When XDS config is converted from protobuf into C++ data structures, those data structures are not always as memory efficient as the protobufs, leading to large memory consumption.

Optional sub-messages are often stored as full blown objects, even when the config for them isn't present.

An example of that is RetryPolicy in RDS:

• Route holds an instance of RetryPolicyImpl class:

  envoy/source/common/router/config_impl.h

  Line 1012 in aa69443

  const RetryPolicyImpl retry_policy_;

• VitrualHost holds an optional instance of the RetryPolicy protobuf:

  envoy/source/common/router/config_impl.h

  Line 316 in aa69443

  absl::optional<envoy::config::route::v3::RetryPolicy> retry_policy_;

Each of them are 200+ bytes.

Another example are HeaderParsers. There are 2 (request/response) in each Route and each VirtualHost. Even if no headers are present in the config, RDS ends up creating HeaderParser instances:

A better approach would be to keep a unique_ptr member and only initialize it when config is present. If not, a reference to a default ConstSingleton object can be used.