Working with big data¶
New in version 1.2.
HyperSpy makes it possible to analyse data larger than the available memory by
providing “lazy” versions of most of its signals and functions. In most cases
the syntax remains the same. This chapter describes how to work with data
larger than memory using the
LazySignal class and
Creating Lazy Signals¶
Lazy Signals from external data¶
If the data is large and not loaded by HyperSpy (for example a
or similar), first wrap it in
dask.array.Array as shown here and then pass it
as normal and call
>>> import h5py >>> f = h5py.File("myfile.hdf5") # Load the file >>> data = f['/data/path'] # Get the data >>> import dask.array as da # Import dask to wrap >>> chunks = (1000,100) # Chunk as appropriate >>> x = da.from_array(data, chunks=chunks) # Wrap the data in dask >>> s = hs.signals.Signal1D(x).as_lazy() # Create the lazy signal
To load the data lazily, pass the keyword
lazy=True. As an example,
loading a 34.9 GB
.blo file on a regular laptop might look like:
>>> s = hs.load("shish26.02-6.blo", lazy=True) >>> s <LazySignal2D, title: , dimensions: (400, 333|512, 512)> >>> s.data dask.array<array-e..., shape=(333, 400, 512, 512), dtype=uint8, chunksize=(20, 12, 512, 512)> >>> print(s.data.dtype, s.data.nbytes / 1e9) uint8 34.9175808 >>> s.change_dtype("float") # To be able to perform decomposition, etc. >>> print(s.data.dtype, s.data.nbytes / 1e9) float64 279.3406464
Loading the dataset in the original unsigned integer format would require around 35GB of memory. To store it in a floating-point format one would need almost 280GB of memory. However, with the lazy processing both of these steps are near-instantaneous and require very little computational resources.
Occasionally the full dataset consists of many smaller files. To combine them
into a one large
LazySignal, we can stack them
lazily (both when loading or afterwards):
>>> siglist = hs.load("*.hdf5") >>> s = hs.stack(siglist, lazy=True) >>> # Or load lazily and stack afterwards: >>> siglist = hs.load("*.hdf5", lazy=True) >>> s = hs.stack(siglist) # no need to pass 'lazy', as signals already lazy >>> # Or do everything in one go: >>> s = hs.load("*.hdf5", lazy=True, stack=True)
Casting signals as lazy¶
To convert a regular HyperSpy signal to a lazy one such that any future
operations are only performed lazily, use the
>>> s = hs.signals.Signal1D(np.arange(150.).reshape((3, 50))) >>> s <Signal1D, title: , dimensions: (3|50)> >>> sl = s.as_lazy() >>> sl <LazySignal1D, title: , dimensions: (3|50)>
Despite the limitations detailed below, most HyperSpy operations can be performed lazily. Important points of note are:
Computing lazy signals¶
Upon saving lazy signals, the result of computations is stored on disk.
In order to store the lazy signal in memory (i.e. make it a normal HyperSpy
signal) it has a
>>> s <LazySignal2D, title: , dimensions: (|512, 512)> >>> s.compute() [########################################] | 100% Completed | 0.1s >>> s <Signal2D, title: , dimensions: (|512, 512)>
Lazy operations that affect the axes¶
When using lazy signals the computation of the data is delayed until
requested. However, the changes to the axes properties are performed
when running a given function that modfies them i.e. they are not
performed lazily. This can lead to hard to debug issues when the result
of a given function that is computed lazily depends on the value of the
axes parameters that may have changed before the computation is requested.
Therefore, in order to avoid such issues, it is reccomended to explicitly
compute the result of all functions that are affected by the axes
paramters. This is the reason why e.g. the result of
shift1D() is not lazy.
Most operations can be performed lazily. However, lazy operations come with a few limitations and constraints that we detail below.
An important limitation when using
LazySignal is the inability to modify
existing data (immutability). This is a logical consequence of the DAG (tree
structure, explained in Behind the scenes –technical details), where a complete history of the
processing has to be stored to traverse later.
In fact, lazy evaluation removes the need for such operation, since only additional tree branches are added, requiring very little resources. In practical terms the following fails with lazy signals:
>>> s = hs.signals.BaseSignal().as_lazy() >>> s += 1 Traceback (most recent call last): File "<ipython-input-6-1bd1db4187be>", line 1, in <module> s += 1 File "<string>", line 2, in __iadd__ File "/home/fjd29/Python/hyperspy3/hyperspy/signal.py", line 1591, in _binary_operator_ruler getattr(self.data, op_name)(other) AttributeError: 'Array' object has no attribute '__iadd__'
However, when operating lazily there is no clear benefit to using in-place operations. So, the operation above could be rewritten as follows:
>>> s = hs.signals.BaseSignal().as_lazy() >>> s = s + 1
Or even better:
>>> s = hs.signals.BaseSignal().as_lazy() >>> s1 = s + 1
Machine learning (decomposition)¶
Decomposition algorithms often performs large matrix manipulations, requiring significantly more memory than the data size. To perform decomposition operation lazily HyperSpy provides several “online” algorithms. These algorithms perform the decomposition by operating serially on chunks of data, enabling the lazy decomposition of large datasets.
In line with the standard HyperSpy signals,
decomposition() offers the following
Other minor differences¶
- Histograms for a
LazySignaldo not support
- CircleROI sets the elements outside the ROI to
np.naninstead of using a masked array, because
daskdoes not support masking. As a convenience,
nan*signal methods were added to mimic the workflow as closely as possible.
Behind the scenes –technical details¶
Standard HyperSpy signals load the data into memory for fast access and processing. While this behaviour gives good performance in terms of speed, it obviously requires at least as much computer memory as the dataset, and often twice that to store the results of subsequent computations. This can become a significant problem when processing very large datasets on consumer-oriented hardware.
HyperSpy offers a solution for this problem by including
LazySignal and its derivatives. The main idea of
these classes is to perform any operation (as the name suggests)
lazily (delaying the
execution until the result is requested (e.g. saved, plotted)) and in a
blocked fashion. This is
achieved by building a “history tree” (formally called a Directed Acyclic Graph
(DAG)) of the computations, where the original data is at the root, and any
further operations branch from it. Only when a certain branch result is
requested, the way to the root is found and evaluated in the correct sequence
on the correct blocks.
The “magic” is performed by (for the sake of simplicity) storing the data not
dask.array.Array (more information here).
dask offers a couple of
- Arbitrary-sized data processing is possible. By only loading a couple of chunks at a time, theoretically any signal can be processed, albeit slower. In practice, this may be limited: (i) some operations may require certain chunking pattern, which may still saturate memory; (ii) many chunks should fit into the computer memory comfortably at the same time.
- Loading only the required data. If a certain part (chunk) of the data is not required for the final result, it will not be loaded at all, saving time and resources.
- Able to extend to a distributed computing environment (clusters).
dask.distributed(documentation here) offers a straightforward way to expand the effective memory for computations to that of a cluster, which allows performing the operations significantly faster than on a single machine.