Thread behavior in 7.8.3
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 ___ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
Irreducible predicates error in Template Haskell
Hello, we were trying to reify a typeclass, which had a ConstraintKind and we hit upon this error: Can't represent irreducible predicates in Template Haskell:. It seems that there is already a ghc bug [ https://ghc.haskell.org/trac/ghc/ticket/7021 ] filed and its status is set as fixed, but there is a comment at the bottom in which the reviewer recommends against merging immediately. Does anybody know when it would get merged in? -- Yours truly, Sreenidhi Nair ___ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
Re: Irreducible predicates error in Template Haskell
This fix will not get merged into the 7.8.x development stream, but it is already available in HEAD and will be available in GHC 7.10.x. We try not to make breaking changes (and this is a breaking change) in the middle of a major version. Richard On Oct 29, 2014, at 11:27 AM, Sreenidhi Nair nair.sreeni...@gmail.com wrote: Hello, we were trying to reify a typeclass, which had a ConstraintKind and we hit upon this error: Can't represent irreducible predicates in Template Haskell:. It seems that there is already a ghc bug [ https://ghc.haskell.org/trac/ghc/ticket/7021 ] filed and its status is set as fixed, but there is a comment at the bottom in which the reviewer recommends against merging immediately. Does anybody know when it would get merged in? -- Yours truly, Sreenidhi Nair ___ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users ___ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
Re: Thread behavior in 7.8.3
Michael Jones m...@proclivis.com writes: 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. Are you using Bas van Dijk's `usb` library by any chance? If so, you should be aware of this [1] issue. 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. Have you looked at the RTS's output when run with `+RTS -sstderr`? Is productivity any different between the two tests? 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. Oh dear, this doesn't sound good at all. Have you tried getting a backtrace out of gdb? Usually this isn't terribly useful but in this case since the event log is involved it might be getting stuck in the RTS which should give a useful backtrace. If not, perhaps strace will give some clues as to what is happening (you'll probably want to hide SIGVTALM to improve signal/noise)? Cheers, - Ben [1] https://github.com/basvandijk/usb/issues/7 pgpGwIoqAmMbI.pgp Description: PGP signature ___ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
Re: Thread behavior in 7.8.3
Ben, I am using Bas van Dijk’s usb, and I am past the -threading issue by using the latest commit. I don’t have any easy way of making comparisons between 7.6 and 7.8 productivity, but from oscilloscope activity, I can’t see any difference. The only difference I see is the thread scheduling on 7.8 for -N1 vs -N2/4. If —sstderr gives some notion of productivity, I’ll have to do an experiment between -N1 and -N2/4. Unchartered territory for me. I’ll setup and experiment tonight. I am not familiar with strace. I’ll fix that soon. Mike On Oct 29, 2014, at 10:24 AM, Ben Gamari bgamari.f...@gmail.com wrote: Michael Jones m...@proclivis.com writes: 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. Are you using Bas van Dijk's `usb` library by any chance? If so, you should be aware of this [1] issue. 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. Have you looked at the RTS's output when run with `+RTS -sstderr`? Is productivity any different between the two tests? 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. Oh dear, this doesn't sound good at all. Have you tried getting a backtrace out of gdb? Usually this isn't terribly useful but in this case since the event log is involved it might be getting stuck in the RTS which should give a useful backtrace. If not, perhaps strace will give some clues as to what is happening (you'll probably want to hide SIGVTALM to improve signal/noise)? Cheers, - Ben [1] https://github.com/basvandijk/usb/issues/7 ___ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
Re: Recursion on TypeNats
I don't think we'll need notation to differentiate: just use overloaded literals, like we do in terms. Something that would operate vaguely like this: type family 3 :: k where 3 :: Nat = ... -- 3 as a Nat 3 :: Integer = ... -- 3 as an Integer I'm not at all suggesting it be implemented this way, but we already have the ability to branch in type families based on result kind, so the mechanism is already around. Unfortunately, this would be unhelpful if the user asked for (3 :: Bool), which would kind-check but be stuck. Richard On Oct 28, 2014, at 8:24 PM, Iavor Diatchki iavor.diatc...@gmail.com wrote: Hello, actually type-level integers are easier to work with than type-level naturals (e.g., one can cancel things by subtracting at will). I agree that ideally we want to have both integers and naturals (probably as separate kinds). I just don't know what notation to use to distinguish the two. -Iavor On Mon, Oct 27, 2014 at 2:13 PM, Barney Hilken b.hil...@ntlworld.com wrote: Ok, I've created a ticket https://ghc.haskell.org/trac/ghc/ticket/9731 Unfortunately I don't know enough about ghc internals to try implementing it. ___ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users ___ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
Re: Thread behavior in 7.8.3
Michael Jones m...@proclivis.com writes: Ben, I am using Bas van Dijk’s usb, and I am past the -threading issue by using the latest commit. Excellent; I hadn't noticed the proclivis in your email address I don’t have any easy way of making comparisons between 7.6 and 7.8 productivity, but from oscilloscope activity, I can’t see any difference. The only difference I see is the thread scheduling on 7.8 for -N1 vs -N2/4. If —sstderr gives some notion of productivity, I’ll have to do an experiment between -N1 and -N2/4. Unchartered territory for me. I’ll setup and experiment tonight. Indeed it does; productivity in this context refers to the fraction of runtime spent in evaluation (as opposed to in the garbage collector, for instance). I am not familiar with strace. I’ll fix that soon. It's often an invaluable tool; that being said it remains to seen whether it yields anything useful in this particular case. Cheers, - Ben pgp2rrHbAUnFW.pgp Description: PGP signature ___ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
Re: Thread behavior in 7.8.3
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 m...@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 ___ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users ___ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
Re: Thread behavior in 7.8.3
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 jwl...@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 m...@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 ___ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users ___ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
Re: Thread behavior in 7.8.3
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 m...@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 jwl...@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 m...@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 ___ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users ___ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
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 m...@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 jwl...@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 m...@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 ___ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users ___ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
Re: Thread behavior in 7.8.3
Yes, that's right. I brought it up because you mentioned that there might still be occasional delays, and those might be caused by a thread not being preemptible for a while. Edward Excerpts from John Lato's message of 2014-10-29 17:31:45 -0700: My understanding is that -fno-omit-yields is subtly different. I think that's for the case when a function loops without performing any heap allocations, and thus would never yield even after the context switch timeout. In my case the looping function does perform heap allocations and does eventually yield, just not until after the timeout. Is that understanding correct? (technically, doesn't it change to yielding after stack checks or something like that?) On Thu, Oct 30, 2014 at 8:24 AM, Edward Z. Yang ezy...@mit.edu wrote: 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 m...@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 jwl...@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 m...@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
