Check out our new GPU Glossary! Read now

Train an SLM from scratch with early-stopping grid search over hyperparameters

Split-Panel Image. Left: AI generated picture of Shakespeare. Right: SLM generated text

When you want a language model that performs well on your task, there are three options, ordered by the degree of customization:

  • Prompt Engineering: large and capable language models understand tasks in natural language, so you can carefully design a natural language “prompt” to elicit the desired behavior.

  • Fine-Tuning: those same language models were trained by gradient descent on data sets representing tasks, and they can be further trained by gradient descent on data sets representative of your task.

  • Training from Scratch: if you have enough data for your task, you can throw the pretrained model away and make your own.

Each step adds additional engineering complexity, but also leads to a superior cost-performance Pareto frontier for your tasks. Fine-tuned models at one-tenth the size regularly outperform more generic models, and models trained from scratch outperform them.

Because these models are so much smaller than the Large Language Models that power generic assistant chatbots like ChatGPT and Claude, they are often called Small Language Models (SLMs).

In this example, we will explore training an SLM from scratch on Modal.

In fact, we’ll train 8 SLMs in parallel with different hyperparameters and then select the best one for additional training.

We’ll monitor this training live and serve our training and trained models as web endpoints and simple browser UIs.

Along the way we’ll use many features of the Modal platform: distributed volumes, multiple web endpoints, and parallel container execution.

Together, these features give every machine learning and AI team the same infrastructural capabilities that the most sophisticated companies have in their internal platforms.

Basic Setup

import logging as L
import urllib.request
from dataclasses import dataclass
from pathlib import Path, PosixPath

import modal
from pydantic import BaseModel

MINUTES = 60  # seconds
HOURS = 60 * MINUTES

app_name = "example-hp-sweep-gpt"
app = modal.App(app_name)

We’ll use A10G GPUs for training, which are able to train the model to recognizably improved performance in ~15 minutes while keeping costs under ~$1.

gpu = "A10G"

Create a Volume to store data, weights, and logs

Since we’ll be coordinating training across multiple machines we’ll use a distributed Volume to store the data, checkpointed models, and TensorBoard logs.

volume = modal.Volume.from_name(
    "example-hp-sweep-gpt-volume", create_if_missing=True
)
volume_path = PosixPath("/vol/data")
model_filename = "nano_gpt_model.pt"
best_model_filename = "best_nano_gpt_model.pt"
tb_log_path = volume_path / "tb_logs"
model_save_path = volume_path / "models"

Define dependencies in container images

The container image for training is based on Modal’s default slim Debian Linux image with torch for defining and running our neural network and tensorboard for monitoring training.

base_image = modal.Image.debian_slim(python_version="3.11").pip_install(
    "pydantic==2.9.1"
)

torch_image = base_image.pip_install(
    "torch==2.1.2",
    "tensorboard==2.17.1",
    "numpy<2",
)

We also have some local dependencies that we’ll need to import into the remote environment. We add them into the remote container.

torch_image = torch_image.add_local_dir(
    Path(__file__).parent / "src", remote_path="/root/src"
)

We’ll serve a simple web endpoint:

web_image = base_image.pip_install(
    "fastapi[standard]==0.115.4", "starlette==0.41.2"
)

And we’ll deploy a web UI for interacting with our trained models using Gradio.

assets_path = Path(__file__).parent / "assets"
ui_image = web_image.pip_install("gradio~=4.44.0").add_local_dir(
    assets_path, remote_path="/assets"
)

We can also “pre-import” libraries that will be used by the functions we run on Modal in a given image using the with image.imports context manager.

with torch_image.imports():
    import glob
    import os
    from timeit import default_timer as timer

    import tensorboard
    import torch
    from src.dataset import Dataset
    from src.logs_manager import LogsManager
    from src.model import AttentionModel
    from src.tokenizer import Tokenizer

Running SLM training on Modal

Here we define the training function, wrapping it in a decorator that specifies the infrastructural parameters, like the container image we want to use, which volume to mount where, the gpu we’re using, and so on.

Training consists of specifying optimization parameters, loading the dataset, building the model, setting up TensorBoard logging & checkpointing, and then finally executing the training_loop itself.

