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The mmap() copy-on-write trick: reducing memory usage of array copies


Let’s voice that it’s possible you’ll per chance furthermore have gotten an array, and it be a must to invent some copies and adjust these copies.
In overall, memory utilization scales with the replacement of copies: if your fashioned array became once 1GB of RAM, every copy will protect 1GB of RAM.
And that can add up.

But continuously, you’re proper changing a shrimp fragment of the array.
Ideally, the memory cost would totally be the parts of the copies that you just changed.

As it appears, there is an working system facility that allows this: mmap()’s copy-on-write functionality.

Listed right here that it’s possible you’ll per chance study:

  1. How standard memory copies work.
  2. Systems to make advise of mmap() copy-on-write with NumPy.
  3. How the underlying mmap() copy-on-write mechanism works, and why it will furthermore be more efficient.

The predicament with copying

Whenever you happen to’d opt on to adjust a copy of an array, the typical methodology is to allocate more memory and copy the contents of the authentic array into the contemporary chunk of memory.
For instance:

>>> import numpy, psutil
>>> def memory_usage(): 
...     current_process = psutil.Task()
...     memory = current_process.memory_info().rss
...     print(int(memory / (1024 * 1024)), "MB")
>>> array1 = numpy.ones((1024, 1024, 50))
>>> memory_usage()
428 MB
>>> array2 = array1.copy()
>>> memory_usage()
827 MB

In visual beget, the allocated memory appears to be luxuriate in this:

Gcluster_array1Array 1cluster_array2Array 2page1Net page 1page2Net page 2page3Net page 3page4page1bNet page 1page2bNet page 2page3bNet page 3page4b

The pages are chunks of 4KB that are the unit of memory administration for the working system.

Saving memory with copy-on-write

In an supreme world, that 2nd array would totally store the diversifications from the principle array: insofar as differences are few, the further memory utilization could per chance per chance per chance be shrimp.
And that’s where mmap()’s copy-on-write functionality comes in (or the the same API on Windows; NumPy wraps them both).

Whenever you happen to’re no longer aware of mmap(), peep my overview evaluating mmap() with HDF5 and Zarr.

To advise mmap() in this mode, we could per chance per chance like a backing file.
Whereas there is a file involved, so lengthy as there’s enough memory available the file is form of an implementation detail; it wants to be there but it gained’t impact efficiency considerable.

Command: On Linux that it’s possible you’ll per chance furthermore dawdle one step further and beget an in-memory file the advise of the memfd_create API, that could per chance per chance furthermore be mature in Python 3.8 and later by doing os.fdopen(os.memfd_create("mymemfile"), "rb+") and then “truncating” the file to be the right dimension.

The numpy.lib.structure.open_memmap() characteristic will launch a file of the right dimension; we’ll launch by increasing our preliminary array:

>>> del array1, array2
>>> memory_usage()
20 MB
>>> open_memmap = numpy.lib.structure.open_memmap
>>> mmap_array1 = open_memmap("/tmp/myarray", mode="w+", form=(1024, 1024, 50))
>>> memory_usage()
22 MB
>>> mmap_array1[:] = 1
>>> mmap_array1[0] = 10
>>> memory_usage()
422 MB

At the delivery the array is proper zeroes (at least on Linux and macOS; Windows could per chance per chance furthermore fluctuate), so the working system is artful enough no longer to allocate any contemporary memory.
When we place some values, memory utilization goes up accordingly.

Subsequent, let’s beget a copy: we’ll mmap() the identical file with mode="c", that contrivance copy-on-write.
On Unix methods luxuriate in Linux or macOS, this interprets to the MAP_PRIVATE flag to the mmap() API.

>>> mmap_array2 = open_memmap("/tmp/myarray", mode="c", form=(1024, 1024, 50))
>>> mmap_array2[0, 0, 0]
>>> mmap_array2[10, 0, 1]
>>> memory_usage()
422 MB

We have now one other copy of the array, with the identical contents… but memory utilization hasn’t changed!

Now let’s adjust that 2nd array, and we’ll peep how memory utilization goes up, however the authentic array is unchanged.

>>> mmap_array2[1:100] = 30
>>> memory_usage()
461 MB
>>> mmap_array1[1, 0, 0]

Now we have efficiently made a copy of an array that:

  1. Doesn’t commerce the authentic array when mutated.
  2. Simplest stores these parts of the copy that have changed from the authentic, allowing us to place memory.

How copy-on-write works

When we mmap() a file with the MAP_PRIVATE flag, right here’s what occurs per the manpage:

Get a non-public copy-on-write mapping.  Updates to the
mapping are no longer visible to other processes mapping the identical
file, and are no longer carried by to the underlying file. It
is unspecified whether or no longer changes made to the file after the
mmap() name are visible within the mapped location.

Stare that changes made to the file could per chance per chance furthermore or could per chance per chance furthermore no longer be visible, that behavior is unspecified.
This ability that, it’s totally no longer to adjust the authentic array.

Returning to our function, we’re saving memory by the advise of copy-on-write.
Which contrivance pages within the 2nd array demonstrate the principle array till some commerce is made to them.
Simplest can have to you write to the obtain page does a copy get made and the writes applied.

At the delivery we mmap()ed /tmp/myarray with MAP_PRIVATE (by the advise of mode="c"), and memory seemed luxuriate in this:

Gcluster_array1Array 1cluster_array2Array 2page1Net page 1page2Net page 2page3Net page 3page4page1bNet page 1page1b->page1page2bNet page 2page2b->page2page3bNet page 3page3b->page3page4bpage4b->page4

That is, we had one other array, but no further memory became once mature.

Then, we made some changes to fragment of the 2nd array.
These pages that were modified get copied, and then modified—the relaxation smooth demonstrate the authentic array.
For instance, if we modified some recordsdata within the principle 4096 bytes within the array’s in-memory illustration, a brand contemporary online page could per chance per chance per chance be allocated that’s a copy of the one within the principle array:

Gcluster_array1Array 1cluster_array2Array 2page1Net page 1page2Net page 2page3Net page 3page4page1bNet page 1page2bNet page 2page2b->page2page3bNet page 3page3b->page3page4bpage4b->page4

Paying totally for what you commerce

The mmap() copy-on-write trick is precious when:

  1. You can have gotten a extraordinarily large array.
  2. You can furthermore very effectively be making copies and totally partially improving these copies.

On this boom, copy-on-write saves memory by totally allocating memory for recordsdata that has undoubtedly changed.
Valid be sure that that no longer to adjust the authentic array; that it’s possible you’ll per chance furthermore have surprising penalties reckoning on your working system.

For other recordsdata structures, luxuriate in dictionaries or lists, that it’s possible you’ll per chance furthermore advise immutable datastructures to lower memory utilization of largely-identical copies; in Python the pyrsistent library is one implementation.

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