Doc: redis memory management / automatic flushing. (#2344)

* Doc: redis memory management / automatic flushing.

* Address comments

* Update redis-memory-management.rst

* Change cross ref style

doc/source/index.rst

@@ -115,6 +115,7 @@ Ray comes with libraries that accelerate deep learning and reinforcement learnin
    fault-tolerance.rst
    plasma-object-store.rst
    resources.rst
+   redis-memory-management.rst
 
 .. toctree::
    :maxdepth: 1

doc/source/redis-memory-management.rst

@@ -0,0 +1,98 @@
+Redis Memory Management (EXPERIMENTAL)
+======================================
+
+Ray stores metadata associated with tasks and objects in one or more Redis
+servers, as described in `An Overview of the Internals
+<internals-overview.html>`_.  Applications that are long-running or have high
+task/object generation rate could risk high memory pressure, potentially leading
+to out-of-memory (OOM) errors.
+
+Here, we describe an experimental feature that transparently flushes metadata
+entries out of Redis memory.
+
+Requirements
+------------
+
+As of early July 2018, the automatic memory management feature requires building
+Ray from source.  We are planning on eliminating this step in the near future by
+releasing official wheels.
+
+Building Ray
+~~~~~~~~~~~~
+
+First, follow `instructions to build Ray from source
+<installation.html#building-ray-from-source>`__ to install prerequisites.  After
+the prerequisites are installed, instead of doing the regular ``pip install`` as
+referenced in that document, pass an additional special flag,
+``RAY_USE_NEW_GCS=on``:
+
+.. code-block:: bash
+
+  git clone https://github.com/ray-project/ray.git
+  cd ray/python
+  RAY_USE_NEW_GCS=on pip install -e . --verbose  # Add --user if you see a permission denied error.
+
+Running Ray applications
+~~~~~~~~~~~~~~~~~~~~~~~~
+
+At run time the environment variables ``RAY_USE_NEW_GCS=on`` and
+``RAY_USE_XRAY=1`` are required.
+
+.. code-block:: bash
+
+  export RAY_USE_NEW_GCS=on
+  export RAY_USE_XRAY=1
+  python my_ray_script.py  # Or launch python/ipython.
+
+Activate memory flushing
+------------------------
+
+After building Ray using the method above, simply add these two lines after
+``ray.init()`` to activate automatic memory flushing:
+
+.. code-block:: python
+
+   ray.init(...)
+
+   policy = ray.experimental.SimpleGcsFlushPolicy()
+   ray.experimental.set_flushing_policy(policy)
+
+   # My awesome Ray application logic follows.
+
+Paramaters of the flushing policy
+---------------------------------
+
+There are three `user-configurable parameters
+<https://github.com/ray-project/ray/blob/8190ff1fd0c4b82f73e2c1c0f21de6bda494718c/python/ray/experimental/gcs_flush_policy.py#L31>`_
+of the ``SimpleGcsFlushPolicy``:
+
+* ``flush_when_at_least_bytes``: Wait until this many bytes of memory usage
+  accumulated in the redis server before flushing kicks in.
+* ``flush_period_secs``: Issue a flush to the Redis server every this many
+  seconds.
+* ``flush_num_entries_each_time``: A hint to the system on the number of entries
+  to flush on each request.
+
+The default values should serve to be non-invasive for lightweight Ray
+applications. ``flush_when_at_least_bytes`` is set to ``(1<<31)`` or 2GB,
+``flush_period_secs`` to 10, and ``flush_num_entries_each_time`` to 10000:
+
+.. code-block:: python
+
+    # Default parameters.
+    ray.experimental.SimpleGcsFlushPolicy(
+        flush_when_at_least_bytes=(1 << 31),
+        flush_period_secs=10,
+        flush_num_entries_each_time=10000)
+
+In particular, these default values imply that
+
+1. the Redis server would accumulate memory usage up to 2GB without any entries
+being flushed, then the flushing would kick in; and
+
+2. generally, "older" metadata entries would be flushed first, and the Redis
+server would always keep the most recent window of metadata of 2GB in size.
+
+**For advanced users.** Advanced users can tune the above parameters to their
+applications' needs; note that the desired flush rate is equal to (flush
+period) * (num entries each flush).