Configuration Settings and Compiling Modes

Configuration

The pytensor.config module contains several attributes that modify PyTensor’s behavior. Many of these attributes are examined during the import of the pytensor module and several are assumed to be read-only.

As a rule, the attributes in the pytensor.config module should not be modified inside the user code.

PyTensor’s code comes with default values for these attributes, but you can override them from your .pytensorrc file, and override those values in turn by the PYTENSOR_FLAGS environment variable.

The order of precedence is:

1. an assignment to pytensor.config.<property>

2. an assignment in PYTENSOR_FLAGS

3. an assignment in the .pytensorrc file (or the file indicated in PYTENSORRC)

You can display the current/effective configuration at any time by printing pytensor.config. For example, to see a list of all active configuration variables, type this from the command-line:

python -c 'import pytensor; print(pytensor.config)' | less

For more detail, see Configuration in the library.

Exercise

Consider the logistic regression:

import numpy as np
import pytensor
import pytensor.tensor as at


rng = np.random.default_rng(2498)

N = 400
feats = 784
D = (rng.standard_normal((N, feats)).astype(pytensor.config.floatX),
rng.integers(size=N,low=0, high=2).astype(pytensor.config.floatX))
training_steps = 10000

# Declare PyTensor symbolic variables
x = at.matrix("x")
y = at.vector("y")
w = pytensor.shared(rng.standard_normal(feats).astype(pytensor.config.floatX), name="w")
b = pytensor.shared(np.asarray(0., dtype=pytensor.config.floatX), name="b")
x.tag.test_value = D[0]
y.tag.test_value = D[1]

# Construct PyTensor expression graph
p_1 = 1 / (1 + at.exp(-at.dot(x, w)-b)) # Probability of having a one
prediction = p_1 > 0.5 # The prediction that is done: 0 or 1
xent = -y*at.log(p_1) - (1-y)*at.log(1-p_1) # Cross-entropy
cost = xent.mean() + 0.01*(w**2).sum() # The cost to optimize
gw,gb = at.grad(cost, [w,b])

# Compile expressions to functions
train = pytensor.function(
    inputs=[x,y],
    outputs=[prediction, xent],
    updates=[(w, w-0.01*gw), (b, b-0.01*gb)],
    name = "train"
)
predict = pytensor.function(
    inputs=[x], outputs=prediction,
    name = "predict"
)

if any(x.op.__class__.__name__ in ['Gemv', 'CGemv', 'Gemm', 'CGemm']
       for x in train.maker.fgraph.toposort()):
    print('Used the cpu')
else:
    print('ERROR, not able to tell if pytensor used the cpu or another device')
    print(train.maker.fgraph.toposort())

for i in range(training_steps):
    pred, err = train(D[0], D[1])

print("target values for D")
print(D[1])

print("prediction on D")
print(predict(D[0]))

Modify and execute this example to run on CPU (the default) with floatX=float32 and time the execution using the command line time python file.py. Save your code as it will be useful later on.

Note

• Apply the PyTensor flag floatX=float32 (through pytensor.config.floatX) in your code.

• Cast inputs before storing them into a shared variable.

• Circumvent the automatic cast of int32 with float32 to float64:

  • Insert manual cast in your code or use [u]int{8,16}.

  • Insert manual cast around the mean operator (this involves division by length, which is an int64).

  • Note that a new casting mechanism is being developed.

Solution

---

Default Modes

Every time pytensor.function is called, the symbolic relationships between the input and output PyTensor variables are rewritten and compiled. The way this compilation occurs is controlled by the value of the mode parameter.

PyTensor defines the following modes by name:

• 'FAST_COMPILE': Apply just a few graph optimizations and only use Python implementations.

• 'FAST_RUN': Apply all optimizations and use C implementations where possible.

• 'DebugMode': Verify the correctness of all optimizations, and compare C and Python

  implementations. This mode can take much longer than the other modes, but can identify several kinds of problems.

• 'NanGuardMode': Same optimization as FAST_RUN, but check if a node generate nans.

