asyn leads to loss of the logs contens

[wanzhen]

i create a spring boot project, based on the spring mvc. every request, print 1 logs. i send request with jmeter, 100 threads, cycle 1000times every thread。 i should get 100*1000 = 100000 logs, bug i get 86000logs. if i use the synchronization mode，i can get 100000logs。 why？