Thread behavior in 7.8.3

[Michael Jones]

Sorry I am reviving an old problem, but it has resurfaced, such that one system behaves different than another.

Using -C0.001 solved problems on a Mac + VM + Ubuntu 14. It worked on a single core 32 bit Atom NUC. But on a dual core Atom MinnowBoardMax, something bad is going on. In summary, the same code that runs on two machines does not run on a third machine. So this indicates I have not made any breaking changes to the code or cabal files. Compiling with GHC 7.8.3.

This bad system has Ubuntu 14 installed, with an updated Linux 3.18.1 kernel. It is a dual core 64 bit I86 Atom processor. The application hangs at startup. If I remove the -C0.00N option and instead use -V0, the application runs. It has bad timing properties, but it does at least run. Note that a hang hangs an IO thread talking USB, and the GUI thread.

When testing with the -C0.00N option, it did run 2 times out of 20 tries, so fail means fail most but not all of the time. When it did run, it continued to run properly. This perhaps indicates some kind of internal race condition.

In the fail to run case, it does some printing up to the point where it tries to create a wxHaskell frame. In another non-UI version of the program it also fails to run. Logging to a file gives a similar indication. It is clear that the program starts up, then fails during the run in some form of lockup, well after the initial startup code.

If I run with the strace command, it always runs with -C0.00N.

All the above was done with profiling enabled, so I removed that and instead enabled eventlog to look for clues.

In this case it lies between good and bad, in that IO to my USB is working, but the GUI comes up blank and never paints. Running this case without -v0 (event log) the gui partially paints and stops, but USB continues.

Questions:

1) Does ghc 7.8.4 have any improvements that might pertain to these kinds of scheduling/thread problems?
2) Is there anything about the nature of a thread using USB, I2C, or wxHaskell IO that leads to problems that a pure calculation app would not have?
3) Any ideas how to track down the problem when changing conditions (compiler or runtime options) affects behavior?
4) Are there other options besides -V and -C for the runtime that might apply?
5) What does -V0 do that makes a problem program run?

Mike




On Oct 29, 2014, at 6:02 PM, Michael Jones <mike at proclivis.com> wrote:

> John,
> 
> Adding -C0.005 makes it much better. Using -C0.001 makes it behave more like -N4.
> 
> Thanks. This saves my project, as I need to deploy on a single core Atom and was stuck.
> 
> Mike
> 
> On Oct 29, 2014, at 5:12 PM, John Lato <jwlato at gmail.com> wrote:
> 
>> By any chance do the delays get shorter if you run your program with `+RTS -C0.005` ?  If so, I suspect you're having a problem very similar to one that we had with ghc-7.8 (7.6 too, but it's worse on ghc-7.8 for some reason), involving possible misbehavior of the thread scheduler.
>> 
>> On Wed, Oct 29, 2014 at 2:18 PM, Michael Jones <mike at proclivis.com> wrote:
>> I have a general question about thread behavior in 7.8.3 vs 7.6.X
>> 
>> I moved from 7.6 to 7.8 and my application behaves very differently. I have three threads, an application thread that plots data with wxhaskell or sends it over a network (depends on settings), a thread doing usb bulk writes, and a thread doing usb bulk reads. Data is moved around with TChan, and TVar is used for coordination.
>> 
>> When the application was compiled with 7.6, my stream of usb traffic was smooth. With 7.8, there are lots of delays where nothing seems to be running. These delays are up to 40ms, whereas with 7.6 delays were a 1ms or so.
>> 
>> When I add -N2 or -N4, the 7.8 program runs fine. But on 7.6 it runs fine without with -N2/4.
>> 
>> The program is compiled -O2 with profiling. The -N2/4 version uses more memory,  but in both cases with 7.8 and with 7.6 there is no space leak.
>> 
>> I tired to compile and use -ls so I could take a look with threadscope, but the application hangs and writes no data to the file. The CPU fans run wild like it is in an infinite loop. It at least pops an unpainted wxhaskell window, so it got partially running.
>> 
>> One of my libraries uses option -fsimpl-tick-factor=200 to get around the compiler.
>> 
>> What do I need to know about changes to threading and event logging between 7.6 and 7.8? Is there some general documentation somewhere that might help?
>> 
>> I am on Ubuntu 14.04 LTS. I downloaded the 7.8 tool chain tar ball and installed myself, after removing 7.6 with apt-get.
>> 
>> Any hints appreciated.
>> 
>> Mike
>> 
>> 
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> 

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