[erlang-questions] prim_inet:ignorefd/2 affecting erts_poll_wait()

Michael Truog <>
Sat Aug 23 03:01:56 CEST 2014


To be a little clearer, my prim_inet:ignorefd/2 usage is to use a file 
descriptor created in the inet source code after passing it through 
gen_tcp:fdopen/2. So, the sequence is:
...
{ok, FileDescriptorInternal} = prim_inet:getfd(Socket),
ok = prim_inet:ignorefd(Socket, true),
{ok, NewSocket} = gen_tcp:fdopen(FileDescriptorInternal, SocketOptions),
...
While this might seem odd, this allows me to dup2 the file descriptor 
without causing obvious problems and handle UNIX domain sockets, which 
are currently unsupported within inet (by avoiding the internal checking 
that prevents their use).

On 08/22/2014 05:46 PM, Michael Truog wrote:
> I have been seeing the erts_poll_wait() thread consume 100% CPU when 
> my configuration makes prim_inet:ignorefd/2 ignore a fd 
> (inet_descriptor has is_ignored set to true) created external to inet 
> (10 file descriptors created this way).  I don't have this problem 
> when using the inet code to create tcp sockets, when 
> prim_inet:ignorefd/2 is not used with the same configuration. When 
> setting "#define INET_DRV_DEBUG 1" in 
> "./erts/emulator/drivers/common/inet_drv.c" and "#define 
> ERTS_POLL_DEBUG_PRINT" in "./erts/emulator/sys/common/erl_poll.c" all 
> the debugging output looks the same when exercising the file 
> descriptors in the same way.  The only difference seems to be the 
> "Entering erts_poll_wait(), timeout=NUMBER" output has non-zero 
> timeout values more often when prim_inet:ignorefd/2 is not used when 
> compare to the output when it is being used.  Also, the NUMBER doesn't 
> seem to go about 1000 for me when prim_inet:ignorefd/2 is used, but it 
> can go above 1000 when prim_inet:ignorefd/2 is not used.
>
> Why would the erts_poll_wait() loop be refusing to timeout?  Is this 
> expected behaviour?  Is there an erts configuration flag which is 
> meant to address the problem?
>
> Thanks,
> Michael




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