Re: GC latency in GCJ

[Andrew Haley <aph at redhat dot com>]

P.O. Gaillard writes:

 > I am testing the latency of programs compiled with GCJ and the GC
 > stops everything when it runs (I run Fedora Core 5, GCJ 4.1 and
 > Linux Trace Toolkit showed me that all the tasks are suspended
 > during GC as described by Hans Boehm in a message on this list).
 > 
 > I would like to test my little test program without GC but could
 > not find any runtime option to stop the garbage collector. Do I
 > really need to recompile GCC or can I change some runtime parameter
 > ?

I don't think so.  Hans will have to answer that one.  You might get
better results on one of the gc lists.

 > Also, I was surprised to see that my test program allocates memory
 > all the time (the size shown by ps increases until the gc starts)
 > when compiled with gcj.  This does not seem to happen with JDK
 > 1.5.

I don't understand what you're saying here.  Until the gc starts,
memory use increases continuously?  What would you expect?

 > So I tried to use valgrind, gdb, oprofile to find where _Jv_Malloc
 > is called but I ran into problems (like the fact that libgc seems
 > to be lacking debugging symbols on my machine).

Well, you have the source.  However, _Jv_Malloc is not what you're
looking for: most of the memory is allocated from 
GC_local_gcj_malloc or GC_gcj_malloc.

 > I would therefore like to ask a few questions on this list :

 > 1) where can I find a documentation of the ways to configure the GC
 > at runtime (environment variables, calls to C functions etc.) ?

boehm-gc/doc

 > 2) is there a way to force the GC to work in 500us/1ms increments ?

I don't think so.  You might get better results on one of the gc
lists.

 > 3) what is the easiest way to find out where memory is allocated
 > with GCJ ? I would like to obtain something like what gnatmem,
 > valgrind, jdk (-Xrunhprof:heap=sites) do. That is obtain backtraces
 > of the call stack for all the allocation sites in a program.

Every time a piece of memory is allocated, you want the stack trace?
That should be easy enough to add to _Jv_AllocObj and
_Jv_AllocPtrFreeObj, simply by doing a backtrace() and printing the
results to a file somewhere.

Andrew.