Thread behavior in 7.8.3

[Michael Jones]

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