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
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
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
Excerpts from Sanket Agrawal's message of Tue Jan 17 23:31:38 -0500 2012: 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-f...