> Hi Thomas, Erik, and all,
> 
> This is an important and timely discussion, and I appreciate the insights on how the gap between SerialGC and G1GC has diminished over time. Based on recent comparative tests of out-of-the-box GC configurations (-Xmx only), I wanted to share some data-backed observations that might help validate this shift.
> 
> I tested G1GC and SerialGC under 1-core/2GB and 2-core/2GB containerized environments (512MB < -Xmx <1.5GB), running SPECJBB2015 with and without stress tests. The key findings:
> 
> Throughput (max_jOPS & critical_jOPS):
> 
> G1GC consistently outperforms SerialGC.
> 1 core: G1GC shows a 1.78× increase in max_jOPS.
> 2 cores: G1GC shows a 2.84× improvement over SerialGC.
> 
> Latency and Stop-the-World (STW) Impact:
> 
> SerialGC struggles under stress, with frequent full GCs leading to long pauses.
> G1GC’s incremental collections keep pause times lower, especially under stress load.
> critical_jOPS, a key SLA metric, is 4.5× higher for G1GC on 2 cores.
> 
> Memory Behavior & Stability:
> 
> In 512MB heap configurations, SerialGC encountered OOM failures due to heap exhaustion.
> 
> Given these results, it seems reasonable to reconsider why SerialGC remains the default in small environments when G1GC offers clear performance and stability advantages.
> 
> Looking forward to thoughts on this.
> 
> Best,
> Monica
> 
> P.S.: I haven’t tested for <512MB heaps yet, as that requires a different test config I’m still working on. I’d also love to hear from anyone running single-threaded, CPU-bound workloads if they have observations to share.
> 
> 
> From: hotspot-gc-dev <hotspot-gc-dev-retn at openjdk.org> on behalf of Thomas Schatzl <thomas.schatzl at oracle.com>
> Sent: Monday, February 24, 2025 2:33 AM
> To: Erik Osterlund <erik.osterlund at oracle.com>
> Cc: hotspot-gc-dev at openjdk.org <hotspot-gc-dev at openjdk.org>
> Subject: [EXTERNAL] Re: RFC: G1 as default collector (for real this time)
>  
> Hi,
> 
> On 21.02.25 15:02, Erik Osterlund wrote:
> > Hi Thomas,
> >
> [...]> There is however a flip side for that argument on the other side
> of the scaling spectrum, where ZGC is probably a better fit on the even
> larger scale. So while it’s true that the effect of a Serial -> G1
> default change is a static default GC, I just think we should mind the
> fact that there is more uncertainty on the larger end of the scale. I’m
> not proposing any changes, just saying that maybe we should be careful
> about stressing the importance of having a static default GC, if we
> don’t know if that is the better strategy on the larger end of the scale
> or not, going forward.
> 
> +1
> 
> Thomas

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://mail.openjdk.org/pipermail/hotspot-gc-dev/attachments/20250317/b062f9bd/attachment-0001.htm>