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Thanks for sharing these results Monica,<br>
<br>
As Thomas mentioned we have done some testing comparing Serial to G1
in small environments as well. Our conclusions are similar to yours,
G1 nowdays handles the small environments pretty good. <br>
<br>
I used SPECjbb2005, and my focus was to compare throughput given a
fixed memory usage. The reason for this is that the low native
memory overhead of Serial (no marking bitmap etc) is often used as
an argument to use it in small environments. On the other hand, the
region based heap layout of G1 can in many cases offer a better out
of the box heap utilization compared to Serial. To test this and to
make a fair comparison I configure Serial to have a slightly larger
heap to get an overall equal memory consumption (using the peak PSS
usage in Linux as the measure). <br>
<br>
SpecJBB2005 by default runs 1 to 8 warehouses, where warehouses
corresponds to worker threads. I did run this in a cgroup
environment with 1CPU and 1G memory. By default this will give G1 a
256m max heap, which I fixed using Xmx and Xms. To let Serial use as
much memory in total as G1 I configured it with a 288MB heap. With
this setup Serial and G1 get a very similar score with a recent JDK
25 build. The calculated score only takes warehouse 1 and 2 into
account and looking at the result/score for 8 warehouses G1 is ~10%
better. So it looks like G1 is able to handle high pressure better
compared to Serial. <br>
<br>
These results are without the new improved barriers for G1, when
using a build with the new barrier the G1 results are improved by
roughly 3%. This is a use-case not at all caring about latency and
the fact the G1 is still performing this good, also points towards
it being a suitable default even for small environments.<br>
<br>
I've also played around a bit with restricting the amount of
concurrent work done with G1, to see how a G1 STW-only mode would
perform, and on a single CPU system this looks beneficial when we
start to run with more worker threads. But I don't suspect it's that
common to run small cloud services at 100% load, so having a default
that can do concurrent work seems reasonable.<br>
<br>
Thanks,<br>
Stefan<br>
<br>
<br>
<div class="moz-cite-prefix">On 2025-03-18 00:59, Monica Beckwith
wrote:<br>
</div>
<blockquote type="cite" cite="mid:IA3PR21MB42212F6DC29D473E820B0AF4E5DF2@IA3PR21MB4221.namprd21.prod.outlook.com">
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Hi Thomas, Erik, and all,</div>
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<br>
</div>
<div class="elementToProof" style="font-family: Aptos, Aptos_EmbeddedFont, Aptos_MSFontService, Calibri, Helvetica, sans-serif; font-size: 11pt; color: rgb(0, 0, 0);">
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.</div>
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<br>
</div>
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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:</div>
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<br>
</div>
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<b>Throughput (max_jOPS & critical_jOPS):</b></div>
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<br>
</div>
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<div class="elementToProof">G1GC consistently outperforms
SerialGC.</div>
</li>
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<div class="elementToProof">1 core: G1GC shows a 1.78×
increase in max_jOPS.</div>
</li>
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<div class="elementToProof">2 cores: G1GC shows a 2.84×
improvement over SerialGC.</div>
</li>
</ul>
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<br>
</div>
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<b>Latency and Stop-the-World (STW) Impact:</b></div>
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<br>
</div>
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<div class="elementToProof">SerialGC struggles under stress,
with frequent full GCs leading to long pauses.</div>
</li>
<li style="font-family: Aptos, Aptos_EmbeddedFont, Aptos_MSFontService, Calibri, Helvetica, sans-serif; font-size: 11pt; color: rgb(0, 0, 0); list-style-type: "- ";">
<div class="elementToProof">G1GC’s incremental collections
keep pause times lower, especially under stress load.</div>
</li>
<li style="font-family: Aptos, Aptos_EmbeddedFont, Aptos_MSFontService, Calibri, Helvetica, sans-serif; font-size: 11pt; color: rgb(0, 0, 0); list-style-type: "- ";">
<div class="elementToProof">critical_jOPS, a key SLA metric,
is 4.5× higher for G1GC on 2 cores.</div>
</li>
</ul>
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<br>
</div>
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<b>Memory Behavior & Stability:</b></div>
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<br>
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<div class="elementToProof">In 512MB heap configurations,
SerialGC encountered OOM failures due to heap exhaustion.</div>
</li>
</ul>
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<br>
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Given these results, it seems reasonable to reconsider why
SerialGC remains the default in small environments when G1GC
offers clear performance and stability advantages.</div>
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<br>
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Looking forward to thoughts on this.</div>
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<br>
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Best,</div>
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Monica</div>
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<br>
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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.</div>
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<br>
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<div id="divRplyFwdMsg" dir="ltr"><span style="font-family: Calibri, sans-serif; font-size: 11pt; color: rgb(0, 0, 0);"><b>From:</b> hotspot-gc-dev
<a class="moz-txt-link-rfc2396E" href="mailto:hotspot-gc-dev-retn@openjdk.org"><hotspot-gc-dev-retn@openjdk.org></a> on behalf of Thomas
Schatzl <a class="moz-txt-link-rfc2396E" href="mailto:thomas.schatzl@oracle.com"><thomas.schatzl@oracle.com></a><br>
<b>Sent:</b> Monday, February 24, 2025 2:33 AM<br>
<b>To:</b> Erik Osterlund <a class="moz-txt-link-rfc2396E" href="mailto:erik.osterlund@oracle.com"><erik.osterlund@oracle.com></a><br>
<b>Cc:</b> <a class="moz-txt-link-abbreviated" href="mailto:hotspot-gc-dev@openjdk.org">hotspot-gc-dev@openjdk.org</a>
<a class="moz-txt-link-rfc2396E" href="mailto:hotspot-gc-dev@openjdk.org"><hotspot-gc-dev@openjdk.org></a><br>
<b>Subject:</b> [EXTERNAL] Re: RFC: G1 as default collector
(for real this time)</span>
<div> </div>
</div>
<div style="font-size: 11pt;">Hi,<br>
<br>
On 21.02.25 15:02, Erik Osterlund wrote:<br>
> Hi Thomas,<br>
><br>
[...]> There is however a flip side for that argument on the
other side<br>
of the scaling spectrum, where ZGC is probably a better fit on
the even<br>
larger scale. So while it’s true that the effect of a Serial
-> G1<br>
default change is a static default GC, I just think we should
mind the<br>
fact that there is more uncertainty on the larger end of the
scale. I’m<br>
not proposing any changes, just saying that maybe we should be
careful<br>
about stressing the importance of having a static default GC, if
we<br>
don’t know if that is the better strategy on the larger end of
the scale<br>
or not, going forward.<br>
<br>
+1<br>
<br>
Thomas<br>
<br>
</div>
</blockquote>
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