@app.function(
    image=torch_image,
    volumes={volume_path: volume},
    gpu=gpu,
    timeout=1 * HOURS,
)
def train_model(
    node_rank,
    n_nodes,
    hparams,
    experiment_name,
    run_to_first_save=False,
    n_steps=3000,
    n_steps_before_eval=None,
    n_steps_before_checkpoint=None,
):
    # optimizer, data, and model prep
    batch_size = 64
    learning_rate = 3e-4

    n_eval_steps = 100
    if n_steps_before_eval is None:
        n_steps_before_eval = int(n_steps / 8)  # eval eight times per run
    if n_steps_before_checkpoint is None:
        n_steps_before_checkpoint = int(n_steps / 4)  # save four times per run

    train_percent = 0.9

    L.basicConfig(
        level=L.INFO,
        format=f"\033[0;32m%(asctime)s %(levelname)s [%(filename)s.%(funcName)s:%(lineno)d] [Node {node_rank+1}] %(message)s\033[0m",
        datefmt="%b %d %H:%M:%S",
    )

    # use GPU if available
    device = "cuda" if torch.cuda.is_available() else "cpu"
    L.info("Remote Device: %s // GPU: %s", device, gpu)

    input_file_path = volume_path / "shakespeare_char.txt"
    text = prepare_data(input_file_path, volume)

    # construct tokenizer & dataset
    tokenizer = Tokenizer(text)
    dataset = Dataset(
        tokenizer.encode(text),
        train_percent,
        batch_size,
        hparams.context_size,
        device,
    )

    # build the model
    model = build_model(hparams, tokenizer.vocab_size, device)
    num_parameters = sum(p.numel() for p in model.parameters())
    L.info(f"Num parameters: {num_parameters}")

    optimizer = setup_optimizer(model, learning_rate)

    # TensorBoard logging & checkpointing prep
    logs_manager = LogsManager(
        experiment_name, hparams, num_parameters, tb_log_path
    )
    L.info(f"Model name: {logs_manager.model_name}")

    model_save_dir = model_save_path / experiment_name / logs_manager.model_name
    if model_save_dir.exists():
        L.info("Loading model from checkpoint...")
        checkpoint = torch.load(str(model_save_dir / model_filename))
        is_best_model = not run_to_first_save
        if is_best_model:
            make_best_symbolic_link(
                model_save_dir, model_filename, experiment_name
            )
        model.load_state_dict(checkpoint["model"])
        start_step = checkpoint["steps"] + 1
    else:
        model_save_dir.mkdir(parents=True, exist_ok=True)
        start_step = 0
        checkpoint = init_checkpoint(
            model, tokenizer, optimizer, start_step, hparams
        )

    checkpoint_path = model_save_dir / model_filename

    out = training_loop(
        start_step,
        n_steps,
        n_steps_before_eval,
        n_steps_before_checkpoint,
        n_eval_steps,
        dataset,
        tokenizer,
        model,
        optimizer,
        logs_manager,
        checkpoint,
        checkpoint_path,
        run_to_first_save,
    )

    return node_rank, float(out["val"]), hparams

Launch a hyperparameter sweep from a local_entrypoint

The main entry point coordinates the hyperparameter optimization. First we specify the default hyperparameters for the model, taken from Andrej Karpathy’s walkthrough. For better performance, you can increase the context_size and scale up the GPU accordingly.

@dataclass
class ModelHyperparameters:
    n_heads: int = 6
    n_embed: int = 384
    n_blocks: int = 6
    context_size: int = 256
    dropout: float = 0.2

Next we define the local entrypoint: the code we run locally to coordinate training.

It will train 8 models in parallel across 8 containers, each with different hyperparameters, varying the number of heads (n_heads), the context_size (called the “block size” by Karpathy), and the dropout rate (dropout). To run in parallel we need to use the starmap method.

We train all of the models until the first checkpoint and then stop early so we can compare the validation losses.

Then we restart training for the best model and train it to completion.

