Each blob in a repository requires one index entry in memory, which is one
of the largest contributors to restic's memory usage. The indexEntry.packIndex
field, an index into the per-Index packs slice, was stored as a machine-word
int (8 bytes on amd64) although the value is always a small array index.
Store it as a uint32 instead. This shrinks indexEntry from 64 to 56 bytes on
64-bit platforms (56 % 8 == 0, so no padding is added), reducing index memory
by roughly 12% -- about 100 MB for a repository with ~13 million blobs, as
confirmed by profiling. uint32 allows 4.29 billion packs (>30 billion blobs at
the assumed 8 blobs/pack), far beyond any real repository; addToPacks guards
against overflow defensively. The on-disk repository format is unaffected.
Matthias Dötsch
e428de5f84