Performance Tips and Tricks

This document will show you how to speed things up and get more out of your GPU/CPU.

Mixed Precision Training

Combined FP16/FP32 training can tremendously improve training speed and use less GPU RAM. For theory behind it see this thread

To deploy it see these instructions.

Faster Image Processing

If you notice a bottleneck in JPEG decoding (decompression) it’s enough to switch to a much faster libjpeg-turbo, using the normal version of Pillow.

If you need faster image resize, blur, alpha composition, alpha premultiplication, division by alpha, grayscale and other image manipulations you need to switch to Pillow-SIMD.

At the moment this section is only relevant if you’re on the x86 platform.


This is a faster compression/decompression libjpeg drop-in replacement.

libjpeg-turbo is a JPEG image codec that uses SIMD instructions (MMX, SSE2, AVX2, NEON, AltiVec). On x86 platforms it accelerates baseline JPEG compression and decompression and progressive JPEG compression. libjpeg-turbo is generally 2-6x as fast as libjpeg, all else being equal.

When you install it system-wide it provides a drop-in replacement for the libjpeg library. Some packages that rely on this library will be able to start using it right away, most will need to be recompiled against the replacement library.

Here is its git-repo.

fastai uses Pillow for its image processing and you have to rebuild Pillow to take advantage of libjpeg-turbo.

To learn how to rebuild Pillow-SIMD or Pillow with libjpeg-turbo see the Pillow-SIMD entry.


There is a faster Pillow version out there.


First, there was PIL (Python Image Library). And then its development was abandoned.

Then, Pillow forked PIL as a drop-in replacement and according to its benchmarks it is significantly faster than ImageMagick, OpenCV, IPP and other fast image processing libraries (on identical hardware/platform).

Relatively recently, Pillow-SIMD was born to be a drop-in replacement for Pillow. This library in its turn is 4-6 times faster than Pillow, according to the same benchmarks. Pillow-SIMD is highly optimized for common image manipulation instructions using Single Instruction, Multiple Data (SIMD approach, where multiple data points are processed simultaneously. This is not parallel processing (think threads), but a single instruction processing, supported by CPU, via data-level parallelism, similar to matrix operations on GPU, which also use SIMD.

Pillow-SIMD currently works only on the x86 platform. That’s the main reason it’s a fork of Pillow and not backported to Pillow - the latter is committed to support many other platforms/architectures where SIMD-support is lacking. The Pillow-SIMD release cycle is made so that its versions are identical Pillow’s and the functionality is identical, except Pillow-SIMD speeds up some of them (e.g. resize).


This section explains how to install Pillow-SIMD w/ libjpeg-turbo (but the very tricky libjpeg-turbo part of it is identically relevant to Pillow - just replace pillow-simd with pillow in the code below).

Here is the tl;dr version to install Pillow-SIMD w/ libjpeg-turbo and w/o TIFF support:

conda uninstall -y --force pillow pil jpeg libtiff libjpeg-turbo
pip   uninstall -y         pillow pil jpeg libtiff libjpeg-turbo
conda install -yc conda-forge libjpeg-turbo
CFLAGS="${CFLAGS} -mavx2" pip install --upgrade --no-cache-dir --force-reinstall --no-binary :all: --compile pillow-simd
conda install -y jpeg libtiff

Here are the detailed instructions, with an optional TIFF support:

  1. First remove pil, pillow, jpeg and libtiff packages. Also remove ‘libjpeg-tubo’ if a previous version is installed:

    conda uninstall -y --force pillow pil jpeg libtiff libjpeg-turbo
    pip   uninstall -y         pillow pil jpeg libtiff libjpeg-turbo

    Both conda packages jpeg and libjpeg-turbo contain a library. jpeg’s library will be replaced later in these instructions with libjpeg-turbo’s one for the duration of the build.

    libtiff is linked against library from the jpeg conda package, and since Pillow will try to link against it, we must remove it too for the duration of the build. If this is not done, import PIL will fail.

    Note, that the --force conda option forces removal of a package without removing packages that depend on it. Using this option will usually leave your conda environment in a broken and inconsistent state. And pip does it anyway. But we are going to fix your environment in the next step. Alternatively, you may choose not to use --force, but then it’ll uninstall a whole bunch of other packages and you will need to re-install them later. It’s your call.

  2. Now we are ready to replace libjpeg with a drop-in replacement of libjpeg-turbo and then replace Pillow with Pillow-SIMD:

    conda install -yc conda-forge libjpeg-turbo
    CFLAGS="${CFLAGS} -mavx2" pip install --upgrade --no-cache-dir --force-reinstall --no-binary :all: --compile pillow-simd

    Do note that since you’re building from source, you may end up not having some of the features that come with the binary Pillow package if the corresponding libraries aren’t available on your system during the build time. For more information see: Building from source.

    If you add -v to the pip install command, you will be able to see all the details of the build, and one useful part of the output is its report of what was enabled and what not, in PIL SETUP SUMMARY:

     --- JPEG support available
     *** OPENJPEG (JPEG2000) support not available
     --- ZLIB (PNG/ZIP) support available
     *** LIBIMAGEQUANT support not available
     *** LIBTIFF support not available
     --- FREETYPE2 support available
     --- LITTLECMS2 support available
     *** WEBP support not available
     *** WEBPMUX support not available
  3. Another nuance is libtiff which we removed, - and that means that Pillow was built without LIBTIFF support and will not be able to read TIFF files.