You can kick off training with the following command:

modal run 06_gpu_and_ml.hyperparameter-sweep.hp_sweep_gpt

The output will look something like this:

Sep 16 21:20:39 INFO [hp_sweep_gpt.py.train_model:127] [Node 1]  Remote Device: cuda // GPU: A10G
Sep 16 21:20:40 INFO [hp_sweep_gpt.py.train_model:149] [Node 1]  Num parameters: 10693697
Sep 16 21:20:40 INFO [hp_sweep_gpt.py.train_model:156] [Node 1]  Model Name: E2024-0916-142031.618259_context_size=8_n_heads=1_dropout=0.1
Sep 16 21:20:41 INFO [hp_sweep_gpt.py.train_model:225] [Node 1]      0) //  1.03s // Train Loss: 3.58 // Val Loss: 3.60
Sep 16 21:20:41 INFO [hp_sweep_gpt.py.train_model:127] [Node 2]  Remote Device: cuda // GPU: A10G
...

The local_entrypoint code is below. Note that the arguments to it can also be passed via the command line. Use --help for details.

@app.local_entrypoint()
def main(
    n_steps: int = 3000,
    n_steps_before_checkpoint: int = None,
    n_steps_before_eval: int = None,
):
    from datetime import datetime
    from itertools import product

    experiment_name = f"E{datetime.now().strftime('%Y-%m-%d-%H%M%S.%f')}"
    default_hparams = ModelHyperparameters()

    # build list of hyperparameters to train & validate
    nheads_options = (1, default_hparams.n_heads)
    context_size_options = (8, default_hparams.context_size)
    dropout_options = (0.1, default_hparams.dropout)

    hparams_list = [
        ModelHyperparameters(n_heads=h, context_size=c, dropout=d)
        for h, c, d in product(
            nheads_options, context_size_options, dropout_options
        )
    ]

    # run training for each hyperparameter setting
    results = []
    stop_early = True  # stop early so we can compare val losses
    print(f"Testing {len(hparams_list)} hyperparameter settings")
    n_nodes = len(hparams_list)
    static_params = (
        experiment_name,
        stop_early,
        n_steps,
        n_steps_before_eval,
        n_steps_before_checkpoint,
    )
    for result in train_model.starmap(
        [(i, n_nodes, h, *static_params) for i, h in enumerate(hparams_list)],
        order_outputs=False,
    ):
        # result = (node_rank, val_loss, hparams)
        node_rank = result[0]
        results.append(result)
        print(
            f"[Node {node_rank+1}/{n_nodes}] Finished."
            f" Early stop val loss result: {result[1:]}"
        )

    # find the model and hparams with the lowest validation loss
    best_result = min(results, key=lambda x: x[1])
    print(f"Best early stop val loss result: {best_result}")
    best_hparams = best_result[-1]

    # finish training with best hparams
    node_rank = 0
    n_nodes = 1  # only one node for final training run
    train_model.remote(
        node_rank,
        n_nodes,
        best_hparams,
        experiment_name,
        not stop_early,
        n_steps,
        n_steps_before_eval,
        n_steps_before_checkpoint,
    )

Monitor experiments with TensorBoard

To monitor our training we will create a TensorBoard WSGI web app, which will display the progress of our training across all 8 models. We’ll use the latest logs for the most recent experiment written to the Volume.

To ensure a unique color per experiment you can click the palette (🎨) icon under TensorBoard > Time Series > Run and use the Regex: E(\d{4})-(\d{2})-(\d{2})-(\d{6})\.(\d{6})

You can deploy this TensorBoard service by running

modal deploy 06_gpu_and_ml.hyperparameter-sweep.hp_sweep_gpt

and visit it at the URL that ends with -monitor-training.modal.run.

After training finishes, your TensorBoard UI will look something like this:

8 lines on a graph, validation loss on y-axis, time step on x-axis. All lines go down over the first 1000 time steps, and one goes to 5000 time steps with a final loss of 1.52

You can also find some sample text generated by the model in the “Text” tab.

@app.function(
    image=torch_image,
    volumes={volume_path: volume},
    allow_concurrent_inputs=1000,
)
@modal.wsgi_app()
def monitor_training():
    import time

    print("📈 TensorBoard: Waiting for logs...")
    ct = 0
    while not tb_log_path.exists():
        ct += 1
        if ct > 10:
            raise Exception("No logs found after 10 seconds.")
        volume.reload()  # make sure we have the latest data.
        time.sleep(1)

    # start TensorBoard server looking at all experiments
    board = tensorboard.program.TensorBoard()
    board.configure(logdir=str(tb_log_path))
    (data_provider, deprecated_multiplexer) = board._make_data_provider()
    wsgi_app = tensorboard.backend.application.TensorBoardWSGIApp(
        board.flags,
        board.plugin_loaders,
        data_provider,
        board.assets_zip_provider,
        deprecated_multiplexer,
    )
    return wsgi_app

Notice that there are 8 models training, and the one with the lowest validation loss at step 600 continues training to 3000 steps.

