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And if yes, are they ordered by the time the request was received (%t) or the time the request was fulfilled (%t+%T)?

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up vote 2 down vote accepted

No. They are ordered by %t+%T so you may have one line A appearing after another line B but A has an earlier %t than B.

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Further question: are they always ordered by %t+%T, even under heavy load with a lot of worker processes/threads? – netvope Dec 30 '10 at 5:02
@netvope No, they are not guaranteed to be ordered by by %t+%T (they just usually are). The time calculation and log writing are not protected by an exclusive and atomic locking mechanism so a context switch may happen. N.B. under apache 1.3 log lines were more likely to in in the "correct" order. You can get the same effect in 2.x by compiling with I_INSIST_ON_EXTRA_CYCLES_FOR_CLF_COMPLIANCE defined. However, there is still a chance for out-of-order lines. – Mark Wagner Dec 30 '10 at 18:08

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