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Garbage collection and caml_adjust_gc_speed
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Date: -- (:)
From: Christopher Kauffman <kauffman@c...>
Subject: Garbage collection and caml_adjust_gc_speed
I am finishing up my first major research application written in OCaml. The code is a scientific 
application which does a moderate amount of floating point computations for which I have employed 
the Bigarray library and the Lacaml package.

I am attempting to tune the performance of the code and to that end, I have examined the native code 
performance using gprof (OCaml manual section 17.4). The first thing that struck me on analyzing the 
profile is that the function 'caml_adjust_gc_speed' is called a lot. The first few lines of the 
profile are:

Flat profile:

Each sample counts as 0.01 seconds.
   %   cumulative   self              self     total
  time   seconds   seconds    calls   s/call   s/call  name
  14.55      6.80     6.80  4841768     0.00     0.00  caml_adjust_gc_speed
  14.41     13.53     6.73                             bigarray_fill
   6.87     16.74     3.21                             lacaml_Dssqr_zero_stub
   6.30     19.68     2.94                             bigarray_offset
   4.30     21.69     2.01                             bigarray_slice
   4.03     23.57     1.88                             bigarray_get_N
   2.68     24.82     1.25     1612     0.00     0.00  sweep_slice
   2.63     26.05     1.23 24714254     0.00     0.00  caml_c_call
   2.10     27.03     0.98  4972572     0.00     0.00  caml_alloc_shr

The actual runtime of the program is about 18 seconds so the gprof cumulative time is off by quite a 
bit. What concerns me is the large overhead I seem to be getting from the first function, 
'caml_adjust_gc_speed', which I assume is related to the garbage collector. Over 4 million calls to 
this function seems a little much. I attempted to play with a garbage collection parameter, the 
value of control.space_overhead in the Gc module. According to the manual, this affects the major GC 
speed and increasing the value is supposed to cut down on the aggressiveness of the GC. Setting 
space_overhead to 99 did not change number of calls to 'caml_adjust_gc_speed'.

I'm looking for someone with a bit more knowledge of the garbage collection in OCaml to enlighten me 
on whether this overhead can be reduced or if it is an unavoidable side-effect of relying on the 
garbage collector. I'd be happy to provide more details on the code if this would be helpful.