leiden and umap not reproducible on different CPUs

[grst]

• I have checked that this issue has not already been reported.
• I have confirmed this bug exists on the latest version of scanpy.
• (optional) I have confirmed this bug exists on the master branch of scanpy.

---

I noticed that running the same single-cell analyses on different nodes of our HPC produces different results.
Starting from the same anndata object with a precomputed X_scVI latent representation, the UMAP and leiden-clustering looks different.

On

• Intel(R) Xeon(R) CPU E5-2699A v4 @ 2.40GHz
• AMD EPYC 7352 24-Core Processor
• Intel(R) Xeon(R) CPU E7-4850 v4 @ 2.10GHz

adata.obs["leiden"].value_counts()

0     4268
1     2132
2     1691
3     1662
4     1659
5     1563
...

On

• Intel(R) Xeon(R) CPU E7- 4870 @ 2.40GHz

0     3856
1     2168
2     2029
3     1659
4     1636
5     1536
...

Minimal code sample (that we can copy&paste without having any data)

A git repository with example data, notebook and a nextflow pipeline is available here:
https://github.com/grst/scanpy_reproducibility

A report of the analysis executed on four different CPU architectures is available here:
https://grst.github.io/scanpy_reproducibility/

Versions

WARNING: If you miss a compact list, please try `print_header`!
-----
anndata     0.7.5
scanpy      1.6.0
sinfo       0.3.1
-----
PIL                 8.0.1
anndata             0.7.5
backcall            0.2.0
cairo               1.20.0
cffi                1.14.4
colorama            0.4.4
cycler              0.10.0
cython_runtime      NA
dateutil            2.8.1
decorator           4.4.2
get_version         2.1
h5py                3.1.0
igraph              0.8.3
ipykernel           5.3.4
ipython_genutils    0.2.0
jedi                0.17.2
joblib              0.17.0
kiwisolver          1.3.1
legacy_api_wrap     0.0.0
leidenalg           0.8.3
llvmlite            0.35.0
matplotlib          3.3.3
mpl_toolkits        NA
natsort             7.1.0
numba               0.52.0
numexpr             2.7.1
numpy               1.19.4
packaging           20.7
pandas              1.1.4
parso               0.7.1
pexpect             4.8.0
pickleshare         0.7.5
pkg_resources       NA
prompt_toolkit      3.0.8
ptyprocess          0.6.0
pycparser           2.20
pygments            2.7.2
pyparsing           2.4.7
pytz                2020.4
scanpy              1.6.0
scipy               1.5.3
setuptools_scm      NA
sinfo               0.3.1
six                 1.15.0
sklearn             0.23.2
sphinxcontrib       NA
storemagic          NA
tables              3.6.1
texttable           1.6.3
tornado             6.1
traitlets           5.0.5
umap                0.4.6
wcwidth             0.2.5
yaml                5.3.1
zmq                 20.0.0
-----
IPython             7.19.0
jupyter_client      6.1.7
jupyter_core        4.7.0
-----
Python 3.8.6 | packaged by conda-forge | (default, Nov 27 2020, 19:31:52) [GCC 9.3.0]
Linux-3.10.0-1160.11.1.el7.x86_64-x86_64-with-glibc2.10
64 logical CPU cores, x86_64
-----
Session information updated at 2021-10-15 09:58

[Zethson]

Hey,

please report back with containerized environments as discussed on Twitter.

[grst]

Hi,

I updated the pipeline to use this singularity container. The problem persists.

[grst]

I tried also with NUMBA_DISABLE_JIT=1. The leiden clustering is stable now (at least across the four nodes), yet the UMAP again looks different to both the previous versions and between the two groups of CPUs:

On

• Intel(R) Xeon(R) CPU E5-2699A v4 @ 2.40GHz
• AMD EPYC 7352 24-Core Processor
• Intel(R) Xeon(R) CPU E7-4850 v4 @ 2.10GHz

On

• Intel(R) Xeon(R) CPU E7- 4870 @ 2.40GHz

[ivirshup]

Do you have any idea if the issue is with our implementation of these, or with the underlying libraries? E.g. if you just run UMAP directly from the UMAP library can you get exact reproducibility? I don't think we would be able to promise more reproducibility than what they offer, and irreproducibility across machines is a known issue there: lmcinnes/umap#525.

