On Tue, Jul 04, 2017 at 12:13:09PM +0200, Ingo Molnar wrote: > > This code on the other hand: > > sa->last_update_time += delta << 10; > > ... in essence creates a whole new absolute clock value that slowly but > surely is > drifting away from the real rq->clock, because 'delta' is always rounded down > to > the nearest 1024 ns boundary, so we accumulate the 'remainder' losses. > > That is because: > > delta >>= 10; > ... > sa->last_update_time += delta << 10; > > Given enough time, ->last_update_time can drift a long way, and this delta: > > delta = now - sa->last_update_time; > > ... becomes meaningless AFAICS, because it's essentially two different clocks > that > get compared.
Thing is, once you drift over 1023 (ns) your delta increases and you catch up again. A B C D E F | | | | | | +----+----+----+----+----+----+----+----+----+----+----+ A: now = 0 sa->last_update_time = 0 delta := (now - sa->last_update_time) >> 10 = 0 B: now = 614 (+614) delta = (614 - 0) >> 10 = 0 sa->last_update_time += 0 (0) sa->last_update_time = now & ~1023 (0) C: now = 1843 (+1229) delta = (1843 - 0) >> 10 = 1 sa->last_update_time += 1024 (1024) sa->last_update_time = now & ~1023 (1024) D: now = 3481 (+1638) delta = (3481 - 1024) >> 10 = 2 sa->last_update_time += 2048 (3072) sa->last_update_time = now & ~1023 (3072) E: now = 5734 (+2253) delta = (5734 - 3072) = 2 sa->last_update_time += 2048 (5120) sa->last_update_time = now & ~1023 (5120) F: now = 6348 (+614) delta = (6348 - 5120) >> 10 = 1 sa->last_update_time += 1024 (6144) sa->last_update_time = now & ~1023 (6144) And you'll see that both are identical, and that both D and F have gotten a spill from sub-chunk accounting.
