Why Should I Care?

After measuring the allocation rate we can understand how the changes in allocation rate affect application throughput by increasing or reducing the frequency of GC pauses. First and foremost, you should notice that only minor GC pauses cleaning the young generation are affected. Neither the frequency nor duration of the GC pauses cleaning the old generation are directly impacted by the allocation rate, but instead by the promotion rate, a term that we will cover separately in the next section.

Knowing that we can focus only on Minor GC pauses, we should next look into the different memory pools inside the young generation. As the allocation takes place in Eden, we can immediately look into how sizing Eden can impact the allocation rate. So we can hypothesize that increasing the size of Eden will reduce the frequency of minor GC pauses and thus allow the application to sustain faster allocation rates.

And indeed, when running the same application with different Eden sizes using -XX:NewSize -XX:MaxNewSize & -XX:SurvivorRatio parameters, we can see a two-fold difference in allocation rates.

  • Re-running with 100 M of Eden reduces the allocation rate to below 100 MB/sec.
  • Increasing Eden size to 1 GB increases the allocation rate to just below 200 MB/sec.

If you are still wondering how this can be true – if you stop your application threads for GC less frequently you can do more useful work. More useful work also happens to create more objects, thus supporting the increased allocation rate.

Now, before you jump to the conclusion that “bigger Eden is better”, you should notice that the allocation rate might and probably does not directly correlate with the actual throughput of your application. It is a technical measurement contributing to throughput. The allocation rate can and will have an impact on how frequently your minor GC pauses stop application threads, but to see the overall impact, you also need to take into account major GC pauses and measure throughput not in MB/sec but in the business operations your application provides.