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a6275648b3
When searching in parallel with many more arguments than threads, the
first arguments are searched last -- unlike in the -j1 case.
This is unexpected for users who know about the parallel nature of rg
and think they can give the scheduler a hint by positioning larger
input files (L1, L2, ..) before smaller ones (█, ██). Instead, this can
result in sub-optimal thread usage and thus longer runtime (simplified
example with 2 threads):
T1: █ ██ █ █ █ █ ██ █ █ █ █ █ ██ ╠═════════════L1════════════╣
T2: █ █ ██ █ █ ██ █ █ █ ██ █ █ ╠═════L2════╣
┏━━━━┳━━━━┳━━━━┳━━━━┓
This is caused by assigning work to ┃ T1 ┃ T2 ┃ T3 ┃ T4 ┃
per-thread stacks in a round-robin ┡━━━━╇━━━━╇━━━━╇━━━━┩
manner, starting here → │ L1 │ L2 │ L3 │ L4 │ ↵
├────├────┼────┼────┤
│ s5 │ s6 │ s7 │ s8 │ ↵
├────┼────┼────┼────┤
╷ .. ╷ .. ╷ .. ╷ .. ╷
├────┼────┼────┼────┤
│ st │ su │ sv │ sw │ ↵
├────┼────┼────┼────┘
│ sx │ sy │ sz │
└────┴────┴────┘
and then processing them bottom-up: ↥ ↥ ↥ ↥
╷ .. ╷ .. ╷ .. ╷ .. ╷
This patch reverses the input order ├────┼────┼────┼────┤
so the two reversals cancel each other │ s7 │ s6 │ s5 │ L4 │ ↵
out. Now at least the first N ├────┼────┼────┼────┘
arguments, N=number-of-threads, are │ L3 │ L2 │ L1 │
processed before any others (then └────┴────┴────┘
work-stealing may happen):
T1: ╠═════════════L1════════════╣ █ ██ █ █ █ █ █ █ ██
T2: ╠═════L2════╣ █ █ ██ █ █ ██ █ █ █ ██ █ █ ██ █ █ █
(With some more shuffling T1 could always be assigned L1 etc., but
that would mostly be for optics).
Closes #2849