datasets/README.md: document srun workflow, PYTHON var, container notes

Update the add_months and Step 6 sections with lessons learned from the
first run attempt:
- Replace salloc with srun (releases node automatically on completion)
- Document the PYTHON variable override needed for parallel/venv
- Note that .zst decompression uses the zstandard library due to
  Singularity container restrictions on the system zstd binary
- Add full srun invocation with bash -l, tee logging, and tmux guidance
- Update Step 6 walkthrough to use srun instead of salloc

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

datasets/README.md

@@ -73,6 +73,22 @@ read all layers together correctly. At a yearly update cadence the number
 of layers stays small; use `merge_layers.sh` to collapse them when
 needed.
 
+#### Environment setup
+
+The Python environment runs inside a Singularity container. Set `PYTHON`
+to the full path of the venv interpreter so that `parallel` jobs use the
+right Python (fresh shells spawned by `parallel` don't inherit the active
+venv):
+
+```sh
+PYTHON=/gscratch/comdata/users/makohill/cdsc_reddit/venv/bin/python3
+```
+
+The `.zst` decompression uses the `zstandard` Python library rather than
+the system `zstd` binary, which is inaccessible from inside the container.
+
+#### Dump directory
+
 The new `.zst` dump files must be accessible at `COMMENTS_DUMPDIR` and
 `SUBMISSIONS_DUMPDIR`. Override the defaults (which match `dumps_helper.py`)
 via environment variables if the files are not in the standard locations:
@@ -80,18 +96,37 @@ via environment variables if the files are not in the standard locations:
 ```sh
 COMMENTS_DUMPDIR=/path/to/new/comments \
 SUBMISSIONS_DUMPDIR=/path/to/new/submissions \
-./add_months.sh 2025-01 2025-02 2025-03
 ```
 
-Part 1 runs directly on a compute node. For Part 2 there are two options:
+#### Running as a Slurm job
 
-- **Single fat node** (simpler, often faster for smaller sorts): `salloc`
-  a `cpu-g2` node (128 cores, ~1 TB RAM) and run the Part 2 script
-  directly with `spark-submit` or `python3`. See Step 6 of the walkthrough
-  below for the `salloc` invocation.
-- **Multi-node Spark cluster**: use `start_spark_and_run.sh` from a login
-  node. It allocates nodes via `salloc` and handles cluster coordination.
-  Pass the number of nodes as the first argument.
+The recommended way to run `add_months.sh` is via `srun` on a fat
+`cpu-g2` node. Using `srun` (rather than `salloc`) means the node is
+released automatically as soon as the script finishes, regardless of the
+walltime. Run from a login node inside a `tmux` session so the terminal
+survives disconnections:
+
+```sh
+tmux new -s add_months
+
+srun -p cpu-g2 -A comdata --nodes=1 --time=72:00:00 -c 112 --mem=400G \
+    bash -l -c "
+    cd /mmfs1/gscratch/comdata/users/makohill/cdsc_reddit && \
+    PYTHON=/gscratch/comdata/users/makohill/cdsc_reddit/venv/bin/python3 \
+    COMMENTS_DUMPDIR=/path/to/new/comments \
+    SUBMISSIONS_DUMPDIR=/path/to/new/submissions \
+    ./datasets/add_months.sh 2025-01 2025-02 ... YYYY-MM
+    " 2>&1 | tee /gscratch/comdata/users/makohill/add_months_run.log
+```
+
+The `bash -l` flag sources `.bashrc` on the compute node so the Spark
+environment is available. The `tee` command writes output to both the
+terminal and a log file so you can review it later.
+
+Detach from tmux with `Ctrl-b d` and reattach with `tmux attach -t add_months`.
+
+For a multi-node Spark cluster instead, use `add_months_multinode.sh`
+from a login node — it takes the number of nodes as its first argument.
 
 ### Merge layers — `merge_layers.sh`
 
@@ -296,25 +331,25 @@ continuous with the most recent data we already have.
 ### Step 6: Part 2 — sorting the `.parquet` files by author and subreddit via Spark
 
 If the `.parquet` files reasonably appear to be complete, we can now
-sort them by author and subreddit. The most efficient way to do so is by
-using one node on `cpu-g2` with 128 CPUs and 994G memory. This one node
-splits into up to six slices (four in our current case) so the tasks
-will still be parallelized (`hyakalloc` or [this Hyak blog][hyak-blog]
-are good resources for further information). Run `tmux` on a login
-node, then grab the whole node for up to a week with:
+sort them by author and subreddit. The most efficient way to do so is via
+`srun` on a `cpu-g2` node (128 CPUs, ~1 TB RAM). Using `srun` releases
+the node automatically when the job finishes. Run from a login node
+inside `tmux`:
 
 ```sh
-salloc -p cpu-g2 -A comdata --nodes=1 --time=168:00:00 -c 128 --mem=994G
+srun -p cpu-g2 -A comdata --nodes=1 --time=72:00:00 -c 112 --mem=400G \
+    bash -l -c "
+    cd /path/to/cdsc_reddit/datasets && \
+    source \$SPARK_CONF_DIR/spark-env.sh && \
+    start_spark_cluster.sh && \
+    spark-submit --master spark://\$(hostname):\$SPARK_MASTER_PORT submissions_part2.py && \
+    spark-submit --master spark://\$(hostname):\$SPARK_MASTER_PORT comments_part2.py && \
+    stop-all.sh
+    "
 ```
 
 [hyak-blog]: https://hyak.uw.edu/blog/g1-vs-g2/
 
-Once Slurm drops you onto the compute node, run
-
-```sh
-./start_spark_and_run.sh submissions_part2.py
-```
-
 Monitor via `htop` (as described in Step 4); the CPUs may not always
 show high usage but you should see that memory is being used. Repeat
 for the comments. Successful jobs will result in