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diff --git a/gc/doc/README b/gc/doc/README deleted file mode 100644 index 09ae43b..0000000 --- a/gc/doc/README +++ /dev/null @@ -1,618 +0,0 @@ -Copyright (c) 1988, 1989 Hans-J. Boehm, Alan J. Demers -Copyright (c) 1991-1996 by Xerox Corporation. All rights reserved. -Copyright (c) 1996-1999 by Silicon Graphics. All rights reserved. -Copyright (c) 1999-2001 by Hewlett-Packard Company. All rights reserved. - -The file linux_threads.c is also -Copyright (c) 1998 by Fergus Henderson. All rights reserved. - -The files Makefile.am, and configure.in are -Copyright (c) 2001 by Red Hat Inc. All rights reserved. - -Several files supporting GNU-style builds are copyrighted by the Free -Software Foundation, and carry a different license from that given -below. - -THIS MATERIAL IS PROVIDED AS IS, WITH ABSOLUTELY NO WARRANTY EXPRESSED -OR IMPLIED. ANY USE IS AT YOUR OWN RISK. - -Permission is hereby granted to use or copy this program -for any purpose, provided the above notices are retained on all copies. -Permission to modify the code and to distribute modified code is granted, -provided the above notices are retained, and a notice that the code was -modified is included with the above copyright notice. - -A few of the files needed to use the GNU-style build procedure come with -slightly different licenses, though they are all similar in spirit. A few -are GPL'ed, but with an exception that should cover all uses in the -collector. (If you are concerned about such things, I recommend you look -at the notice in config.guess or ltmain.sh.) - -This is version 6.1alpha5 of a conservative garbage collector for C and C++. - -You might find a more recent version of this at - -http://www.hpl.hp.com/personal/Hans_Boehm/gc - -OVERVIEW - - This is intended to be a general purpose, garbage collecting storage -allocator. The algorithms used are described in: - -Boehm, H., and M. Weiser, "Garbage Collection in an Uncooperative Environment", -Software Practice & Experience, September 1988, pp. 807-820. - -Boehm, H., A. Demers, and S. Shenker, "Mostly Parallel Garbage Collection", -Proceedings of the ACM SIGPLAN '91 Conference on Programming Language Design -and Implementation, SIGPLAN Notices 26, 6 (June 1991), pp. 157-164. - -Boehm, H., "Space Efficient Conservative Garbage Collection", Proceedings -of the ACM SIGPLAN '91 Conference on Programming Language Design and -Implementation, SIGPLAN Notices 28, 6 (June 1993), pp. 197-206. - -Boehm H., "Reducing Garbage Collector Cache Misses", Proceedings of the -2000 International Symposium on Memory Management. - - Possible interactions between the collector and optimizing compilers are -discussed in - -Boehm, H., and D. Chase, "A Proposal for GC-safe C Compilation", -The Journal of C Language Translation 4, 2 (December 1992). - -and - -Boehm H., "Simple GC-safe Compilation", Proceedings -of the ACM SIGPLAN '96 Conference on Programming Language Design and -Implementation. - -(Some of these are also available from -http://www.hpl.hp.com/personal/Hans_Boehm/papers/, among other places.) - - Unlike the collector described in the second reference, this collector -operates either with the mutator stopped during the entire collection -(default) or incrementally during allocations. (The latter is supported -on only a few machines.) On the most common platforms, it can be built -with or without thread support. On a few platforms, it can take advantage -of a multiprocessor to speed up garbage collection. - - Many of the ideas underlying the collector have previously been explored -by others. Notably, some of the run-time systems developed at Xerox PARC -in the early 1980s conservatively scanned thread stacks to locate possible -pointers (cf. Paul Rovner, "On Adding Garbage Collection and Runtime Types -to a Strongly-Typed Statically Checked, Concurrent Language" Xerox PARC -CSL 84-7). Doug McIlroy wrote a simpler fully conservative collector that -was part of version 8 UNIX (tm), but appears to not have received -widespread use. - - Rudimentary tools for use of the collector as a leak detector are included -(see http://www.hpl.hp.com/personal/Hans_Boehm/gc/leak.html), -as is a fairly sophisticated string package "cord" that makes use of the -collector. (See doc/README.cords and H.-J. Boehm, R. Atkinson, and M. Plass, -"Ropes: An Alternative to Strings", Software Practice and Experience 25, 12 -(December 1995), pp. 1315-1330. This is very similar to the "rope" package -in Xerox Cedar, or the "rope" package in the SGI STL or the g++ distribution.) - -Further collector documantation can be found at - -http://www.hpl.hp.com/personal/Hans_Boehm/gc - - -GENERAL DESCRIPTION - - This is a garbage collecting storage allocator that is intended to be -used as a plug-in replacement for C's malloc. - - Since the collector does not require pointers to be tagged, it does not -attempt to ensure that all inaccessible storage is reclaimed. However, -in our experience, it is typically more successful at reclaiming unused -memory than most C programs using explicit deallocation. Unlike manually -introduced leaks, the amount of unreclaimed memory