Serving SLMs on Modal during and after training

Because our weights are stored in a distributed Volume, we can deploy an inference endpoint based off of them without any extra work — and we can even check in on models while we’re still training them!

Remote inference with Modal Clses

We wrap our inference in a Modal Cls called ModelInference. The user of ModelInference can control which model is used by providing the experiment_name. Each unique choice creates a separate auto-scaling deployment. If the user does not specify an experiment_name, the latest experiment is used.

@app.cls(image=torch_image, volumes={volume_path: volume}, gpu=gpu)
class ModelInference:
    experiment_name: str = modal.parameter(default="")

    def get_latest_available_model_dirs(self, n_last):
        """Find the latest models that have a best model checkpoint saved."""
        save_model_dirs = glob.glob(f"{model_save_path}/*")
        sorted_model_dirs = sorted(
            save_model_dirs, key=os.path.getctime, reverse=True
        )

        valid_model_dirs = []
        for latest_model_dir in sorted_model_dirs:
            if Path(f"{latest_model_dir}/{best_model_filename}").exists():
                valid_model_dirs.append(Path(latest_model_dir))
            if len(valid_model_dirs) >= n_last:
                return valid_model_dirs
        return valid_model_dirs

    @modal.method()
    def get_latest_available_experiment_names(self, n_last):
        return [d.name for d in self.get_latest_available_model_dirs(n_last)]

    def load_model_impl(self):
        from .src.model import AttentionModel
        from .src.tokenizer import Tokenizer

        if self.experiment_name != "":  # user selected model
            use_model_dir = f"{model_save_path}/{self.experiment_name}"
        else:  # otherwise, pick latest
            try:
                use_model_dir = self.get_latest_available_model_dirs(1)[0]
            except IndexError:
                raise ValueError("No models available to load.")

        if self.use_model_dir == use_model_dir and self.is_fully_trained:
            return  # already loaded fully trained model.

        print(f"Loading experiment: {Path(use_model_dir).name}...")
        checkpoint = torch.load(f"{use_model_dir}/{best_model_filename}")

        self.use_model_dir = use_model_dir
        hparams = checkpoint["hparams"]
        key = (  # for backwards compatibility
            "unique_chars" if "unique_chars" in checkpoint else "chars"
        )
        unique_chars = checkpoint[key]
        steps = checkpoint["steps"]
        val_loss = checkpoint["val_loss"]
        self.is_fully_trained = checkpoint["finished_training"]

        print(
            f"Loaded model with {steps} train steps"
            f" and val loss of {val_loss:.2f}"
            f" (fully_trained={self.is_fully_trained})"
        )

        self.tokenizer = Tokenizer(unique_chars)
        self.device = "cuda" if torch.cuda.is_available() else "cpu"

        self.model = AttentionModel(
            self.tokenizer.vocab_size, hparams, self.device
        )
        self.model.load_state_dict(checkpoint["model"])
        self.model.to(self.device)

    @modal.enter()
    def load_model(self):
        self.use_model_dir = None
        self.is_fully_trained = False
        self.load_model_impl()

    @modal.method()
    def generate(self, prompt):
        self.load_model_impl()  # load updated model if available

        n_new_tokens = 1000
        return self.model.generate_from_text(
            self.tokenizer, prompt, n_new_tokens
        )

Adding a simple web_endpoint

The ModelInference class above is available for use from any other Python environment with the right Modal credentials and the modal package installed — just use lookup.

But we can also expose it as a web endpoint for easy access from anywhere, including other programming languages or the command line.

class GenerationRequest(BaseModel):
    prompt: str


@app.function(image=web_image)
@modal.web_endpoint(method="POST", docs=True)
def web_generate(request: GenerationRequest):
    output = ModelInference().generate.remote(request.prompt)
    return {"output": output}

This endpoint can be deployed on Modal with modal deploy. That will allow us to generate text via a simple curl command like this:

curl -X POST -H 'Content-Type: application/json' --data-binary '{"prompt": "\n"}' https://your-workspace-name--modal-nano-gpt-web-generate.modal.run

which will return something like:

{
"output":
   "BRUTUS:
    The broy trefore anny pleasory to
    wip me state of villoor so:
    Fortols listhey for brother beat the else
    Be all, ill of lo-love in igham;
    Ah, here all that queen and hould you father offer"
}

It’s not exactly Shakespeare, but at least it shows our model learned something!

