Spicing up your app’s efficiency – a easy recipe for GC tuning | Weblog | bol.com



The rubbish collector is a posh piece of equipment that may be tough to tune. Certainly, the G1 collector alone has over 20 tuning flags. Not surprisingly, many builders dread touching the GC. For those who don’t give the GC just a bit little bit of care, your complete utility may be working suboptimal. So, what if we let you know that tuning the GC doesn’t need to be arduous? Actually, simply by following a easy recipe, your GC and your complete utility may already get a efficiency enhance.

This weblog submit exhibits how we acquired two manufacturing purposes to carry out higher by following easy tuning steps. In what follows, we present you the way we gained a two occasions higher throughput for a streaming utility. We additionally present an instance of a misconfigured high-load, low-latency REST service with an abundantly massive heap. By taking some easy steps, we diminished the heap dimension greater than ten-fold with out compromising latency. Earlier than we accomplish that, we’ll first clarify the recipe we adopted that spiced up our purposes’ efficiency.

A easy recipe for GC tuning

Let’s begin with the elements of our recipe:

ingredients of performance recipe

In addition to your utility that wants spicing, you need some method to generate a production-like load on a take a look at atmosphere – except feeling courageous sufficient to make performance-impacting adjustments in your manufacturing atmosphere.

To guage how good your app does, you want some metrics on its key efficiency indicators. Which metrics depend upon the particular objectives of your utility. For instance, latency for a service and throughput for a streaming utility. In addition to these metrics, you additionally need details about how a lot reminiscence your app consumes. We use Micrometer to seize our metrics, Prometheus to extract them, and Grafana to visualise them.

Along with your app metrics, your key efficiency indicators are lined, however ultimately, it’s the GC we like to boost. Except being fascinated with hardcore GC tuning, these are the three key efficiency indicators to find out how good of a job your GC is doing:

  • Latency – how lengthy does a single rubbish accumulating occasion pause your utility.
  • Throughput – how a lot time does your utility spend on rubbish accumulating, and the way a lot time can it spend on doing utility work.
  • Footprint – the CPU and reminiscence utilized by the GC to carry out its job

This final ingredient, the GC metrics, may be a bit tougher to search out. Micrometer exposes them. (See for instance this weblog submit for an outline of metrics.) Alternatively, you might get hold of them out of your utility’s GC logs. (You may confer with this text to discover ways to get hold of and analyze them.)

Now we’ve got all of the elements we’d like, it’s time for the recipe:

recipe for performance

Let’s get cooking. Fireplace up your efficiency assessments and hold them working for a interval to heat up your utility. At this level it’s good to write down down issues like response occasions, most requests per second. This manner, you’ll be able to examine completely different runs with completely different settings later.

Subsequent, you establish your app’s dwell information dimension (LDS). The LDS is the dimensions of all of the objects remaining after the GC collects all unreferenced objects. In different phrases, the LDS is the reminiscence of the objects your app nonetheless makes use of. With out going into an excessive amount of element, you have to:

  • Set off a full rubbish acquire, which forces the GC to gather all unused objects on the heap. You may set off one from a profiler similar to VisualVM or JDK Mission Management.
  • Learn the used heap dimension after the total acquire. Beneath regular circumstances it is best to have the ability to simply acknowledge the total acquire by the large drop in reminiscence. That is the dwell information dimension.

The final step is to recalculate your utility’s heap. Most often, your LDS ought to occupy round 30% of the heap (Java Efficiency by Scott Oaks). It’s good apply to set your minimal heap (Xms) equal to your most heap (Xmx). This prevents the GC from doing costly full collects on each resize of the heap. So, in a method: Xmx = Xms = max(LDS) / 0.3

Spicing up a streaming utility

Think about you’ve got an utility that processes messages which can be printed on a queue. The appliance runs within the Google cloud and makes use of horizontal pod autoscaling to robotically scale the variety of utility nodes to match the queue’s workload. The whole lot appears to run nice for months already, however does it?

The Google cloud makes use of a pay-per-use mannequin, so throwing in additional utility nodes to spice up your utility’s efficiency comes at a worth. So, we determined to check out our recipe on this utility to see if there’s something to achieve right here. There actually was, so learn on.

Earlier than

To determine a baseline, we ran a efficiency take a look at to get insights into the appliance’s key efficiency metrics. We additionally downloaded the appliance’s GC logs to study extra about how the GC behaves. The under Grafana dashboard exhibits what number of parts (merchandise) every utility node processes per second: max 200 on this case.

grafana graph

These are the volumes we’re used to, so all good. Nonetheless, whereas inspecting the GC logs, we discovered one thing that shocked us.

GC LogGC Log

The typical pause time is 2,43 seconds. Recall that in pauses, the appliance is unresponsive. Lengthy delays don’t should be a problem for a streaming utility as a result of it doesn’t have to reply to shoppers’ requests. The stunning half is its throughput of 69%, which signifies that the appliance spends 31% of its time wiping out reminiscence. That’s 31% not being spent on area logic. Ideally, the throughput needs to be a minimum of 95%.

