[erlang-questions] Mysterious gen_server timeouts in MIX

[Bob Ippolito]

gen_server has a default timeout of 2 seconds, so if your call takes long
enough for whatever reason this can happen. Try changing all if your
gen_server calls to explicitly specify a longer or infinite timeout.

On Friday, November 1, 2013, John R. Ashmun wrote:

> I have implemented an emulator for MIX, Donald Knuth's notional 1960's
> computer, as an Erlang application, as a way of learning to use Erlang.
>
> As part of the emulator's startup currently, MIX loads Knuth's Program P
> into its memory, then jumps to the starting location of Program P.  Program
> P calculates the first 500 prime numbers, pops them into a table, and then
> prints 10 prime numbers at a time on what it considers to be its line
> printer.
>
> The emulator usually runs fine, and when it does, it carries out Program P
> perfectly.
>
> Sometimes, however, things get stuck, and I have been unable to learn what
> goes wrong.  When there is a problem, I usually see these two error reports:
>
> =ERROR REPORT==== 1-Nov-2013::11:33:54===
> ** Generic server <0.38.0> terminating
> ** Last message in was timeout
> ** When server state == []
> ** Reason for termination ==
> ** {timeout,{gen_server,call,[io_controller,{wait_until_not_busy,18}]}}
>
> =ERROR REPORT==== 1-Nov-2013::11:33:54===
> ** Generic server <0.4095.0> terminating
> ** Last message in was {'$gen_cast',{write,1995}}
> ** When Server state == {state,18,<0.4058.0>,24}
> ** Reason for termination ==
> ** {timeout,{gen_server,call,[io_controller,{set_ready,18}]}}
>
> The first one seems clearly to tell me that MIX's io_controller
> gen_server's API wait_until_not_busy( ) function sent the message that it
> should have, but the handle_call( ) never found the readiness status for
> MIX device 18, its line printer, not to be busy, and then the gen_server
> was timed out.  This should never happen, but then there is the second
> report to consider.
>
> The second report confuses me.  Program P attempts to write a title (FIRST
> FIVE HUNDRED PRIMES) to MIX's line printer.  This is what the $gen_cast is
> attempting to do (1995 is the MIX address of the first character of the
> title).  A MIX I/O operation (in this case, the write) begins by waiting
> until the I/O unit that's addressed is not busy.  Apparently that's not
> true over the timeout period on this run; here is the mystery:  the
> emulator's I/O device gen_servers are all initialized with their readiness
> state set to ready, their readiness is changed to busy only by a request to
> perform an I/O operation, and the operation is actually carried out by an
> io_operation gen_server that is started by the I/O device gen_server.  That
> io_operation gen_server is the entity that requests the io_controller to
> set the readiness of the device back to ready, once the io_operation has
> performed the I/O, or the output, in this case, to the file that plays the
> role of the line printer, in this case.  It shouldn't be possible for
> io_controller:set_ready( Device ) to be called before io_controller:write(
> Device, Address ).  Is this what the error report is telling me happened?
>
> All right, you say, you've programmed your emulator incompetently, and I
> suppose I may have.  Why, then, does all this work perfectly, over and over
> again, on another machine, or even on the same machine, before going into
> failure mode?  (I have been unable to identify anything causing things to
> start failing nor to start working, and I'm not making changes other than
> sometimes running using application:start( 'MIX' ) and sometimes booting
> MIX as an Erlang release -- sometimes compiling using +debug_info (or not)
> has seemed to change from success to failure (or, equally likely, the
> opposite).
>
> I need advice, please.
>
> Regards,
> John Ashmun
>
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