typically stays -bounded. - - In the following, an "object" is defined to be a region of memory allocated -by the routines described below. - - Any objects not intended to be collected must be pointed to either -from other such accessible objects, or from the registers, -stack, data, or statically allocated bss segments. Pointers from -the stack or registers may point to anywhere inside an object. -The same is true for heap pointers if the collector is compiled with - ALL_INTERIOR_POINTERS defined, as is now the default. - -Compiling without ALL_INTERIOR_POINTERS may reduce accidental retention -of garbage objects, by requiring pointers from the heap to to the beginning -of an object. But this no longer appears to be a significant -issue for most programs. - -There are a number of routines which modify the pointer recognition -algorithm. GC_register_displacement allows certain interior pointers -to be recognized even if ALL_INTERIOR_POINTERS is nor defined. -GC_malloc_ignore_off_page allows some pointers into the middle of large objects -to be disregarded, greatly reducing the probablility of accidental -retention of large objects. For most purposes it seems best to compile -with ALL_INTERIOR_POINTERS and to use GC_malloc_ignore_off_page if -you get collector warnings from allocations of very large objects. -See README.debugging for details. - - WARNING: pointers inside memory allocated by the standard "malloc" are not -seen by the garbage collector. Thus objects pointed to only from such a -region may be prematurely deallocated. It is thus suggested that the -standard "malloc" be used only for memory regions, such as I/O buffers, that -are guaranteed not to contain pointers to garbage collectable memory. -Pointers in C language automatic, static, or register variables, -are correctly recognized. (Note that GC_malloc_uncollectable has semantics -similar to standard malloc, but allocates objects that are traced by the -collector.) - - WARNING: the collector does not always know how to find pointers in data -areas that are associated with dynamic libraries. This is easy to -remedy IF you know how to find those data areas on your operating -system (see GC_add_roots). Code for doing this under SunOS, IRIX 5.X and 6.X, -HP/UX, Alpha OSF/1, Linux, and win32 is included and used by default. (See -README.win32 for win32 details.) On other systems pointers from dynamic -library data areas may not be considered by the collector. -If you're writing a program that depends on the collector scanning -dynamic library data areas, it may be a good idea to include at least -one call to GC_is_visible() to ensure that those areas are visible -to the collector. - - Note that the garbage collector does not need to be informed of shared -read-only data. However if the shared library mechanism can introduce -discontiguous data areas that may contain pointers, then the collector does -need to be informed. - - Signal processing for most signals may be deferred during collection, -and during uninterruptible parts of the allocation process. -Like standard ANSI C mallocs, by default it is unsafe to invoke -malloc (and other GC routines) from a signal handler while another -malloc call may be in progress. Removing -DNO_SIGNALS from Makefile -attempts to remedy that. But that may not be reliable with a compiler that -substantially reorders memory operations inside GC_malloc. - - The allocator/collector can also be configured for thread-safe operation. -(Full signal safety can also be achieved, but only at the cost of two system -calls per malloc, which is usually unacceptable.) -WARNING: the collector does not guarantee to scan thread-local storage -(e.g. of the kind accessed with pthread_getspecific()). The collector -does scan thread stacks, though, so generally the best solution is to -ensure that any pointers stored in thread-local storage are also -stored on the thread's stack for the duration of their lifetime. -(This is arguably a longstanding bug, but it hasn't been fixed yet.) - -INSTALLATION AND PORTABILITY - - As distributed, the macro SILENT is defined in Makefile. -In the event of problems, this can be removed to obtain a moderate -amount of descriptive output for each collection. -(The given statistics exhibit a few peculiarities. -Things don't appear to add up for a variety of reasons, most notably -fragmentation losses. These are probably much more significant for the -contrived program "test.c" than for your application.) - - Note that typing "make test" will automatically build the collector -and then run setjmp_test and gctest. Setjmp_test will give you information -about configuring the collector, which is useful primarily if you have -a machine that's not already supported. Gctest is a somewhat superficial -test of collector functionality. Failure is indicated by a core dump or -a message to the effect that the collector is broken. Gctest takes about -35 seconds to run on a SPARCstation 2. It may use up to 8 MB of memory. (The -multi-threaded version will use more. 64-bit versions may use more.) -"Make test" will also, as its last step, attempt to build and test the -"cord" string library. This