Limit threads per zenoh session

Signed-off-by: Yadunund <[email protected]>

rmw_zenoh_cpp/config/DEFAULT_RMW_ZENOH_SESSION_CONFIG.json5

@@ -185,6 +185,9 @@
           /// Higher values lead to a more aggressive batching but it will introduce additional latency.
           backoff: 100,
         },
+        // Number of threads dedicated to transmission
+        // By default, the number of threads is calculated as follows: 1 + ((#cores - 1) / 4)
+        threads: 1,
       },
       /// Configure the zenoh RX parameters of a link
       rx: {

zenoh_c_vendor/CMakeLists.txt

@@ -10,9 +10,24 @@ endif()
 find_package(ament_cmake REQUIRED)
 find_package(ament_cmake_vendor_package REQUIRED)
 
+# Disable default features and only enable tcp transport for zenoh. This reduces
+# build time but more importantly allows us to set the number of threads that
+# the zenoh session can spin via session config as well as ASYNC_STD_THREAD_COUNT
+# envar. Without this limit, applications with multiple zenoh sessions can
+# encounter to system resource limits when trying to create new threads.
+# Once zenoh migrates to relying on tokio for async runtime, we can consider
+# removing these flags since tokio allows better control over threads spawned with
+# the help of thread pools.
+# Note: We separate the two args needed for cargo with "$<SEMICOLON>" and not ";" as the
+# latter is a list separater in cmake and hence the string will be split into two
+# when expanded.
+set(ZENOHC_CARGO_FLAGS "--no-default-features$<SEMICOLON>--features=zenoh/transport_tcp")
+
 ament_vendor(zenoh_c_vendor
   VCS_URL https://github.com/eclipse-zenoh/zenoh-c.git
   VCS_VERSION 0.10.1-rc
+  CMAKE_ARGS
+    "-DZENOHC_CARGO_FLAGS=${ZENOHC_CARGO_FLAGS}"
 )
 
 # set(INSTALL_DIR "${CMAKE_CURRENT_BINARY_DIR}/${PROJECT_NAME}-prefix/install")