tree 5cd5c567da7275f959ea4babd21be7cbd1f2b63a
parent 7649aacf7f17410b88f258b2b2eddea9e8445922
author Willy Tarreau <w@1wt.eu> 1511445148 +0100
committer Willy Tarreau <w@1wt.eu> 1511451152 +0100
encoding latin1

BUG/MEDIUM: threads/time: maintain a common time reference between all threads

During high loads it becomes visible that the time drifts between threads,
sometimes showing tens of seconds after several minutes. The root cause is
the per-thread correction which is performed based on a local offset and
local time. But we can't use a unique global time either as we need the
thread-local time to be stable between two poll() calls.

This commit takes a stab at this problem by proceeding this way :

  - a global "global_now" date is monotonous and common between all threads.
  - each thread has its own local <now> which is resynced with <global_now>
    on each invocation of tv_update_date()
  - each thread detects its own drift based on its poll() timeout and its
    local <now>, and recalculates its adjusted local time
  - each thread then ensures its new local time is no older than the current
    global time, otherwise it readjusts its local time to match this one
  - finally threads do atomically update the global time to match its own
    local one

This guarantees a monotonous global time and a monotonous+stable local time.

It is still possible by definition for two threads to report a minor time
variation on subsequent events but that variation will only be caused by
the moment they watched the time and are very small. When a common global
time is needed between all threads, global_now could be used as a reference
(with care). The wallclock time used in logs is still <date> anyway.