我在heroku日志中看到很多超时警告，导致问题的原因是什么？以及如何解决此问题。
这里是日志：
» 23 Jun 2016 18:48:32.402 190 <45>1 2016-06-23T13:18:32.004373+00:00 heroku web.1 - - source=web.1 dyno=heroku.18360441.8ab71785-b4e8-4599-80f7-97b8011028f8 sample#load_avg_1m=0.00
» 23 Jun 2016 18:48:32.488 367 <45>1 2016-06-23T13:18:32.004373+00:00 heroku web.1 - - source=web.1 dyno=heroku.18360441.8ab71785-b4e8-4599-80f7-97b8011028f8 sample#memory_total=159.71MB sample#memory_rss=159.69MB sample#memory_cache=0.02MB sample#memory_swap=0.00MB sample#memory_pgpgin=41822pages sample#memory_pgpgout=937pages sample#memory_quota=512.00MB
» 23 Jun 2016 18:48:33.312 134 <45>1 2016-06-23T13:18:33.079856+00:00 heroku worker.1 - - Stopping all processes with SIGTERM
» 23 Jun 2016 18:48:41.123 196 <45>1 2016-06-23T13:18:40.812500+00:00 heroku worker.1 - - source=worker.1 dyno=heroku.18360441.dacfe326-ce70-4c44-89c5-37f2cf9643fe sample#load_avg_1m=0.35
» 23 Jun 2016 18:48:41.221 374 <45>1 2016-06-23T13:18:40.812619+00:00 heroku worker.1 - - source=worker.1 dyno=heroku.18360441.dacfe326-ce70-4c44-89c5-37f2cf9643fe sample#memory_total=162.64MB sample#memory_rss=162.64MB sample#memory_cache=0.00MB sample#memory_swap=0.00MB sample#memory_pgpgin=45774pages sample#memory_pgpgout=4139pages sample#memory_quota=512.00MB
» 23 Jun 2016 18:48:53.503 377 <45>1 2016-06-23T13:18:53.136240+00:00 heroku worker.1 - - source=worker.1 dyno=heroku.18360441.2d4cce2d-6ded-43b0-830d-f182a0251f35 sample#memory_total=299.26MB sample#memory_rss=292.47MB sample#memory_cache=4.30MB sample#memory_swap=2.49MB sample#memory_pgpgin=423417pages sample#memory_pgpgout=397522pages sample#memory_quota=512.00MB
» 23 Jun 2016 18:48:53.503 245 <45>1 2016-06-23T13:18:53.136240+00:00 heroku worker.1 - - source=worker.1 dyno=heroku.18360441.2d4cce2d-6ded-43b0-830d-f182a0251f35 sample#load_avg_1m=0.00 sample#load_avg_5m=0.00 sample#load_avg_15m=0.00
» 23 Jun 2016 18:48:54.817 190 <45>1 2016-06-23T13:18:54.528272+00:00 heroku web.1 - - source=web.1 dyno=heroku.18360441.8ab71785-b4e8-4599-80f7-97b8011028f8 sample#load_avg_1m=0.00
» 23 Jun 2016 18:48:54.856 367 <45>1 2016-06-23T13:18:54.528368+00:00 heroku web.1 - - source=web.1 dyno=heroku.18360441.8ab71785-b4e8-4599-80f7-97b8011028f8 sample#memory_total=159.86MB sample#memory_rss=159.84MB sample#memory_cache=0.02MB sample#memory_swap=0.00MB sample#memory_pgpgin=41861pages sample#memory_pgpgout=938pages sample#memory_quota=512.00MB
» 23 Jun 2016 18:49:01.790 140 <45>1 2016-06-23T13:19:01.740975+00:00 heroku worker.1 - - Stopping remaining processes with SIGKILL
» 23 Jun 2016 18:49:02.085 191 <45>1 2016-06-23T13:19:01.740975+00:00 **heroku worker.1 - - Error R12 (Exit timeout) -> At least one process failed to exit within 30 seconds of SIGTERM Warning Context**
» 23 Jun 2016 18:49:02.714 374 <45>1 2016-06-23T13:19:02.467061+00:00 heroku worker.1 - - source=worker.1 dyno=heroku.18360441.dacfe326-ce70-4c44-89c5-37f2cf9643fe sample#memory_total=163.35MB sample#memory_rss=163.35MB sample#memory_cache=0.00MB sample#memory_swap=0.00MB sample#memory_pgpgin=45956pages sample#memory_pgpgout=4139pages sample#memory_quota=512.00MB
» 23 Jun 2016 18:49:02.741 196 <45>1 2016-06-23T13:19:02.466774+00:00 heroku worker.1 - - source=worker.1 dyno=heroku.18360441.dacfe326-ce70-4c44-89c5-37f2cf9643fe sample#load_avg_1m=0.24
» 23 Jun 2016 18:49:03.415 129 <45>1 2016-06-23T13:19:03.102528+00:00 heroku worker.1 - - Process exited with status 137