Hi Edward,

I was just going to get back to you about it. I did find out that the issue was indeed one GHC thread dealing with 5 C threads for callback (1:5 mapping) - so, the C threads were blocking on callback waiting for the only GHC thread to be available. I updated the code to do 1:1 mapping - 5 GHC threads for 5 C threads. That proved to be almost linearly scalable.

John Latos suggested the above approach two days back, but I didn't get to test the idea until now.

It doesn't seem to matter whether number of GHC threads are increased, if the mapping between GHC threads and C threads is not 1:1. I got 1:1 mapping by doing forkIO for each C thread. Is it really possible to do 7:5 mapping (that is 7 GHC threads to choose from, for 5 C threads during callback)? I can't think of a way to do it. Not that I need it. I am just curious if that is possible.

Thanks,
Sanket

On Fri, Jan 20, 2012 at 11:16 PM, Edward Z. Yang <ezyang@mit.edu> wrote:
Hello Sanket,

What happens if you run this experiment with 5 threads in the C function,
and have GHC run RTS with -N7? (e.g. five C threads + seven GHC threads = 12
threads on your 12-core box.)

Edward

Excerpts from Sanket Agrawal's message of Tue Jan 17 23:31:38 -0500 2012:
> I posted this issue on StackOverflow today. A brief recap:
>
>  In the case when C FFI calls back a Haskell function, I have observed
> sharp increase in total time when multi-threading is enabled in C code
> (even when total number of function calls to Haskell remain same). In my
> test, I called a Haskell function 5M times using two scenarios (GHC 7.0.4,
> RHEL5, 12-core box):
>
>
>    - Single-threaded C function: call back Haskell function 5M times -
>    Total time 1.32s
>    - 5 threads in C function: each thread calls back the Haskell function 1M
>    times - so, total is still 5M - Total time 7.79s - Verified that pthread
>    didn't contribute much to the overhead by having the same code call a C
>    function instead, and compared with single-threaded version. So, almost all
>    of the increase in overhead seems to come from GHC runtime.
>
> What I want to ask is if this is a known issue for GHC runtime? If not,  I
> will file a bug report for GHC team with code to reproduce it. I don't want
> to file a duplicate bug report if this is already known issue. I searched
> through GHC trac using some keywords but didn't see any bugs related to it.
>
> StackOverflow post link (has code and details on how to reproduce the
> issue):
> http://stackoverflow.com/questions/8902568/runtime-performance-degradation-for-c-ffi-callback-when-pthreads-are-enabled