Runtime performance degradation for multi-threaded C FFI callback

[Sanket Agrawal]

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
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