postgresql log indicate too many execute S_2:COMMIT

Hello:
    I test postgresql database performance recently,i set
log_min_duration_statement=1000;
i monitor pg_log file,i find too many execue S_2:commit
10.0.44.21:30170:2012-04-28 11:38:46.340 CSTLOG:  duration: 1050.679 ms
execute S_2: COMMIT

10.0.44.21:30208:2012-04-28 11:38:46.340 CSTLOG:  duration: 1050.576 ms
execute S_2: COMMIT

10.0.44.21:30318:2012-04-28 11:38:46.340 CSTLOG:  duration: 1028.171 ms
execute S_2: COMMIT

........

please explain what meaning,how to cause it ,how to deal with it?thank you !

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