[erlang-bugs] Scheduler thread spins in futex_wait and sched_yield

[Jebu Ittiachen]

Hi,
  I seem to have hit upon a weird bug in the Erlang scheduler. I'm running
R15B01 on Linux 64bit, Erlang compiled with HiPE disabled. Erlang starts up
with 4 scheduler threads and everything is ok for a while. After a period
of time the CPU usage drops on the machine and things start going slow. top
-H shows 2 threads of the 4 running at around 15% and the other 2 at 95%.
Typically all 4 threads are more or less in the same CPU utilization
figures. strace on the process shows the two sluggish threads alternating
between calls to futex_wait and sched_yield while the other two are doing a
lot of other stuff.

  Here is a sample of strace -f -p <pid> |grep <thread id>

20292 sched_yield( <unfinished ...>
20292 <... sched_yield resumed> )       = 0
20292 sched_yield( <unfinished ...>
20292 <... sched_yield resumed> )       = 0
20292 sched_yield()                     = 0
20292 sched_yield()                     = 0
20292 sched_yield()                     = 0
20292 sched_yield()                     = 0
20292 sched_yield()                     = 0
20292 sched_yield()                     = 0
20292 sched_yield()                     = 0
20292 sched_yield()                     = 0
20292 sched_yield()                     = 0
20292 sched_yield()                     = 0
20292 futex(0x1bf2220, FUTEX_WAIT_PRIVATE, 4294967295, NULL <unfinished ...>
20292 <... futex resumed> )             = 0
20292 sched_yield()                     = 0
20292 sched_yield()                     = 0
20292 sched_yield()                     = 0
20292 sched_yield()                     = 0
20292 sched_yield()                     = 0
20292 sched_yield()                     = 0
20292 sched_yield()                     = 0
20292 sched_yield()                     = 0
20292 sched_yield()                     = 0
20292 sched_yield()                     = 0

  My only option out of this now is to restart the node, when it again runs
happily for a while before scheduler threads start dropping off. I'd be
happy to provide any more dumps/info that maybe needed to get to the bottom
of this.

Thanks
--
Jebu Ittiachen
jebu.ittiachen@REDACTED
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