By default xlsxwriter
holds all cell data in memory. This is to allow future
features where formatting is applied separately from the data.
The effect of this is that for large files xlsxwriter
can consume a lot of memory and it is
even possible to run out of memory.
Fortunately, this memory usage can be reduced almost completely by setting the
Workbook:new() 'constant_memory'
property:
workbook = Workbook:new(filename, {constant_memory = true})
The optimisation works by flushing each row after a subsequent row is written. In this way the largest amount of data held in memory for a worksheet is the amount of memory required to hold a single row of data.
Since each new row flushes the previous row, data must be written in sequential
row order when 'constant_memory'
mode is on:
-- With 'constant_memory' you must write data in row column order.
for row = 0, row_max do
for col = 0, col_max do
worksheet:write(row, col, some_data)
end
end
-- With 'constant_memory' the following would only write the first column.
for col = 0, col_max do -- !!
for row = 0, row_max do
worksheet:write(row, col, some_data)
end
end
Another optimisation that is used to reduce memory usage is that cell strings aren’t stored in an Excel structure call “shared strings” and instead are written “in-line”. This is a documented Excel feature that is supported by most spreadsheet applications. One known exception is Apple Numbers for Mac where the string data isn’t displayed.
The trade-off when using 'constant_memory'
mode is that you won’t be able
to take advantage of any features that manipulate cell data after it is
written. Currently there aren’t any such features.
For larger files 'constant_memory'
mode also gives an increase in execution
speed, see below.
The performance figures below show execution time and memory usage for
worksheets of size N
rows x 50 columns with a 50/50 mixture of strings and
numbers. The figures are taken from an arbitrary, mid-range, machine. Specific
figures will vary from machine to machine but the trends should be the same.
Xlsxwriter in normal operation mode: the execution time and memory usage increase more of less linearly with the number of rows:
Rows | Columns | Time (s) | Memory (bytes) |
---|---|---|---|
200 | 50 | 0.20 | 2071819 |
400 | 50 | 0.40 | 4149803 |
800 | 50 | 0.86 | 8305771 |
1600 | 50 | 1.87 | 16617707 |
3200 | 50 | 3.84 | 33271579 |
6400 | 50 | 8.02 | 66599323 |
12800 | 50 | 16.54 | 133254811 |
Xlsxwriter in constant_memory
mode: the execution time still increases
linearly with the number of rows but the memory usage remains small and
mainly constant:
Rows | Columns | Time (s) | Memory (bytes) |
---|---|---|---|
200 | 50 | 0.18 | 41119 |
400 | 50 | 0.36 | 24735 |
800 | 50 | 0.69 | 24735 |
1600 | 50 | 1.41 | 24735 |
3200 | 50 | 2.83 | 41119 |
6400 | 50 | 5.83 | 41119 |
12800 | 50 | 11.29 | 24735 |
These figures were generated using the perf_tester.lua
program in the
examples
directory of the xlsxwriter repo.
Note, there will be further optimisation in both modes in later releases.