Subversion Repositories openrisc

The collector supports both incremental collection and threads under

Solaris 2.  The incremental collector normally retrieves page dirty information

through the appropriate /proc calls.  But it can also be configured

(by defining MPROTECT_VDB instead of PROC_VDB in gcconfig.h) to use mprotect

and signals.  This may result in shorter pause times, but it is no longer

safe to issue arbitrary system calls that write to the heap.

Under other UNIX versions,

the collector normally obtains memory through sbrk.  There is some reason

to expect that this is not safe if the client program also calls the system

malloc, or especially realloc.  The sbrk man page strongly suggests this is

not safe: "Many library routines use malloc() internally, so use brk()

and sbrk() only when you know  that malloc() definitely will not be used by

any library routine."  This doesn't make a lot of sense to me, since there

seems to be no documentation as to which routines can transitively call malloc.

Nonetheless, under Solaris2, the collector now (since 4.12) allocates

memory using mmap by default.  (It defines USE_MMAP in gcconfig.h.)

You may want to reverse this decisions if you use -DREDIRECT_MALLOC=...

SOLARIS THREADS:

The collector must be compiled with -DGC_SOLARIS_THREADS (thr_ functions)

or -DGC_SOLARIS_PTHREADS (pthread_ functions) to be thread safe.

It is also essential that gc.h be included in files that call thr_create,

thr_join, thr_suspend, thr_continue, or dlopen.  Gc.h macro defines

these to also do GC bookkeeping, etc.  Gc.h must be included with

one or both of these macros defined, otherwise

these replacements are not visible.

A collector built in this way way only be used by programs that are

linked with the threads library.

In this mode, the collector contains various workarounds for older Solaris

bugs.  Mostly, these should not be noticeable unless you look at system

call traces.  However, it cannot protect a guard page at the end of

a thread stack.  If you know that you will only be running Solaris2.5

or later, it should be possible to fix this by compiling the collector

with -DSOLARIS23_MPROTECT_BUG_FIXED.

Since 5.0 alpha5, dlopen disables collection temporarily,

unless USE_PROC_FOR_LIBRARIES is defined.  In some unlikely cases, this

can result in unpleasant heap growth.  But it seems better than the

race/deadlock issues we had before.

If solaris_threads are used on an X86 processor with malloc redirected to

GC_malloc a deadlock is likely to result.

It appears that there is a problem in using gc_cpp.h in conjunction with

Solaris threads and Sun's C++ runtime.  Apparently the overloaded new operator

is invoked by some iostream initialization code before threads are correctly

initialized.  As a result, call to thr_self() in garbage collector

initialization  segfaults.  Currently the only known workaround is to not

invoke the garbage collector from a user defined global operator new, or to

have it invoke the garbage-collector's allocators only after main has started.

(Note that the latter requires a moderately expensive test in operator

delete.)

Hans-J. Boehm

(The above contains my personal opinions, which are probably not shared

by anyone else.)