Mention likely multi-threading issues on BVAR notebook

[emanuele]

Describe the issue:

When running the Bayesian Vector Autoregressive Models — PyMC example gallery 1 on x86_64 (tested on Ubuntu 20.04 + fresh install of PyMC via official installation instructions), the sampling time explodes to tens of hours instead of the expected few minutes. I can reproduce the issue on multiple machines.

Differently, on arm64 (M1 Macbook) the issue does not occur, and the sampling time is a few minutes as expected.

Below is a minimal example, extracted from the notebook above, that consistently shows the issue on x86_64.

The critical line is l.113, where pm.sample() has default values and the NUTS sampler is auto-assigned.

By manually specifying pm.sample(cores=1, ... or using a non-default NUTS sampler (nuts_sampler="blackjax", or nuts_sampler="numpyro"), the issue disappears on x86_64.

Related discussion on Discord here.

Screenshot of the issue:

Reproduceable code example:

import numpy as np
import pandas as pd
import pymc as pm


RANDOM_SEED = 8927
rng = np.random.default_rng(RANDOM_SEED)

def simulate_var(
    intercepts, coefs_yy, coefs_xy, coefs_xx, coefs_yx, noises=(1, 1), *, warmup=100, steps=200
):
    draws_y = np.zeros(warmup + steps)
    draws_x = np.zeros(warmup + steps)
    draws_y[:2] = intercepts[0]
    draws_x[:2] = intercepts[1]
    for step in range(2, warmup + steps):
        draws_y[step] = (
            intercepts[0]
            + coefs_yy[0] * draws_y[step - 1]
            + coefs_yy[1] * draws_y[step - 2]
            + coefs_xy[0] * draws_x[step - 1]
            + coefs_xy[1] * draws_x[step - 2]
            + rng.normal(0, noises[0])
        )
        draws_x[step] = (
            intercepts[1]
            + coefs_xx[0] * draws_x[step - 1]
            + coefs_xx[1] * draws_x[step - 2]
            + coefs_yx[0] * draws_y[step - 1]
            + coefs_yx[1] * draws_y[step - 2]
            + rng.normal(0, noises[1])
        )
    return draws_y[warmup:], draws_x[warmup:]


### Define a helper function that will construct our autoregressive step for the marginal contribution of each lagged
### term in each of the respective time series equations
def calc_ar_step(lag_coefs, n_eqs, n_lags, df):
    ars = []
    for j in range(n_eqs):
        ar = pm.math.sum(
            [
                pm.math.sum(lag_coefs[j, i] * df.values[n_lags - (i + 1) : -(i + 1)], axis=-1)
                for i in range(n_lags)
            ],
            axis=0,
        )
        ars.append(ar)
    beta = pm.math.stack(ars, axis=-1)

    return beta


### Make the model in such a way that it can handle different specifications of the likelihood term
### and can be run for simple prior predictive checks. This latter functionality is important for debugging of
### shape handling issues. Building a VAR model involves quite a few moving parts and it is handy to
### inspect the shape implied in the prior predictive checks.
def make_model(n_lags, n_eqs, df, priors, mv_norm=True, prior_checks=True):
    coords = {
        "lags": np.arange(n_lags) + 1,
        "equations": df.columns.tolist(),
        "cross_vars": df.columns.tolist(),
        "time": [x for x in df.index[n_lags:]],
    }

    with pm.Model(coords=coords) as model:
        lag_coefs = pm.Normal(
            "lag_coefs",
            mu=priors["lag_coefs"]["mu"],
            sigma=priors["lag_coefs"]["sigma"],
            dims=["equations", "lags", "cross_vars"],
        )
        alpha = pm.Normal(
            "alpha", mu=priors["alpha"]["mu"], sigma=priors["alpha"]["sigma"], dims=("equations",)
        )
        data_obs = pm.Data("data_obs", df.values[n_lags:], dims=["time", "equations"], mutable=True)

        betaX = calc_ar_step(lag_coefs, n_eqs, n_lags, df)
        betaX = pm.Deterministic(
            "betaX",
            betaX,
            dims=[
                "time",
            ],
        )
        mean = alpha + betaX

        if mv_norm:
            n = df.shape[1]
            ## Under the hood the LKJ prior will retain the correlation matrix too.
            noise_chol, _, _ = pm.LKJCholeskyCov(
                "noise_chol",
                eta=priors["noise_chol"]["eta"],
                n=n,
                sd_dist=pm.HalfNormal.dist(sigma=priors["noise_chol"]["sigma"]),
            )
            obs = pm.MvNormal(
                "obs", mu=mean, chol=noise_chol, observed=data_obs, dims=["time", "equations"]
            )
        else:
            ## This is an alternative likelihood that can recover sensible estimates of the coefficients
            ## But lacks the multivariate correlation between the timeseries.
            sigma = pm.HalfNormal("noise", sigma=priors["noise"]["sigma"], dims=["equations"])
            obs = pm.Normal(
                "obs", mu=mean, sigma=sigma, observed=data_obs, dims=["time", "equations"]
            )

