python-mpi-bcast

An HPC friendly python environment that deploys packages to computing nodes via MPI.

Benchmark on Cray systems

We performed benchmarks on the Cray XC-30 system Edison at NERSC and the Cray XT system BlueWaters at NCSA.

See the following figure. python-mpi-bcast demonstrates excellent scaling properties. We can start 12,288 ranks, each import scipy, and doing all of these in 50 seconds.

The overhead of python-mpi-bcast at full machine capability is comparable to the ALPS applicationg launching overhead.

Cray XC30 Edison	Cray XT BlueWaters

Why is Python slow on a supercomputer?

One problem with large scale parallel application written in Python is the slow startup time. The Python interpreter may spend half an hour before even start processing any useful user logic.

Python does a lot of file operations upon startup. This is not an issue for small scale applications -- but on applications at a massive scale (10K+ MPI ranks), these file operations become a burden to the shared file system, just like the shared library burden, described in [Hopper-UG]

For example, on a typical python installation with numpy the number of file operations to :

$ strace -ff -e file python -c '' 2>&1 |wc -l
917

$ strace -ff -e file python -c 'import numpy.fft' 2>&1 |wc -l

4557
$ strace -ff -e file python -c 'import numpy.fft; import scipy.interpolate' 2>&1|wc -l
8089

Now multiply this number by the number of ranks, 1024, for example.

Keep in mind that in a massively parallel application, the payload may in fact only access a few very large files. The overhead here is a headache.

What do we do about this?

People have thought that python just can never work well on HPC systems. This is not true. We can start 1024 Python ranks on edison.nersc.gov in 40 seconds, consistently as long as we follow the principles in this page. We will need the help of a tool 'bcast' that is provided here.

The idea is simple:

• Avoid meta-data requests from slow filesystems (e.g. home directories);
• Avoid as much as possible meta-data requests on (even fast) shared filesystems;

If these two are done, spinning up thousands of python ranks is no slower than spinning up the same number of C ranks; and no modifications on the user programs needs to be done.

The biggest part is from bcast provided here, which deploys selected packages to the computing node, and properly set up the python environment to avoid most of the meta-data requests on the shared filesystem.

Here is the TODO list that enables the full benefits of the python-mpi implementation provided here. These steps can be implemented either by the computing faciliaties, or by a user.

1. Install Conda/Anaconda, and create a tar ball of the entire installation with the supplied 'tar-anaconda.sh'

wget http://repo.continuum.io/miniconda/Miniconda-latest-Linux-x86_64.sh -O miniconda.sh
chmod +x miniconda.sh
./miniconda.sh -b -p $HOME/miniconda
export PATH=$HOME/miniconda/bin:$PATH
conda update --yes conda
conda create --yes -n test python=2.7
source activate test
conda install --yes numpy=1.9 nose mpi4py # install other packages as well
bash tar-anaconda.sh anaconda.tar.gz $HOME/miniconda/envs/test

Note

On some systems, an anaconda based installation is already supplied by the vendor. (e.g. Edison and Cori). In that case, find the location of that installation via the module file, and directly use tar-anaconda.sh to generate a tar ball.

Attention

copy the tar ball file to a fast file system, e.g. scratch or project directory.

We will assume the location is $SCRATCH/2.7-anaconda.tar.gz

2. Alternatively, prepackage individual python packages to .tar.gz files. On some systems where the conda prebuilt packages are not an option, this will be the only feasible way. We provide a script tar-pip.sh for this:

# build a fitsio bundle

bash tar-pip.sh fitsio-0.9.8rc2.tar.gz https://github.com/esheldon/fitsio/archive/v0.9.8rc2.zip

# build a bundle for locally checked out code with a setup.py

bash tar-pip.sh my-package.tar.gz .

# you get the idea

Note

Still, the installation of some packages may not be this trivial. Luckily, usually the vendor must have compiled most python packages, and it is worthwhile to inspect the module files and directly run the tar command there, skipping the installation part.

3. Reset PYTHONHOME PYTHONBASE, PYTHONUSERBASE, and PATH, LD_LIBRARY_PATH to /dev/shm/local.

This can be done by sourcing 'activate.sh'. activate.sh takes 2 arguments, the prefix of the new python environment, and the command prefix to launch 'bcast'. activate.sh also provide a 'bcast' function to the shell script, which will simply run bcast with the provided prefix. A good choice of the prefix is /dev/shm/local. If the computing nodes contain private scratch hardrives, that would be a good location as well.

Warning

All packages install in ~/.local is unavailable during the session.

4. Copy the relevant python scripts to a fast filesystem.

Especially be aware of starting a python script in HOME directory. It can be very slow. (recall sometimes ls on home directory takes for ever?)

Here is a full job script example on Edison following all of the guidelines. Notice that on Edison, I have already created the tar ball of the 2.7 and 3.4 version of anaconda installation at /project/projectdirs/m779/python-mpi

#PBS -j eo
#PBS -l mppwidth=1024
#PBS -q debug

set -x
export OMP_NUM_THREADS=1

source /project/projectdirs/m779/python-mpi/activate.sh /dev/shm/local "aprun -n 1024 -d 1"

cd $PBS_O_WORKDIR

# send the anaconda packages
bcast -v /project/projectdirs/m779/python-mpi/2.7-anaconda.tar.gz 

# testpkg contains the tar-ed version of the script;
# if the script is sufficiently complicated, it helps to treat it like 
# another package.

bcast -v testpkg.tar.gz

time aprun -n 1024 -d 1 python-mpi /dev/shm/local/testpkg/main.py

Yu Feng - BCCP / BIDS.