    You can safely skip this step if you don’t care for TIFF files.

    This can be fixed by installing a libtiff library, linked against libjpeg-turbo.

    conda install -y -c zegami libtiff-libjpeg-turbo

    and then rebuilding Pillow as explained in the stage above. Only linux version of the libtiff-libjpeg-turbo package is available at the moment.

    XXX: The libtiff-libjpeg-turbo package could be outdated - it’s currently only available on someone’s personal channel. Alternatively, it’ll need to be built from scratch. Pypi’s libtiff package doesn’t help - it doesn’t place under conda environment’s lib directory.

    The other option is to install system-wide libjpeg-turbo and libtiff linked against the former.

  4. Assuming that libjpeg-turbo and jpeg’s don’t collide you can now reinstall back the jpeg package - other programs most likely need it. And libtiff too:

    conda install -y jpeg libtiff
  5. Since this is a forked drop-in replacement, however, the package managers don’t know they have Pillow-replacement package installed, so if any update happens that triggers an update of Pillow, conda/pip will overwrite Pillow-SIMD reverting to the less speedy Pillow solution. So it’s worthwhile checking your run-time setup that you’re indeed using Pillow-SIMD in your code.

    That means that every time you update the fastai conda package you will have to rebuild Pillow-SIMD.

How to check whether you’re running Pillow or Pillow-SIMD?

> "python -c "import PIL; print(PIL.__version__)"
'8.1.0' or 7.0.0.post3

According to the author, if PILLOW_VERSION has a postfix, it is Pillow-SIMD. (Assuming that Pillow will never make a .postX release).

Is JPEG compression SIMD-optimized?

libjpeg-turbo replacement for libjpeg is SIMD-optimized. In order to get Pillow or its faster fork Pillow-SIMD to use libjpeg-turbo, the latter needs to be already installed during the former’s compilation time. Once Pillow is compiled/installed, it no longer matters which libjpeg version is installed in your virtual environment or system-wide, as long as the same libjpeg library remains at the same location as it was during the compilation time (it’s dynamically linked).

However, if at a later time something triggers a conda or pip update on Pillow it will fetch a pre-compiled version which most likely is not built against libjpeg-turbo and replace your custom built Pillow or Pillow-SIMD.

Here is how you can see that the PIL library is dynamically linked to

cd ~/anaconda3/envs/fastai/lib/python3.6/site-packages/PIL/
ldd | grep libjpeg => ~/anaconda3/envs/fastai/lib/

and ~/anaconda3/envs/fastai/lib/ was installed by conda install -c conda-forge libjpeg-turbo. We know that from:

cd  ~/anaconda3/envs/fastai/conda-meta/
grep libjpeg-turbo-2.0.1-h470a237_0.json

If I now install the normal libjpeg and do the same check on the jpeg’s package info:

conda install jpeg
cd  ~/anaconda3/envs/fastai/conda-meta/
grep jpeg-9b-h024ee3a_2.json

I find that it’s lib/ (~/anaconda3/envs/fastai/lib/

Also, if libjpeg-turbo and libjpeg happen to have the same version number, even if you built Pillow or Pillow-SIMD against libjpeg-turbo, but then later replaced it with the default jpeg with exactly the same version you will end up with the slower version, since the linking happens at build time. But so far that risk appears to be small, as of this writing, libjpeg-turbo releases are in the 8.x versions, whereas jpeg’s are in 9.x’s.

How to tell whether Pillow or Pillow-SIMD is using libjpeg-turbo?

You need Pillow>=5.4.0 to accomplish the following (install from github until then: pip install git+

python -c "from PIL import features; print(features.check_feature('libjpeg_turbo'))"

And a version-proof check:

from PIL import features, Image
from packaging import version

try:    ver = Image.__version__     # PIL >= 7
except: ver = Image.PILLOW_VERSION  # PIL <  7

if version.parse(ver) >= version.parse("5.4.0"):
    if features.check_feature('libjpeg_turbo'):
        print("libjpeg-turbo is on")
        print("libjpeg-turbo is not on")
    print(f"libjpeg-turbo' status can't be derived - need Pillow(-SIMD)? >= 5.4.0 to tell, current version {ver}")

Conda packages

The fastai conda (test) channel has an experimental pillow package built against a custom build of libjpeg-turbo. There are python 3.6 and 3.7 linux builds:

To install:

conda uninstall -y --force pillow libjpeg-turbo
conda install -c fastai/label/test pillow

There is also an experimental pillow-simd-5.3.0.post0 conda package built against libjpeg-turbo and compiled with avx2. Try it only for python 3.6 on linux.

conda uninstall -y --force pillow libjpeg-turbo
conda install -c fastai/label/test pillow-simd

It probably won’t work on your setup unless its CPU has the same capability as the one it was built on (Intel). So if it doesn’t work, install pillow-simd from source instead.

Note that pillow-simd will get overwritten by pillow through update/install of any other package depending on pillow. You can fool pillow-simd into believing it is pillow and then it’ll not get wiped out. You will have to make a local build for that.

If you have problems with these experimental packages please post here, including the output of python -m fastai.utils.check_perf and python -m fastai.utils.show_install and the exact problem/errors you encountered.

GPU Performance

See GPU Memory Notes.