[erlang-bugs] Scheduler Wall Time Statistics live|dead locking a process.

[Fred Hebert]

Hi there,

If you go on erlang-questions, you'll find the following thread I 
started regarding one of my gen_servers locking up forever until I try 
to connect to the VM: 
http://erlang.org/pipermail/erlang-questions/2012-July/068097.html

And the information following it in 
http://erlang.org/pipermail/erlang-questions/2012-July/068099.html

The gist of it is that apparently, the gen_server gets stuck while 
calling erlang:statistics(scheduler_wall_time). A process info dump on 
it returns:

[{registered_name,vmstats_server},
  {current_function,{erlang,sched_wall_time,3}},
  {initial_call,{proc_lib,init_p,5}},
  {status,waiting},
  {message_queue_len,2},
  {messages,[{system,{<5998.7341.243>,#Ref<5998.0.3810.221818>},get_status},
{system,{<5998.28757.800>,#Ref<5998.0.3811.260443>},get_status}]},
{links,[<5998.918.0>]},
  {dictionary,[{random_seed,{17770,13214,15044}},
               {'$ancestors',[vmstats_sup,<5998.917.0>]},
               {'$initial_call',{vmstats_server,init,1}}]},
  {trap_exit,false},
  {error_handler,error_handler},
  {priority,normal},
  {group_leader,<5998.916.0>},
  {total_heap_size,122003},
  {heap_size,121393},
  {stack_size,21},
  {reductions,314325681},
  {garbage_collection,[{min_bin_vheap_size,46368},
                       {min_heap_size,233},
                       {fullsweep_after,65535},
                       {minor_gcs,23774}]},
  {suspending,[]}]
ok

with the interesting parts:
{current_function,{erlang,sched_wall_time,3}},
{status,waiting},

I'm unsure what exactly causes the problem, and we're running the VM 
with default arguments when it comes to scheduling and layout. It 
happens even when the virtual machine is under relatively low load 
(scheduler active wall time is less than 5%, but more than 2% of the 
total wall time when averaging all cores) and can also happen under 
higher load.

Only that process appears affected.