db: mergingIter.heap.items recycling does not work

[nvanbenschoten]

There seems to be an effort made to recycle the mergingIter.heap.items slice here:

pebble/merging_iter.go

Lines 317 to 321 in 56834cb

	if cap(m.heap.items) < len(levels) {
	m.heap.items = make([]*mergingIterLevel, 0, len(levels))
	} else {
	m.heap.items = m.heap.items[:0]
	}

This doesn't play well with the iterAlloc memory pooling:

pebble/db.go

Lines 971 to 980 in 56834cb

	type iterAlloc struct {
	dbi Iterator
	keyBuf []byte
	boundsBuf [2][]byte
	prefixOrFullSeekKey []byte
	merging mergingIter
	mlevels [3 + numLevels]mergingIterLevel
	levels [3 + numLevels]levelIter
	levelsPositioned [3 + numLevels]bool
	}

That's because the mergingIter field is cleared before these structs are placed back into the iterAllocPool:

pebble/iterator.go

Lines 2330 to 2335 in 56834cb

	*alloc = iterAlloc{
	keyBuf: alloc.keyBuf,
	boundsBuf: alloc.boundsBuf,
	prefixOrFullSeekKey: alloc.prefixOrFullSeekKey,
	}
	iterAllocPool.Put(alloc)

As a result, I see 1 memory allocation in each call to (*DB).NewIterWithContext:

This may be working as intended. If so, feel free to close.