You can choose which model to use by specifying the experiment_name in the query parameters of the request URL.

Serving a Gradio UI with asgi_app

Second, we create a Gradio web app for generating text via a graphical user interface in the browser. That way our fellow team members and stakeholders can easily interact with the model and give feedback, even when we’re still training the model.

You should see the URL for this UI in the output of modal deploy or on your Modal app dashboard for this app.

The Gradio UI will look something like this:

Image of Gradio Web App. Top shows model selection dropdown. Left side shows input prompt textbox. Right side shows SLM generated output. Bottom has button for starting generation process

@app.function(
    image=ui_image,
    concurrency_limit=1,
    volumes={volume_path: volume},
    allow_concurrent_inputs=1000,
)
@modal.asgi_app()
def ui():
    import gradio as gr
    from fastapi import FastAPI
    from fastapi.responses import FileResponse
    from gradio.routes import mount_gradio_app

    # call out to the inference in a separate Modal environment with a GPU
    def generate(text="", experiment_name=""):
        if not text:
            text = "\n"
        generated = ModelInference(
            experiment_name=experiment_name
        ).generate.remote(text)
        return text + generated

    example_prompts = [
        "DUKE OF YORK:\nWhere art thou Lucas?",
        "ROMEO:\nWhat is a man?",
        "CLARENCE:\nFair is foul and foul is fair, but who are you?",
        "Brevity is the soul of wit, so what is the soul of foolishness?",
    ]

    web_app = FastAPI()

    # custom styles: an icon, a background, and a theme
    @web_app.get("/favicon.ico", include_in_schema=False)
    async def favicon():
        return FileResponse("/assets/favicon.svg")

    @web_app.get("/assets/background.svg", include_in_schema=False)
    async def background():
        return FileResponse("/assets/background.svg")

    with open("/assets/index.css") as f:
        css = f.read()

    n_last = 20
    experiment_names = (
        ModelInference().get_latest_available_experiment_names.remote(n_last)
    )
    theme = gr.themes.Default(
        primary_hue="green", secondary_hue="emerald", neutral_hue="neutral"
    )

    # add a Gradio UI around inference
    with gr.Blocks(theme=theme, css=css, title="SLM") as interface:
        # title
        gr.Markdown("# GPT-style Shakespeare text generation.")

        # Model Selection
        with gr.Row():
            gr.Markdown("## Model Version")
        with gr.Row():
            experiment_dropdown = gr.Dropdown(
                experiment_names, label="Select Model Version"
            )

        # input and output
        with gr.Row():
            with gr.Column():
                gr.Markdown("## Input:")
                input_box = gr.Textbox(  # input text component
                    label="",
                    placeholder="Write some Shakespeare like text or keep it empty!",
                    lines=10,
                )
            with gr.Column():
                gr.Markdown("## Output:")
                output_box = gr.Textbox(  # output text component
                    label="",
                    lines=10,
                )

        # button to trigger inference and a link to Modal
        with gr.Row():
            generate_button = gr.Button("Generate", variant="primary", scale=2)
            generate_button.click(
                fn=generate,
                inputs=[input_box, experiment_dropdown],
                outputs=output_box,
            )  # connect inputs and outputs with inference function

            gr.Button(  # shameless plug
                " Powered by Modal",
                variant="secondary",
                link="https://modal.com",
            )

        # example prompts
        with gr.Column(variant="compact"):
            # add in a few examples to inspire users
            for ii, prompt in enumerate(example_prompts):
                btn = gr.Button(prompt, variant="secondary")
                btn.click(
                    fn=lambda idx=ii: example_prompts[idx], outputs=input_box
                )

    # mount for execution on Modal
    return mount_gradio_app(
        app=web_app,
        blocks=interface,
        path="/",
    )

Addenda

The remainder of this code is boilerplate.

