While the extraordinarily large shipping container, Ever Given, ran aground in the Suez Canal, halting a major trade route that has caused losses in the billions, our solution engineers at Aspen Mesh have been stuck diagnosing a tricky Istio and Envoy performance bottleneck on their own island for the past few weeks. Though the scale and global impacts of these two problems is quite different, it has presented an interesting way to correlate a global shipping event with the metaphorical nautical themes used by Istio. To elaborate on this theme, let’s switch from containers carrying dairy, and apparently everything else under the sun, to containers shuttling network packets.

To unlock the most from containers and microservices architecture, Istio (and Aspen Mesh) uses a sidecar proxy model. Adding sidecar proxies into your mesh provides a host of benefits, from uniform identity to security to metrics and advanced traffic routing. As Aspen Mesh customers range from large enterprises all the way to service providers, the performance impacts of adding these sidecars is as important to us as the benefits outlined above. The performance experiment that I’m going to cover in this blog is geared toward evaluating the impact of adding sidecar proxies in high throughput scenarios on the server or client, or both sides.

We have encountered workloads, especially in the service provider space, where there are high requests or transactions-per-second requirements for a particular service. Also, scaling up — i.e., adding more CPU/memory — is preferable to scaling out. We wanted to test the limits of sidecar proxies with regards to the maximum achievable throughput so that we can tune and optimize our model to meet the performance requirements of the wide variety of workloads used by our customers.

Throughput Test Setup

The test setup we used for this experiment was rather simple: a Fortio client and server running on Kubernetes on large AWS node instance types like burstable t3.2xlarge with 8 vCPUs and 32 GB of memory or dedicated m5.8xlarge instance types which have 32 vCPUs and 128 GB of memory. The test was running a single instance of the Fortio client and server pod with no resource constraints on their own dedicated nodes. The Fortio client was run in a mode to maximize throughput like this:

The above command runs the test for 60 seconds with queries per second (QPS) 0 (i.e. maximum throughput with a varying number of simultaneous parallel connections). With this setup on a t3.2xlarge machine, we were able to achieve around 100,000 QPS. Further increasing the number of parallel connections didn’t result in throughput beyond ~100K QPS, signaling a possible CPU bottleneck. Running the same experiment on an m5.8xlarge instance, we could achieve much higher throughput around 300,000 QPS or higher depending upon the parallel connection settings.

This was sufficient proof of CPU throttling. As adding more CPUs increased the QPS, we felt that we had a reasonable baseline to start evaluating the effects of adding sidecar proxies in this setup.

Adding Sidecar Proxies on Both Ends

Next, with the same setup on t3.2xlarge instances, we added Istio sidecar proxies on both Fortio client and server pods with Aspen Mesh default settings; mTLS STRICT setting, access logging enabled and the default concurrency (worker threads) of 2. With these parameters, and running the same command as before, we could only get a maximum throughput of around ~10,000 QPS.

This is a factor of 10 reduction in throughput. This was expected as we had only configured two worker threads, which were hopefully running at their maximum capacity but could not keep up with client load.

So, the logical next step for us was to increase the concurrency setting to run more worker threads to accept more connections and achieve higher throughput. In Istio and Aspen Mesh, you can set the proxy concurrency globally via the concurrency setting in proxy config under mesh config or override them via pod annotations like this:

Note that using the value “0” for concurrency configures it to use all the available cores on the machine. We increased the concurrency setting from two to four to six and saw a steady increase in maximum throughput from 10K QPS to ~15K QPS to ~20K QPS as expected. However, these numbers were still quite low (by a factor of five) as compared to the results with no sidecar proxies.

To eliminate the CPU throttling factor, we ran the same experiment on m5.8xlarge instances with even higher concurrency settings but the maximum throughput we could achieve was still around ~20,000 QPS.

This degradation was far from acceptable, so we dug into why the throughput was low even with sufficient worker threads configured on the sidecar proxies.

Peeling the Onion

To investigate this issue, we looked at the CPU utilization metrics in the server pod and noticed that the CPU utilization as a percentage of total requested CPUs was not very high. This seemed odd as we expected the proxy worker threads to be spinning as fast as possible to achieve the maximum throughput, so we needed to investigate further to understand the root cause.

To get a better understanding of low CPU utilization, we inspected the connections received by the server sidecar proxy. Envoy’s concurrency model relies on the kernel to distribute connections between the different worker threads listening on the same socket. This means that if the number of connections received at the server sidecar proxy is less than the number of worker threads, you can never fully use all CPUs.

As this investigation was purely on the server-side, we ran the above experiment again with the Fortio client pod, but this time without the sidecar proxy injected and only the Fortio server pod with the proxy injected. We found that the maximum throughput was still limited to around ~20K QPS as before, thereby hinting at issues on the server sidecar proxy.

