Runtime performance degradation for multi-threaded C FFI callback

Edward Z. Yang ezyang at MIT.EDU
Sat Jan 21 05:16:26 CET 2012

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.)


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):

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