The default mode is typically FAST_RUN, but it can be controlled via the configuration variable config.mode, which can be overridden by passing the keyword argument to pytensor.function.

short name	Full constructor	What does it do?
FAST_COMPILE	compile.mode.Mode(linker='py', optimizer='fast_compile')	Python implementations only, quick and cheap graph transformations
FAST_RUN	compile.mode.Mode(linker='cvm', optimizer='fast_run')	C implementations where available, all available graph transformations.
DebugMode	compile.debugmode.DebugMode()	Both implementations where available, all available graph transformations.

Note

For debugging purpose, there also exists a MonitorMode (which has no short name). It can be used to step through the execution of a function: see the debugging FAQ for details.

Default Linkers

A Mode object is composed of two things: an optimizer and a linker. Some modes, like NanGuardMode and DebugMode, add logic around the optimizer and linker. DebugMode uses its own linker.

You can select which linker to use with the PyTensor flag config.linker. Here is a table to compare the different linkers.

linker	gc [1]	Raise error by op	Overhead	Definition
cvm	yes	yes	“++”	As c|py, but the runtime algo to execute the code is in c
cvm_nogc	no	yes	“+”	As cvm, but without gc
c|py [2]	yes	yes	“+++”	Try C code. If none exists for an op, use Python
c|py_nogc	no	yes	“++”	As c|py, but without gc
c	no	yes	“+”	Use only C code (if none available for an op, raise an error)
py	yes	yes	“+++”	Use only Python code
NanGuardMode	yes	yes	“++++”	Check if nodes generate NaN
DebugMode	no	yes	VERY HIGH	Make many checks on what PyTensor computes

[1]

Garbage collection of intermediate results during computation. Otherwise, their memory space used by the ops is kept between PyTensor function calls, in order not to reallocate memory, and lower the overhead (make it faster…).

[2]

Default

For more detail, see Mode in the library.

Default Optimizers

PyTensor allows compilations with a number of predefined rewrites that are expected to improve graph evaluation performance on average. An optimizer is technically just a Rewriter, or an object that indicates a particular set of rewrites (e.g. a string used to query optdb for a Rewriter).

The optimizers PyTensor provides are summarized below to indicate the trade-offs one might make between compilation time and execution time.

These optimizers can be enabled globally with the PyTensor flag: optimizer=name or per call to pytensor functions with function(...mode=Mode(optimizer="name")).

optimizer	Compile time	Execution time	Description
None	“++++++”	“+”	Applies none of PyTensor’s rewrites
o1 (fast_compile)	“+++++”	“++”	Applies only basic rewrites
o2	“++++”	“+++”	Applies few basic rewrites and some that compile fast
o3	“+++”	“++++”	Applies all rewrites except ones that compile slower
o4 (fast_run)	“++”	“+++++”	Applies all rewrites
unsafe	“+”	“++++++”	Applies all rewrites, and removes safety checks
stabilize	“+++++”	“++”	Only applies stability rewrites

For a detailed list of the specific rewrites applied for each of these optimizers, see Optimizations. Also, see Unsafe Rewrites and Faster PyTensor Function Compilation for other trade-off.

Using DebugMode

While normally you should use the FAST_RUN or FAST_COMPILE mode, it is useful at first–especially when you are defining new kinds of expressions or new rewrites–to run your code using the DebugMode (available via mode='DebugMode). The DebugMode is designed to run several self-checks and assertions that can help diagnose possible programming errors leading to incorrect output. Note that DebugMode is much slower than FAST_RUN or FAST_COMPILE, so use it only during development.

DebugMode is used as follows:

x = at.dvector('x')

f = pytensor.function([x], 10 * x, mode='DebugMode')

f([5])
f([0])
f([7])

If any problem is detected, DebugMode will raise an exception according to what went wrong, either at call time (e.g. f(5)) or compile time ( f = pytensor.function(x, 10 * x, mode='DebugMode')). These exceptions should not be ignored; talk to your local PyTensor guru or email the users list if you cannot make the exception go away.

Some kinds of errors can only be detected for certain input value combinations. In the example above, there is no way to guarantee that a future call to, say f(-1), won’t cause a problem. DebugMode is not a silver bullet.

If you instantiate DebugMode using the constructor (see DebugMode) rather than the keyword DebugMode you can configure its behaviour via constructor arguments. The keyword version of DebugMode (which you get by using mode='DebugMode') is quite strict.

For more detail, see DebugMode in the library.