To investigate further, we had to look at connection level metrics reported by Envoy proxy. Later in this article, we’ll see what happens to this experiment with Envoy metrics exposed. (By default, Istio and Aspen Mesh don’t expose the connection-level metrics from Envoy.)

These metrics can be enabled in Istio version 1.8 and above by following this guide and adding the appropriate pod annotations corresponding to the metrics you want to be exposed. Envoy has many low-level metrics emitted at high resolution that can easily overwhelm your metrics backend for a moderately sized cluster, so you should enable this cautiously in production environments.

Additionally, it can be quite a journey to find the right Envoy metrics to enable, so here’s what you will need to get connection-level metrics. On the server-side pod, add the following annotation:

This will enable reporting for all listeners configured by Istio, which can be a lot depending upon the number of services in your cluster, but only enable the downstream connections total counter and downstream connections active gauge metrics.

To look at these metrics, you can use your Prometheus dashboard, if it’s enabled, or port-forward to the server pod under test to port 15000 and navigate to http://localhost:15000/stats/prometheus. As there are many listeners configured by Istio, it can be tricky to find the correct one. Here’s a quick primer on how Istio sets up Envoy configuration. (You can find the complete list of Envoy listener metrics here.)

For any inbound connections to a pod from clients outside of the pod, Istio configures a virtual inbound listener at, which receives all the traffic from iptables’ redirect rules. This is the only listener that’s actually configured to receive connections from the kernel, and after the connection is received, it is matched against filter chain attributes to proxy the traffic to the correct application port on localhost. This means that even though the Fortio client above is targeting port 8080, we need to look at the total and active connections for the virtual inbound listener at instead of Looking at this metric, we found that the number of active connections were close to the configured number of simultaneous connections on the Fortio client side. This invalidated our theory about the number of connections being less than worker threads.

The next step in our debugging journey was to look at the number of connections received on each worker thread. As I had alluded to earlier, Envoy relies on the kernel to distribute the accepted connections to different worker threads, and for all the worker threads to be fully utilizing the allotted CPUs, the connections also need to be fairly balanced. Luckily, Envoy has per-worker metrics for listeners that can be enabled to understand the distribution. Since these metrics are rooted at listener.<address>.<handler>.<metric name>, the regex provided in the annotation above should also expose these metrics. The per-worker metrics looked like this:

As you can see from the above image, the connections were far from being evenly distributed among the worker threads. One thread, worker 10, had 11.5K active connections as compared to some threads which had around ~1-1.5K active connections, and others were even lower. This explains the low CPU utilization numbers as most of the worker threads just didn’t have enough connections to do useful work.

In our Envoy research, we quickly stumbled upon this issue, which very nicely sums up the problem and the various efforts that have been made to fix it.

Image via Pixabay.

So, next, we went looking for a solution to fix this problem. It seemed like, for the moment, our own Ever Given was stuck as some diligent worker threads struggled to find balance. We needed an excavator to start digging.

While our intrepid team tackled the problem of scaling for high-throughput workloads by adding sidecar proxies, we encountered a bottleneck not entirely unlike what the Ever Given experienced not long ago in the Suez Canal.

Luckily, we had a few more things to try, and we were ready to take a closer look at the listener metrics.

Let There Be Equality Among Threads!

After parsing through the conversations in the issue, we found the pull request that enabled a configuration option to turn on a feature to achieve better balancing across worker threads. At this point, trying this out seemed worthwhile, so we looked at how to enable this in Istio. (Note that as part of this PR, the per-worker thread metrics were added, which was useful in diagnosing this problem.)

For all the ignoble things EnvoyFilter can do in Istio, it’s useful in situations like these to quickly try out new Envoy configuration knobs without making code changes in “istiod” or the control plane. To turn the “exact balance” feature on, we created an EnvoyFilter resource like this:

With this configuration applied and with bated breath, we ran the experiment again and looked at the per-worker thread metrics. Voila! Look at the perfectly balanced connections in the image below:

Measuring the throughput with this configuration set, we could achieve around ~80,000 QPS, which is a significant improvement over the earlier results. Looking at CPU utilization, we saw that all the CPUs were fully pegged at or near 100%. This meant that we were finally seeing the CPU throttling. At this point, by adding more CPUs and a bigger machine, we could achieve much higher numbers as expected. So far so good.

As you may recall, this experiment was purely to test the effects of server sidecar proxy, so we removed the client sidecar proxy for these tests. It was now time to measure performance with both sidecars added.