Training Loop

There’s quite a lot of code for just the training loop! If you’d rather not write this stuff yourself, consider a training framework like PyTorch Lightning or Hugging Face.

def training_loop(
    start_step,
    n_steps,
    n_steps_before_eval,
    n_steps_before_checkpoint,
    n_eval_steps,
    dataset,
    tokenizer,
    model,
    optimizer,
    logs_manager,
    checkpoint,
    checkpoint_path,
    run_to_first_save,
):
    @torch.no_grad()
    def eval_model(model, dataset, tokenizer, n_eval_steps):
        """Evaluate model on train and validation data."""
        out = {}
        model.eval()  # Turn off gradients
        for split in ("train", "val"):
            losses = torch.zeros(n_eval_steps)
            for k in range(n_eval_steps):
                xb, yb = dataset.get_batch(split)
                logits, loss = model.forward(xb, yb)
                losses[k] = loss
            out[split] = losses.mean()

        # Generate some output samples
        out["sample"] = model.generate_from_text(tokenizer, "\n", 1000)

        model.train()  # Turn on gradients
        return out

    t_last = timer()
    for step in range(start_step, n_steps + 1):
        # sample a batch of data
        xb, yb = dataset.get_batch("train")

        # evaluate the loss, calculate & apply gradients
        logits, loss = model.forward(xb, yb)
        optimizer.zero_grad(set_to_none=True)
        loss.backward()
        optimizer.step()

        # log training loss
        logs_manager.add_train_scalar("Cross Entropy Loss", loss.item(), step)

        # evaluate model on validation set
        if step % n_steps_before_eval == 0:
            out = eval_model(model, dataset, tokenizer, n_eval_steps)
            log_evals(out, step, t_last, logs_manager)
            t_last = timer()

        # save model with checkpoint information
        if step > 0 and step % n_steps_before_checkpoint == 0:
            checkpoint["steps"] = step
            checkpoint["val_loss"] = out["val"]

            # mark as finished if we hit n steps.
            checkpoint["finished_training"] = step >= n_steps

            L.info(
                f"Saving checkpoint to {checkpoint_path}"
                f"\t {checkpoint['finished_training']})"
            )
            save_checkpoint(checkpoint, checkpoint_path)

            if run_to_first_save:
                L.info("Stopping early...")
                break
    return out


def save_checkpoint(checkpoint, checkpoint_path):
    torch.save(checkpoint, checkpoint_path)
    volume.commit()


def build_model(hparams, vocab_size, device):
    """Initialize the model and move it to the device."""
    model = AttentionModel(vocab_size, hparams, device)
    model.to(device)
    return model


def setup_optimizer(model, learning_rate):
    """Set up the optimizer for the model."""
    return torch.optim.AdamW(model.parameters(), lr=learning_rate)

Miscellaneous

The remaining code includes small helper functions for training the model.

def prepare_data(input_file_path: Path, volume: modal.Volume) -> str:
    """Download and read the dataset."""
    volume.reload()
    if not input_file_path.exists():
        L.info("Downloading Shakespeare dataset...")
        data_url = "https://raw.githubusercontent.com/karpathy/char-rnn/master/data/tinyshakespeare/input.txt"
        urllib.request.urlretrieve(data_url, input_file_path)
        volume.commit()
    return input_file_path.read_text()


def make_best_symbolic_link(model_save_dir, model_filename, experiment_name):
    # create symlink to the best model so it's easy to find for web serving
    os.symlink(
        str(model_save_dir / model_filename),
        str(model_save_path / experiment_name / best_model_filename),
    )
    volume.commit()  # commit the symlink


def init_checkpoint(model, tokenizer, optimizer, start_step, hparams):
    return {
        "model": model.state_dict(),
        "unique_chars": tokenizer.unique_chars,
        "optimizer": optimizer.state_dict(),
        "val_loss": float("inf"),
        "steps": start_step,
        "hparams": hparams,
        "finished_training": False,
    }


def log_evals(result, step, t_last, logs_manager):
    runtime_s = timer() - t_last
    L.info(
        f"{step:5d}) // {runtime_s:>5.2f}s"
        f" // Train Loss: {result['train']:.2f} // Val Loss:"
        f" {result['val']:.2f}"
    )
    logs_manager.add_val_scalar("Cross Entropy Loss", result["val"], step)
    logs_manager.add_val_text("Sample Output", result["sample"], step)
    logs_manager.flush()
    volume.commit()  # Make sure TensorBoard container will see it.

    return result