Figuring out the dwell information dimension

Allow us to see if we are able to make this higher. We decide the LDS by triggering a full rubbish acquire whereas the appliance is underneath load. Our utility was performing so dangerous that it already carried out full collects – this sometimes signifies that the GC is in bother. On the intense aspect, we do not have to set off a full acquire manually to determine the LDS.

We distilled that the max heap dimension after a full GC is roughly 630MB. Making use of our rule of thumb yields a heap of 630 / 0.3 = 2100MB. That’s virtually twice the dimensions of our present heap of 1135MB!


Inquisitive about what this could do to our utility, we elevated the heap to 2100MB and fired up our efficiency assessments as soon as extra. The outcomes excited us.

GC LogGC Log

After rising the heap, the common GC pauses decreased quite a bit. Additionally, the GC’s throughput improved dramatically – 99% of the time the appliance is doing what it’s meant to do. And the throughput of the appliance, you ask? Recall that earlier than, the appliance processed 200 parts per second at most. Now it peaks at 400 per second!

Grafana graph

Spicing up a high-load, low-latency REST service

Quiz query. You have got a low-latency, high-load service working on 42 digital machines, every having 2 CPU cores. Sometime, you migrate your utility nodes to 5 beasts of bodily servers, every having 32 CPU cores. Given that every digital machine had a heap of 2GB, what dimension ought to or not it’s for every bodily server?

So, you have to divide 42 * 2 = 84GB of whole reminiscence over 5 machines. That boils right down to 84 / 5 = 16.8GB per machine. To take no probabilities, you spherical this quantity as much as 25GB. Sounds believable, proper? Effectively, the proper reply seems to be lower than 2GB, as a result of that’s the quantity we acquired by calculating the heap dimension based mostly on the LDS. Can’t imagine it? No worries, we couldn’t imagine it both. Subsequently, we determined to run an experiment.

Experiment setup

We have now 5 utility nodes, so we are able to run our experiment with 5 differently-sized heaps. We give node one 2GB, node two 4GB, node three 8GB, node 4 12GB, and node 5 25GB. (Sure, we’re not courageous sufficient to run our utility with a heap underneath 2GB.)

As a subsequent step, we fireplace up our efficiency assessments producing a steady, production-like load of a baffling 56K requests per second. All through the entire run of this experiment, we measure the variety of requests every node receives to make sure that the load is equally balanced. What’s extra, we measure this service’s key efficiency indicator – latency.

As a result of we acquired weary of downloading the GC logs after every take a look at, we invested in Grafana dashboards to indicate us the GC’s pause occasions, throughput, and heap dimension after a rubbish acquire. This manner we are able to simply examine the GC’s well being.


This weblog is about GC tuning, so let’s begin with that. The next determine exhibits the GC’s pause occasions and throughput. Recall that pause occasions point out how lengthy the GC freezes the appliance whereas sweeping out reminiscence. Throughput then specifies the share of time the appliance shouldn’t be paused by the GC.

2 Grafana graphs

As you’ll be able to see, the pause frequency and pause occasions don’t differ a lot. The throughput exhibits it finest: the smaller the heap, the extra the GC pauses. It additionally exhibits that even with a 2GB heap the throughput remains to be OK – it doesn’t drop underneath 98%. (Recall {that a} throughput greater than 95% is taken into account good.)

So, rising a 2GB heap by 23GB will increase the throughput by virtually 2%. That makes us surprise, how important is that for the general utility’s efficiency? For the reply, we have to have a look at the appliance’s latency.

If we have a look at the 99-percentile latency of every node – as proven within the under graph – we see that the response occasions are actually shut.

Grafana graph

Even when we think about the 999-percentile, the response occasions of every node are nonetheless not very far aside, as the next graph exhibits.

Grafana graph

How does the drop of virtually 2% in GC throughput have an effect on our utility’s total efficiency? Not a lot. And that’s nice as a result of it means two issues. First, the straightforward recipe for GC tuning labored once more. Second, we simply saved a whopping 115GB of reminiscence!


We defined a easy recipe of GC tuning that served two purposes. By rising the heap, we gained two occasions higher throughput for a streaming utility. We diminished the reminiscence footprint of a REST service greater than ten-fold with out compromising its latency. All of that we achieved by following these steps:
• Run the appliance underneath load.
• Decide the dwell information dimension (the dimensions of the objects your utility nonetheless makes use of).
• Measurement the heap such that the LDS takes 30% of the overall heap dimension.

Hopefully, we satisfied you that GC tuning does not should be daunting. So, carry your personal elements and begin cooking. We hope the outcome can be as spicy as ours.


Many because of Alexander Bolhuis, Ramin Gomari, Tomas Sirio and Deny Rubinskyi for serving to us run the experiments. We couldn’t have written this weblog submit with out you guys.