Question
Memory allocation and mutation consistency limits of sequtils.chunks pagination
Lax Vale
0 reputation · 02 Apr 2024, 12:37 UTC
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0 reputation · 02 Apr 2024, 12:37 UTC
Nim's sequtils.chunks procedure provides a documented way to bound an in-memory sequence into fixed-size sub-sequences, which can act as a simple pagination layer when the dataset fits in RAM. For sequences that approach available heap, however, the practical boundaries of this approach remain unclear.
Because each chunk produced by chunks is a new sequence, repeatedly paginating a very large seq may generate many allocations and increase GC pressure. The standard library also does not appear to document whether mutating the source sequence during chunk iteration is supported, or what consistency guarantees apply if elements are added or removed mid-iteration.
Is there a documented allocation or size threshold beyond which chunks becomes unsuitable for pagination? What consistency guarantees exist when the source sequence is mutated during iteration? Are there standard-library patterns that stream bounded results without materializing each chunk as a separate sequence?
35775 reputation · 02 Apr 2024, 17:03 UTC
There is no documented size or element‑count threshold that makes sequtils.chunks unsuitable for pagination; suitability depends on the available heap and GC tuning. Mutating the source sequence while iterating yields undefined behavior and does not preserve slice validity. For streaming bounded results without materializing separate chunks, use manual index slicing (seq[offset:offset+size]) or itertools.take/drop.
nimchunks allocates a new seq header; total allocation cost grows with the number of chunks, not with the total data size.itertools.take/drop to produce bounded views without new seq objects.Because chunks creates a fresh sequence header for every slice, the number of allocations scales with the number of chunks. In a memory‑constrained environment this can cause high allocation rates, leading to more frequent GC cycles and potential pause times. The lack of documented thresholds means developers must rely on empirical measurements rather than language guarantees.
Please confirm which Nim compiler version you are using and the GC settings in place; this information will let us tailor the recommendation.
Use comments to ask for clarification. Post a solution as an answer.
35,775 reputation · 02 Apr 2024, 16:06 UTC
The standard library does not expose a documented size threshold for sequtils.chunks; the practical limit is the transient 2× heap spike when the outer seq[seq[T]] and all inner chunks are allocated simultaneously. For in-RAM pagination that avoids this spike entirely, Nim ≥1.4 treats src[i .. j] as an openArray view (no copy) as long as the source seq is not resized. The idiomatic pattern is:
for i in countup(0, src.len - 1, pageSize):
let page = src[i .. min(i + pageSize - 1, src.high)]
process(page) # page is an openArray[T], zero allocation
This keeps memory flat and works with any proc process(x: openArray[T]). Caveat: any add, setLen, or newSeq on src invalidates all outstanding slices, so the source must remain stable for the duration of the loop.