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Using Cloud TPUs

Setting up Colab with a TPU Pod backend

Colab requires a backend (a.k.a., runtime) and a frontend. In this setup, we are going to start a Jupyter backend on Cloud VMs with acesss to TPUs and use the existing frontend colab.research.google.com.

The main challenge of the setup is to get colab.research.google.com to connect to the Jupyter VM over the network. This is a challenge because of Cloud security measures including these:

  1. colab.research.google.com doesn't connect to arbitrary machines.
  2. Jupyter by default allows only local connections.
  3. Cloud firewalls block all but a few ports by default.

Rather than undoing these measures, in this setup we are going to have Jupyter listen on port 8888 and establish an ssh tunnel for 8888 all the way from the machine running the browser to Jupyter.

  1. Using a machine where you have installed gcloud, create the TPU pod. The example below creates a pod of 4 TPUv3 hosts for a total of 32 TPUs. You can also use the cloud console for this step and for figuring out what TPU topologies are available for your project.

    NOTE: The example below sets --xla_jf_conv_full_precision to true which forces TPU operations (convolutions and dot products) to use the highest float precision. To use low precision floats replace the TPU_FLAGS line below with TPU_FLAGS=--xla_jf_conv_full_precision=false.

    For an explanation of how TPU_FLAGS works see the section Setting libtpu.so flags on pod TPUs.

    PROJECT=<YOUR_PROJECT_ID>
TPU=<A_UNIQUE_TPU_VM_NAME>
ZONE=europe-west4-a
TYPE=v3-32
TPU_FLAGS=--xla_jf_conv_full_precision=true

gcloud compute tpus tpu-vm create $TPU --project=$PROJECT --zone=$ZONE \
--accelerator-type=$TYPE --version=tpu-vm-tf-2.11.0-pod \
--metadata="tensorflow-env-vars=-e LIBTPU_INIT_ARGS=$TPU_FLAGS"

    Same example using the Cloud console:

    Cloud console

  2. ssh to a VM in the pod and tunnel port 8888. By default this command connects to the first worker.

    gcloud alpha compute tpus tpu-vm ssh --zone=$ZONE --project=$PROJECT $TPU \
  -- -L 8888:localhost:8888

  3. In the worker VM, install and set up Jupyter along with a few common python packages.

    pip install jupyter jupyter_http_over_ws matplotlib scipy

# This allows the colab frontend to talk to Jupyter.
.local/bin/jupyter serverextension enable --py jupyter_http_over_ws

# This stops the main process from seeing the local TPUs - we want the
# local TPUs to be accessed as part of the pod.
export TPU_LOAD_LIBRARY=0

  4. Start Jupyter.

    .local/bin/jupyter notebook \
--NotebookApp.allow_origin='https://colab.research.google.com' \
--port=8888 \
--NotebookApp.port_retries=0

  5. When Jupyter starts it will print out a localhost URL with a security token. Copy this URL.

    Jupyter URL

    Then use it to connect to a "local" runtime in colab.research.google.com.

    Colab Local Runtime

  6. When you are done, delete the TPU pod. Unlike non-pod VMs, it is not possible to stop the VMs and later start them back again, so deleting is the only option. Run this command back on your workstation.

    gcloud compute tpus tpu-vm delete $TPU --zone=$ZONE --project=$PROJECT

Configuring TPU libraries via flags

libtpu.so

On Cloud, code that interacts with TPU hardware is linked into binaries as a dynamically-loaded library, named libtpu.so.

One implication of loading libtpu.so dynamically is that flags that are defined in libtpu.so are not available to the main process. The startup code of the main process that deals with command line arguments is not aware of the flags defined in libtpu.so and cannot initialize them.

TensorFlow has a workaround for this issue. The code that loads libtpu.so uses the value of the environment variable LIBTPU_INIT_ARGS to initialize the flags defined libtpu.so.

Setting libtpu.so flags on non-pod TPUs

When you run a TensorFlow binary on non-pod TPUs (e.g., <=8 TPUv3 TPUs), the main process loads libtpu.so, so you can set libtpu.so flags via the environment variable LIBTPU_INIT_ARGS. For example, to run main.py and with the flag --xla_jf_conv_full_precision set to true:

LIBTPU_INIT_ARGS=--xla_jf_conv_full_precision=true python3 main.py

libtpu.so in TPU Pods

In a pod configuration, the TPU devices are connected to multiple machines (e.g., 4 machines for 32 TPUv3 TPUs). On Cloud, these TPUs are managed by a separate process, tpu-runtime, that is automatically started when the machine starts. The main process communicates with tpu-runtime via gRPC.

In this configuration, the main process does not load libtpu.so, but tpu-runtime does, since it is tpu-runtime that interacts with the TPU hardware.

tpu-runtime and libtpu.so in pod

Because it is now tpu-runtime that loads libtpu.so, it is also tpu-runtime that reads the environment variable LIBTPU_INIT_ARGS to initialize libtpu.so's flags. In fact tpu-runtime goes through the exact same steps as the main process in the non-pod configuration.

This presents a new problem because tpu-runtime is started automatically at boot up before the main TensorFlow process starts. So the main process has no way to control the environment in which tpu-runtime runs.

But TensorFlow has a workaround for this issue, too. The startup script for tpu-runtime reads tensorflow-env-vars, a metadata variable from the machine's metadata configuration, and sets environment variables for tpu-runtime.

Setting libtpu.so flags on pod TPUs

You can set tensorflow-env-vars as a metadata variable while creating the TPU pod. For example, to set xla_jf_conv_full_precision to true:

gcloud compute tpus tpu-vm create $TPU --project=$PROJECT --zone=$ZONE \
  --accelerator-type=v3-32 --version=tpu-vm-tf-2.11.0-pod \
  --metadata="tensorflow-env-vars=-e LIBTPU_INIT_ARGS=--xla_jf_conv_full_precision=true"

If you've already created the TPU pod and want to change some libtpu.so flag, you need to update tensorflow-env-vars and then restart tpu-runtime on all the machines.

  1. First set tensorflow-env-vars:

    curl -X PATCH \
  -H "Authorization: Bearer $(gcloud auth print-access-token)" \
  -H "Content-Type: application/json" \
  -d "{metadata: {'tensorflow-env-vars': '-e LIBTPU_INIT_ARGS=--xla_jf_conv_full_precision=true'}}" \
  https://tpu.googleapis.com/v2/projects/$PROJECT/locations/$ZONE/nodes/$TPU?updateMask=metadata

  2. Then restart tpu-runtime:

    gcloud alpha compute tpus tpu-vm ssh $TPU --zone=$ZONE --project=$PROJECT \
  --worker=all --command="sudo systemctl restart tpu-runtime"