Configuring Proxy Concurrency
The Linkerd data plane’s proxies are multithreaded, and are capable of running a variable number of worker threads so that their resource usage matches the application workload.
In a vacuum, of course, proxies will exhibit the best throughput and lowest latency when allowed to use as many CPU cores as possible. However, in practice, there are other considerations to take into account.
A real world deployment is not a load test where clients and servers perform no other work beyond saturating the proxy with requests. Instead, the service mesh model has proxy instances deployed as sidecars to application containers. Each proxy only handles traffic to and from the pod it is injected into. This means that throughput and latency are limited by the application workload. If an application container instance can only handle so many requests per second, it may not actually matter that the proxy could handle more. In fact, giving the proxy more CPU cores than it requires to keep up with the application may harm overall performance, as the application may have to compete with the proxy for finite system resources.
Therefore, it is more important for individual proxies to handle their traffic efficiently than to configure all proxies to handle the maximum possible load. The primary method of tuning proxy resource usage is limiting the number of worker threads used by the proxy to forward traffic. There are multiple methods for doing this.
The simplest way to configure the proxy’s thread pool is using the
config.linkerd.io/proxy-cpu-limit annotation. This annotation configures the
proxy injector to set an environment variable that controls the number of CPU
cores the proxy will use.
When installing Linkerd using the
linkerd install CLI
--proxy-cpu-limit argument sets this
annotation globally for all proxies injected by the Linkerd installation. For
# first, install the Linkerd CRDs linkerd install --crds | kubectl apply -f - # install Linkerd, with a proxy CPU limit configured. linkerd install --proxy-cpu-limit 2 | kubectl apply -f -
For more fine-grained configuration, the annotation may be added to any injectable Kubernetes resource, such as a namespace, pod, or deployment.
For example, the following will configure any proxies in the
deployment to use two CPU cores:
kind: Deployment apiVersion: apps/v1 metadata: name: my-deployment # ... spec: template: metadata: annotations: config.linkerd.io/proxy-cpu-limit: '1' # ...
Using Kubernetes CPU Limits and Requests
CPU limits and CPU requests
to configure the resources assigned to any pod or container. These may also be
used to configure the Linkerd proxy’s CPU usage. However, depending on how the
kubelet is configured, using Kubernetes resource limits rather than the
proxy-cpu-limit annotation may not be ideal.
The kubelet uses one of two mechanisms for enforcing pod CPU limits. This is
determined by the
--cpu-manager-policy kubelet option.
With the default CPU manager policy,
the kubelet uses
CFS quotas to enforce
CPU limits. This means that the Linux kernel is configured to limit the amount
of time threads belonging to a given process are scheduled. Alternatively, the
CPU manager policy may be set to
In this case, the kubelet will use Linux
cgroups to enforce CPU limits for
containers which meet certain criteria.
When the environment variable configured by the
proxy-cpu-limit annotation is
unset, the proxy will run a number of worker threads equal to the number of CPU
cores available. This means that with the default
none CPU manager policy, the
proxy may spawn a large number of worker threads, but the Linux kernel will
limit how often they are scheduled. This is less efficient than simply reducing
the number of worker threads, as
proxy-cpu-limit does: more time is spent on
context switches, and each worker thread will run less frequently, potentially
On the other hand, using
will limit the number of CPU cores available to the process. In essence, it will
appear to the proxy that the system has fewer CPU cores than it actually does.
This will result in similar behavior to the
However, it’s worth noting that in order for this mechanism to be used, certain criteria must be met:
- The kubelet must be configured with the
staticCPU manager policy
- The pod must be in the Guaranteed QoS class. This means that all containers in the pod must have both a limit and a request for memory and CPU, and the limit for each must have the same value as the request.
- The CPU limit and CPU request must be an integer greater than or equal to 1.
If you’re not sure whether these criteria will all be met, it’s best to use the
proxy-cpu-limit annotation in addition to any Kubernetes CPU limits and
When using Helm, users must take care to set the
proxy.cores Helm variable in addition to
the criteria for cgroup-based CPU limits
described above are not met.