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  • Metaspace is the location where he JVM stores application metadata (class meta-information, method and field tables, HIT optimisations, etc). It is unlimited by default.

    It is important to note that if not sized appropriately then if the JVM resizes Metaspace, a full GC must
  • be done, which is GC savings can be made by determining a Metaspace size that is appropriate for your applicaton, to reduce continued expansion. This is because the JVM always performs a full GC before it resizes Metaspace, which can be expensive.


    GSA: I don't expect this kind of memory to need a lot of tuning, maybe just remove that section ?

    WM: Ok, but if Metaspace is resized (too small) then we require a time-consuming full-GC. I think it's not that well-known so we should mention it. I'll reduce it, to reduce its importance but keep the section with that detail in it.

    GSA: Metaspace is where JVM stores classes, it's not OldGen, so the sizing should be dictated by the size of the application (all the classes that need to be loaded in memory), or am I mistaken ?

    WM: You're right, but may have missed my point. Metaspace isn't OldGen - and is where we store class info, method tables, JIT optimisations, etc. With PermGen it used to be limited but with Metaspace it's now unlimited. However, if the initial allocation isn't large enough then Metaspace needs to be increased. If it's increased then the JVM does a full GC first. So, the point is, you want it to be sized appropriately first to avoid the full GC. It should be easy enough ( (smile) ) to size and is a cost saving.

    WM: I've reworded it slightly to explain why setting this to a sensible value is a good thing.

    (Will remove Tuesday 30/6/2020 if no further comment)


-serverEnsures the JVM uses server-optimised configuration, compilation and execution. This is the default with a 64-bit JVM.



Configure the initial and maximum heap space:

  • Equivalent to -Xms and -Xmx flags respectively.
  • Oracle recommend that these figures are set the same to avoid expensive allocation operations. However, recent improvements in G1GC G1 with its adaptive optimization algorithm (see below) means that this may limit vertical scaling. Though it is still recommended to follow this practice, if using G1GCG1, consider testing individual initial and maximum values too.
  • As a start point, IG has been shown to operate well in performance testing with a heap size of 5Gb. That is, with this sized heap, even with more memory available, a memory increment was never done.



Configure the initial and maximum YoungGen space:

  • This configuration is important to IG as a large consumer of Eden space.
  • Incrementally test to optimise, with anticipated concurrent load in a pre-production environment. The maximum should be within the overall allocated maximum heap-size (-Xmx), leaving space for non-heap memory.
  • If using G1GCG1, it is advised not to set these values and instead allow G1GC G1 to optimise based on use. As ever, this should be tested.



Prevent String duplication and so conserve memory (java 8u20+), reducing GC needs. The threshold can be provided to specify the age after which a String becomes a candidate for deduplication.


GSA: why not, but do we think that this can have a notable impact on our performances ?

WM: It's documented everywhere as advisable. It's also advised in the DJ tuning guide for 7.0. It's basically the inlining of Strings, as used to be the case pre-java8.

(Will remove Tuesday 30/6/2020 if no further comment)


Configure the number of transitions between survivor spaces before an object is moved to OldGen.

Because IG largely consumes YoungGen space, we can determine that anything that eventually would live in OldGen space could be moved there early to free up Eden space and reduce objects transitioning between survivor spaces. We could therefore set -MaxTenuringThreshold to a low value, possibly as low as 1 to accomplish this.


GSA: Good, this is exactly the kind of description that is useful to our users: it explains why this is worth considering this option

WM: Thanks

(Will remove Tuesday 30/6/2020 if no further comment)



Configure initial and maximum Metaspace size (from java 8). IG  GC savings can be made by determining a Metaspace size that is appropriate for your applicaton, to reduce continual dynamic expansion to claim more memory. This is because the JVM always performs a full GC before it resizes Metaspace, which can be expensive.


GSA: Unsure if we should keep this one in the doc, for sure not in the first positions, it's relatively less important than young/old gen settings

WM: Probably not vital but it is included in other guides and I think it's important to note we only need a smallish Metaspace. I've moved it to last position, as I want to describe it as II've provided a recommended default.

GSA: Smallish until we use SAML (smile)

WM: Lol, ok (smile) Is there anything I need to document there then?

WM: Added note why it's beneficial - to avoid full GC.

(Will remove Tuesday 30/6/2020 if no further comment)


  • Default Hotspot collector in java 8 - in resource-rich environment (multiprocessor, memory availability).
  • Multiple threads for managing heap-space speeds up collection process.
  • Aims to maximise throughput.
  • Requires more full-GCs ("stop-the-world"), freezing application threads while in progress.
  • It is advisable to configure the -XX:InitialHeapSize  and -XX:MaxHeapSize to the same value to prevent unnecessary memory increases.
  • It is advisable to configure the -XX:NewSize  and -XX:MaxNewSize to weigh the ratio in favour of YoungGen memory, which IG makes more use of.

The following Parallel GC options are worth considering:



GSA: If we don't provide added values, I would not bother adding these things in our doc, a pointer to the official doc page would be enough (and easier to maintain)

Added value like: why this is interesting for IG, or return of experience like we observed a noticeable impact when ...

WM: When you say added value though, the below is taken from looking into multiple GC tests and guides, and is intended only as a guide for IG to consolidate info (without having done exhaustive testing). I think it's therefore still very useful. 

GSA, when I say added value, I mean we found a correlation that we can explain, otherwise it's not really more that just copy paste of the GC doc (hoping there is one somewhere)

WM: Ok, well, I can remove them but they are the key useful tuning flags.

(Will remove Tuesday 30/6/2020 if no further comment)

Adaptive sizing of each generation can be controlled using these parameters.
-XX:+UseParallelGCUse the Parallel garbage collector.
-XX:MaxGCPauseMillis=<duration in milliseconds>Supports configuration of the target max time to pause in GC. This is purely a goal and not guaranteed. It does, however, allow some configuration between throughput (longer pauses) and latency (shorter pauses). Test values between 500ms - 2000ms.
-XX:GCPauseTimeInterval=<duration in milliseconds>Supports configuration of the ideal max time between pauses in GC. This is purely a goal and not guaranteed. It does, however, allow some configuration between throughput (longer pauses) and latency (shorter pauses). Test values between 500ms - 2000ms.
-XX:GCTimeRatio=<percent value>Supports configuration of the optimal ratio between time in GC and application time. This ratio is 1% by default and should not be configured to be greater than 5%.
-XX:ParallelGCThreads=<size> Number of threads to use in parallel operations






The following is an example set of JVM options using the Parallel GC (GC logging omitted):



Care should be taken in selecting an appropriate garbage collector, depending on your identified performance targets.

G1 is designed for multiprocessor environments with large available memory with the intent of good overall performance without the need to specify additional options. It is designed to reduce garbage collection through low-GC latency. It is largely self-tuning, with an adaptive optimisation algorithm, but there are a number of options to consider to suite the needs of the protected web application. See Oracle G1 collector documentationsdocumentation and tuning guide.

The Parallel GC aims to improve garbage collection by following a high-throughput strategy, but requires more full GCs. See Oracle Parallel GC documentation.

Generally, the recent addition of the G1 garbage collector is largely promoted by Oracle to satisfy most needs, but testing should be done to compare the results of G1 and the Parallel collector in typical business use cases.