Question
Zsh Array Slicing: Memory Overhead During Large Dataset Pagination
Dev Indigo
0 reputation · 24 Dec 2020, 06:55 UTC
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0 reputation · 24 Dec 2020, 06:55 UTC
Zsh 5.x provides native array slicing via the array[start,end] syntax, which allows for bounding datasets returned by command substitutions without relying on external utilities like less. This mechanism is often used to implement windowed data views within shell scripts.
When managing exceptionally large datasets stored in memory, there is a potential performance trade-off between using standard parameter expansion and the zsh/parameter module for slicing. Specifically, it is unclear how the shell handles memory allocation when creating slices of arrays that contain hundreds of thousands of elements, especially when combined with glob qualifiers for filtering.
Zsh array slicing (arr[start,end] or ${arr[@]:offset:length}) does not create a view; it builds a new array by copying the selected elements. Each slice therefore allocates memory roughly equal to the total size of the copied elements.
When you repeatedly slice a large array to implement pagination, the shell allocates a fresh array for every page. If the array holds hundreds of thousands of elements, the cumulative RSS can grow linearly with the number of pages times the page size, which may become noticeable compared to streaming the data through an external paginator.
The manual states that parameter expansions like ${arr[@]:offset:length} or arr[offset+1,length] produce a new array; the original array is left unchanged and the result is a separate copy, not a reference or view.
zshpage=(${arr[$i,$((i+99))]}).ps -o rss= -p $$ or /usr/bin/time -v.page size × average element size.Instead of materialising a slice, access elements by index:
for (( i = start; i <= end; i++ )); do
elem=${arr[i]}
# process $elem
doneThis keeps memory usage constant because no new array is allocated.
If your array elements are very large (e.g., multi‑megabyte strings), the per‑slice overhead scales with the total byte size of the slice. Knowing the average element size would let you estimate the exact memory cost, but the recommendation to use index‑based access remains unchanged regardless of element size.
Use comments to ask for clarification. Post a solution as an answer.
32,775 reputation · 24 Dec 2020, 11:52 UTC
Even when you immediately unset a sliced array, the memory used for that slice may not be returned to the operating system right away. Most malloc implementations keep freed blocks in their internal pools for reuse, so the resident set size (RSS) observed with ps -o rss= can stay elevated across many pagination iterations, giving the impression of a leak.
This effect is independent of how the slice is created: arr[start,end], ${arr[@]:offset:length}, or assigning via typeset -a slice=${arr[@]:offset:length} all result in a fresh copy of the selected elements. The same holds for negative indices (e.g., arr[-10,-1]) and for associative arrays – slicing the values still copies the scalar data.
If you need to keep memory usage truly constant, avoid materialising a slice altogether. Iterating over indices with a for (( i = start; i <= end; i++ )) loop or using while read -r line; do ... done < <(printf '%s\n' "${arr[@]}") (where the process substitution reads the original array without copying) prevents the allocator from allocating new blocks for each page.