        if prior_checks:
            idata = pm.sample_prior_predictive()
            return model, idata
        else:
            idata = pm.sample_prior_predictive()
            idata.extend(pm.sample(draws=2000, random_seed=130))
            pm.sample_posterior_predictive(idata, extend_inferencedata=True, random_seed=rng)
    return model, idata


if __name__=="__main__":

    
    var_y, var_x = simulate_var(
        intercepts=(18, 8),
        coefs_yy=(-0.8, 0),
        coefs_xy=(0.9, 0),
        coefs_xx=(1.3, -0.7),
        coefs_yx=(-0.1, 0.3),
    )
    df = pd.DataFrame({"x": var_x, "y": var_y})
    df.head()
    
    n_lags = 2
    n_eqs = 2
    priors = {
        "lag_coefs": {"mu": 0.3, "sigma": 1},
        "alpha": {"mu": 15, "sigma": 5},
        "noise_chol": {"eta": 1, "sigma": 1},
        "noise": {"sigma": 1},
    }
    model, idata_fake_data = make_model(n_lags, n_eqs, df, priors, prior_checks=False)

Error message:

No error message, just the code hanging (almost) forever.

PyMC version information:

Fresh install of PyMC via official installation instructions.

%load_ext watermark
%watermark -n -u -v -iv -w -p pytensor,aeppl,xarray
----
Last updated: Tue May 23 2023

Python implementation: CPython
Python version       : 3.11.3
IPython version      : 8.13.2

pytensor: 2.11.2
aeppl   : not installed
xarray  : 2023.5.0

pymc     : 5.3.0
watermark: 2.3.1
pandas   : 2.0.1
numpy    : 1.24.3

Watermark: 2.3.1

Context for the issue:

1. This example in the PyMC documentation becomes unusable for a fair share of users.
2. The reason behind the issue may affect PyMC users in general, irrespective of the example.

[welcome]

🎉 Welcome to PyMC! 🎉 We're really excited to have your input into the project! 💖

If you haven't done so already, please make sure you check out our Contributing Guidelines and Code of Conduct.

[ricardoV94]

I can reproduce in my (4x2) core CPU. I think it boils down to BLAS/ LAPACK multithreading.
By default it saturates all the cores.

Setting the MKL threads to 1 fixes it for me:

%env MKL_NUM_THREADS=1
%env OPENBLAS_NUM_THREADS=1

Or setting the number of threads to 2 and reducing the number of chains to 2/3 is also fine.

More context for this can be found in:
https://discourse.pymc.io/t/regarding-the-use-of-multiple-cores/4249/3
https://discourse.pymc.io/t/nuts-uses-all-cores/909/10

Maybe it's useful to add a warning message at the top of that notebook?

Part of this may be helped by fixing pymc-devs/pymc#6717

[emanuele]

Good points @ricardoV94 . But why this happens only on Linux? Why specifically with this problem?

Anedoctically, it is the first time I see multiprocessing/multithreading being a problem on Linux and not on Windows or MacOS :D

[ricardoV94]

My guess is that neither of those are actually triggering multi-threading. For instance, I think OpenMP (which could be another indirect source of multi-threading) is not installed by default on MacOs. Not sure without having a machine to try those out.

The way Python multi-processes are created is also different across those 3 operating systems IIRC, that could also matter.

[ricardoV94]

Another user afflicted with this: https://discourse.pymc.io/t/bayesian-var-multivariate-slow-performance/12463/2?u=ricardov94

I think we should change the flags explicitly on the pymc example and have a warning message on why we are doing this. Not everyone needs it, but many users do. We can mention users can try to turn it off and see if it still samples fast.

[OriolAbril]

Updating the flags sounds great

[emanuele]

What is the suggested way to update the flags just in the case of Linux? A simple cell at the beginning of the notebook like

%env MKL_NUM_THREADS=1
%env OPENBLAS_NUM_THREADS=1

is not conditional on the operating system. Is checking via sys.platform the way to go? Or PyMC has a more structured way?

[ricardoV94]

I think those flags are fine in most OSes?

[ricardoV94]

It would also be worth testing on the lastest PyMC (5.6.0) release as we managed to remove some useless Blas operations on MvNormal models. The problem may not exist anymore. We would need someone who had problems before to test it out.

[emanuele]

Just tested on Linux with PyMC v5.6.0 with the code above without setting the flags and, unfortunately, the problem still persists. Maybe the sampling time diverges less dramatically: sampling reaches 10% in 3 minutes and all cores are completely filled (mostly in red color with htop, which means kernel time, so not what we want here). But after setting the flags as above, 10% of sampling requires <30 seconds.

%load_ext watermark
%watermark -n -u -v -iv -w -p pytensor,aeppl,xarray

gives:

Last updated: Wed Jul 12 2023

Python implementation: CPython
Python version       : 3.9.7
IPython version      : 8.14.0

pytensor: 2.12.3
aeppl   : not installed
xarray  : 2023.6.0

pymc  : 5.6.0
numpy : 1.25.1
pandas: 2.0.3

Watermark: 2.4.3

[emanuele]

I think those flags are fine in most OSes?

I really have no idea :)

[ricardoV94]

Yes I think they work in other OSes, and won't hurt if not