Re: Thread behavior in 7.8.3

I don't think this is directly related to the problem, but if you have a thread that isn't yielding, you can force it to yield by using -fno-omit-yields on your code. It won't help if the non-yielding code is in a library, and it won't help if the problem was that you just weren't setting timeouts finely enough (which sounds like what was happening). FYI. Edward Excerpts from John Lato's message of 2014-10-29 17:19:46 -0700:

I guess I should explain what that flag does...

The GHC RTS maintains capabilities, the number of capabilities is specified by the `+RTS -N` option. Each capability is a virtual machine that executes Haskell code, and maintains its own runqueue of threads to process.

A capability will perform a context switch at the next heap block allocation (every 4k of allocation) after the timer expires. The timer defaults to 20ms, and can be set by the -C flag. Capabilities perform context switches in other circumstances as well, such as when a thread yields or blocks.

My guess is that either the context switching logic changed in ghc-7.8, or possibly your code used to trigger a switch via some other mechanism (stack overflow or something maybe?), but is optimized differently now so instead it needs to wait for the timer to expire.

The problem we had was that a time-sensitive thread was getting scheduled on the same capability as a long-running non-yielding thread, so the time-sensitive thread had to wait for a context switch timeout (even though there were free cores available!). I expect even with -N4 you'll still see occasional delays (perhaps <5% of calls).

We've solved our problem with judicious use of `forkOn`, but that won't help at N1.

We did see this behavior in 7.6, but it's definitely worse in 7.8.

Incidentally, has there been any interest in a work-stealing scheduler? There was a discussion from about 2 years ago, in which Simon Marlow noted it might be tricky, but it would definitely help in situations like this.

John L.

On Thu, Oct 30, 2014 at 8:02 AM, Michael Jones 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 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 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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