Measuring the Impacts of a Client Sidecar Proxy

With this exact balancing configuration enabled on the inbound port (server side only), we ran the experiment with sidecars on both ends. We were hoping to achieve high throughputs that could only be limited by the number of CPUs dedicated to Envoy worked threads. If only things were that simple.

We found that the maximum throughput was once again capped at around ~20K QPS.

A bit disappointing, but since we then knew about the issue of connection imbalance on the server side, we reasoned that the same could happen on the client side between the application and the sidecar proxy container on localhost. First, we enabled the following metrics on the client-side proxy:

In addition to the listener metrics, we also enabled cluster-level metrics, which emit total and active connections for any upstream cluster. We wanted to verify that the client sidecar proxy was sending a sufficient number of connections to the upstream Fortio server cluster to keep the server worker threads occupied. We found that the number of active connections mirrored the number of connections used by the Fortio client in our command. This was a good sign. Note that Envoy doesn’t report cluster-level metrics at the per-worker level, but these are all aggregated, so there’s no way for us to know how the connections were distributed on the outbound side.

Next, we inspected the listener connection statistics on the client side similar to the server side to ensure that we were not having connection imbalance issues. The outbound listeners, or the listeners set up to handle traffic originating from the application in the same pod as the sidecar proxy, are set up a bit differently in Istio as compared to the inbound side. For outbound traffic, a virtual listener “” is created similar to the listener on “,” which is the target for iptables redirect rules. Unlike the inbound side, the virtual listener hands off the connection to the more specific listener like “” based on the original destination address. If there are no specific matches, then the listener configuration in the virtual outbound takes effect. This can block or allow all traffic depending on your configured outbound traffic policy. In the traffic flow from the Fortio client to server, we expected the listener at “” to be handling connections on the client-side proxy, so we inspected connections metrics at this listener. The listener metrics looked like this:

The above image shows the connection imbalance issue between worker threads as we saw it on the server side. However, the connections on the outbound client-side proxy were only getting handled by one worker thread which explains the poor throughput QPS numbers. Having fixed this on the server-side, we applied a similar EnvoyFilter configuration with minor tweaks for context and port to address this imbalance:

Surely, applying this resource would fix our issue and we would be able to achieve high QPS with both client and server sidecar proxies with sufficient CPUs allocated to them. Well, we ran the experiment again and saw no difference in the throughput numbers. Checking the listener metrics again, we saw that even with this EnvoyFilter resource applied, only one worker thread was handling all the connections. We also tried applying the exact balance config on both virtual outbound port 15001 and outbound port 8080, but the throughput was still limited to 20K QPS.

This warranted the next round of investigations.

Original Destination Listeners, Exact Balance Issues

We went around looking in Envoy code and opened Github issues to understand why the client-side exact balance configuration was not taking effect, while the server side was working wonders. The key difference between the two listeners, other than the directionality, was that the virtual outbound listener “” was an original destination listener, which hands over connections to other listeners matched on the original destination address. With help from the Istio community (thanks, Yuchen Dai from Google), we found this open issue, which explains this behavior in a rather cryptic way.

Basically, the current exact balance implementation relies on connection counters per worker thread to fix the imbalance. When the original destination is enabled on the virtual outbound listener, the connection counter on the worker thread is incremented when a connection is received, but as the connection is immediately handed to the more specific listener like “,” it is decremented again. This quick increase and decrease in the internal count spoofs the exact balancer into thinking the balance is perfect as all these counters are always at zero. It also appears that applying the exact balance on the listener that handles the connection, “” in this case, but doesn’t accept the connection from the kernel has no effect due to current implementation limitations.

Fortunately, the fix for this issue is in progress, and we’ll be working with the community to get this addressed as quickly as possible. In the meantime, if you’re getting hit by these performance issues on the client side, scaling out with a lower concurrency setting is a better approach to reach higher throughput QPS numbers than scaling up with higher concurrency and worker threads. We are also working with the Istio community to provide configuration knobs for enabling exact balance in Envoy to optionally switch default settings so that everyone can benefit from our findings.

Working on this performance analysis was interesting and a challenge in its own way, like the small tractor next to the giant ship trying to make it move.

Well, maybe not exactly, but it was a learning experience for me and my team, and I’m glad we are able to share our learnings with the rest of the community as this aspect of Istio is often overlooked by the broader vendor ecosystem. We will run and publish performance numbers related to the impact of turning on various features such as mTLS, access logging and tracing in high-throughout scenarios in future blogs, so if you’re interested in this topic, subscribe to our blog to get updates or reach out to us with any questions.

I would like to give a special mention to my team members Pawel and Bart who patiently and diligently ran various test scenarios, collected data and were uncompromising in their pursuit to get the last bit out of Istio and Aspen Mesh. It’s not surprising. After all, being part of F5, taking performance seriously is just part of our DNA.