Enable server SSH pool by default

mkdocs/docs/guides/server-deployment.md

@@ -5,7 +5,10 @@ description: Deploying the dstack server
 
 The `dstack` server can run on your laptop or any environment with access to the cloud and on-prem clusters you plan to use.
 
-The minimum hardware requirements for running the server are 1 CPU and 1GB of RAM.
+??? info "Hardware requirements"
+    The minimum hardware requirements for running the server are 1 CPU and 1GB of RAM. The recommended RAM is
+    "8MB × number of active instances". For example, a server with 1000 active instances should have 8GB of RAM.
+    You can set the `DSTACK_SERVER_SSH_POOL_DISABLED` env var to minimize RAM usage at the expense of slower processing.
 
 === "pip"
     > The server can be set up via `pip` on Linux, macOS, and Windows (via WSL 2). It requires Git and OpenSSH.
@@ -43,7 +46,7 @@ The minimum hardware requirements for running the server are 1 CPU and 1GB of RA
     </div>
 
 === "Docker"
-     > To deploy the server most reliably, it's recommended to use `dstackai/dstack` Docker image.
+     > For production deployments, it's recommended to use `dstackai/dstack` Docker image.
 
     <div class="termy">
 

mkdocs/docs/reference/env.md

@@ -146,6 +146,8 @@ For more details on the options below, refer to the [server deployment](../guide
 - `DSTACK_SERVER_SSHPROXY_ENFORCED`{ #DSTACK_SERVER_SSHPROXY_ENFORCED } – When set to any value, restricts all SSH connections to go through the SSH proxy.
 - `DSTACK_SERVER_JOB_NETWORK_MODE`{ #DSTACK_SERVER_JOB_NETWORK_MODE } – Controls the network mode assigned to jobs. Accepts an integer value: `1` forces bridge networking for single-node jobs while distributed tasks still use host networking; `2` uses host networking whenever the job occupies a full instance (default); `3` forces bridge networking for all jobs including distributed tasks.
 - `DSTACK_SERVER_SSH_CONNECT_TIMEOUT`{ #DSTACK_SERVER_SSH_CONNECT_TIMEOUT } – The SSH `ConnectTimeout` for server-instance connections, in seconds. Defaults to `3`. Increase if there are high-latency links between the server and instances.
+- `DSTACK_SERVER_SSH_POOL_DISABLED`{ #DSTACK_SERVER_SSH_POOL_DISABLED } – Disables the reuse of server SSH connections to instances. If set, significantly decreases server RAM usage, but
+slows down processing and may cause CPU spikes due to frequent SSH-connection establishment.
 
 ??? info "Internal environment variables"
      The following environment variables are intended for development purposes:

src/dstack/_internal/server/app.py

@@ -171,6 +171,8 @@ async def lifespan(app: FastAPI):
         init_default_storage()
     if settings.SERVER_SSH_POOL_ENABLED:
         await run_async(instance_connection_pool.startup_cleanup)
+    else:
+        logger.info("Server SSH pool is disabled")
     scheduler = None
     pipeline_manager = None
     if settings.SERVER_BACKGROUND_PROCESSING_ENABLED:

src/dstack/_internal/server/settings.py

@@ -149,9 +149,8 @@
     os.getenv("DSTACK_SERVER_LOG_QUOTA_PER_JOB_HOUR", 50 * 1024 * 1024)  # 50 MB
 )
 
-# TODO: Replace DSTACK_SERVER_SSH_POOL_ENABLED with DSTACK_SERVER_SSH_POOL_DISABLED
-# as pool becomes opt-out and document the env var.
-SERVER_SSH_POOL_ENABLED = os.getenv("DSTACK_SERVER_SSH_POOL_ENABLED") is not None
+SERVER_SSH_POOL_DISABLED = os.getenv("DSTACK_SERVER_SSH_POOL_DISABLED") is not None
+SERVER_SSH_POOL_ENABLED = not SERVER_SSH_POOL_DISABLED
 SERVER_SSH_CONNECT_TIMEOUT = int(os.getenv("DSTACK_SERVER_SSH_CONNECT_TIMEOUT", 3))
 
 # Development settings