[grst]

That's a good point, and it is not:

reducer = umap.UMAP(min_dist=0.5)
embedding = reducer.fit_transform(adata.obsm["X_scVI"])
adata.obsm["X_umap"] = embedding

again produces stable results on only 3/4 CPUs.

Ok, let's forget about UMAP. It's only a nice figure to get an overview of the data and I don't use it for downstream stuff. Irreproducible clustering, on the other hand, is quite a deal-breaker, as for instance cell-type annotations depend on it. I mean, why would I even bother releasing the source code of an analysis alongside the paper if it is not reproducible anyway?

I found out a few more things:

• the leiden algorithm itself seems deterministic on all 4 nodes, when started from a pre-computed adata.obsp["connectivities"].
• when running pp.neighbors with NUMBA_DISABLE_JIT=1, the clustering is stable on all four nodes (but terribly slow, ofc)
• when rounding the connectivities to 3-4 digits, the clustering is also stable (plus the total runtime is reduced from 2:30 to 1:50min)

adata.obsp["connectivities"] = np.round(adata.obsp["connectivities"], decimals=3)
adata.obsp["distances"] = np.round(adata.obsp["distances"], decimals=3)

[Zethson]

So should we add a deterministic flag for Leiden clustering which enforces NUMBA_DISABLE_JIT=1 and is disabled by default? Users then have the option to enable this for publication code.

[chlee-tabin]

I have yet to install the most latest scanpy and I do not have CPUs to test for this specific case, but I had some issue of reproducing leiden results from scanpy from a published paper, and found that running leiden 10 times (per n_iteration option) resolved any discrepancy. Wonder whether it adds to the discussion.

[ivirshup]

@grst I don't think leiden is the issue here, but pynndescent.

My guess is this is going to have to do with the CPU that gives different results being much older using a different instruction set that the other intel processors. This could be triggered by either use of any parallelism at all or pynndescent being pretty liberal with the use of numba's fastmath, and different CPUs having different features.

Do you get the same graph out of pynndescent if you are make the computation single threaded? If not, we may be able to look at the assembly to see which feature sets give different results. It's possible these can be turned off with a flag. But we may then have to consider what kind of a performance hit we'd be willing to take for exact reproducibility on discontinued chip sets.

[grst]

I don't think leiden is the issue here, but pynndescent.

💯

Do you get the same graph out of pynndescent if you are make the computation single threaded

I have already been exporting those four env vars before running the analysis. Is there anything else that might be threaded?

export MKL_NUM_THREADS="1"
export OPENBLAS_NUM_THREADS="1"
export OMP_NUM_THREADS="1"
export NUMBA_NUM_THREADS="1"

[ivirshup]

threadpoolctl does a nice job of accounting for most possible options. But do you see more than a single thread being used?

[grst]

Just to be sure, I additionally included

from threadpoolctl import threadpool_limits
threadpool_limits(1)

I also confirmed that only a single thread was actually used. The results didn't change.

[ivirshup]

Alright, if you would like to achieve reproducibility the next things to play around with would probably be these CPU feature flag variables for numba. In particular: NUMBA_CPU_NAME=generic.

[grst]

another 💯

with NUMBA_CPU_NAME=generic the leiden results are the same on all four nodes and also the UMAPs look a lot more similar (but not identical).

In terms of speed I didn't notice a bit difference (maybe 10s slower, but that would require proper benchmarking ofc).

[ivirshup]

What do you think the right thing to do here is?

I don't think we can modify the global environment for our users by setting those numba flags by default. Is this just something we should document?

[grst]

How about creating a page about reproducibility in the docs, similar to the one by pytorch? It could gather all information around reproducibility with scanpy, such as

• use of containers
• numba flags

and also state the limits, e.g.

• UMAP is not reproducible across systems.

In addition to that, @Zethson, what do you think of creating a mlf-core template for single-cell analyses that sets the right defaults?