will fail without an ANSI C compiler, but -the garbage collector itself should still be usable. - - The Makefile will generate a library gc.a which you should link against. -Typing "make cords" will add the cord library to gc.a. -Note that this requires an ANSI C compiler. - - It is suggested that if you need to replace a piece of the collector -(e.g. GC_mark_rts.c) you simply list your version ahead of gc.a on the -ld command line, rather than replacing the one in gc.a. (This will -generate numerous warnings under some versions of AIX, but it still -works.) - - All include files that need to be used by clients will be put in the -include subdirectory. (Normally this is just gc.h. "Make cords" adds -"cord.h" and "ec.h".) - - The collector currently is designed to run essentially unmodified on -machines that use a flat 32-bit or 64-bit address space. -That includes the vast majority of Workstations and X86 (X >= 3) PCs. -(The list here was deleted because it was getting too long and constantly -out of date.) - It does NOT run under plain 16-bit DOS or Windows 3.X. There are however -various packages (e.g. win32s, djgpp) that allow flat 32-bit address -applications to run under those systemsif the have at least an 80386 processor, -and several of those are compatible with the collector. - - In a few cases (Amiga, OS/2, Win32, MacOS) a separate makefile -or equivalent is supplied. Many of these have separate README.system -files. - - Dynamic libraries are completely supported only under SunOS -(and even that support is not functional on the last Sun 3 release), -Linux, IRIX 5&6, HP-PA, Win32 (not Win32S) and OSF/1 on DEC AXP machines. -On other machines we recommend that you do one of the following: - - 1) Add dynamic library support (and send us the code). - 2) Use static versions of the libraries. - 3) Arrange for dynamic libraries to use the standard malloc. - This is still dangerous if the library stores a pointer to a - garbage collected object. But nearly all standard interfaces - prohibit this, because they deal correctly with pointers - to stack allocated objects. (Strtok is an exception. Don't - use it.) - - In all cases we assume that pointer alignment is consistent with that -enforced by the standard C compilers. If you use a nonstandard compiler -you may have to adjust the alignment parameters defined in gc_priv.h. - - A port to a machine that is not byte addressed, or does not use 32 bit -or 64 bit addresses will require a major effort. A port to plain MSDOS -or win16 is hard. - - For machines not already mentioned, or for nonstandard compilers, the -following are likely to require change: - -1. The parameters in gcconfig.h. - The parameters that will usually require adjustment are - STACKBOTTOM, ALIGNMENT and DATASTART. Setjmp_test - prints its guesses of the first two. - DATASTART should be an expression for computing the - address of the beginning of the data segment. This can often be - &etext. But some memory management units require that there be - some unmapped space between the text and the data segment. Thus - it may be more complicated. On UNIX systems, this is rarely - documented. But the adb "$m" command may be helpful. (Note - that DATASTART will usually be a function of &etext. Thus a - single experiment is usually insufficient.) - STACKBOTTOM is used to initialize GC_stackbottom, which - should be a sufficient approximation to the coldest stack address. - On some machines, it is difficult to obtain such a value that is - valid across a variety of MMUs, OS releases, etc. A number of - alternatives exist for using the collector in spite of this. See the - discussion in gcconfig.h immediately preceding the various - definitions of STACKBOTTOM. - -2. mach_dep.c. - The most important routine here is one to mark from registers. - The distributed file includes a generic hack (based on setjmp) that - happens to work on many machines, and may work on yours. Try - compiling and running setjmp_t.c to see whether it has a chance of - working. (This is not correct C, so don't blame your compiler if it - doesn't work. Based on limited experience, register window machines - are likely to cause trouble. If your version of setjmp claims that - all accessible variables, including registers, have the value they - had at the time of the longjmp, it also will not work. Vanilla 4.2 BSD - on Vaxen makes such a claim. SunOS does not.) - If your compiler does not allow in-line assembly code, or if you prefer - not to use such a facility, mach_dep.c may be replaced by a .s file - (as we did for the MIPS machine and the PC/RT). - At this point enough architectures are supported by mach_dep.c - that you will rarely need to do more than adjust for assembler - syntax. - -3. os_dep.c (and gc_priv.h). - Several kinds of operating system dependent routines reside here. - Many are optional. Several are invoked only through corresponding - macros in gc_priv.h, which may also be redefined as appropriate. - The routine GC_register_data_segments is crucial. It registers static - data areas that must be traversed by the collector. (User calls to - GC_add_roots may sometimes be used for similar effect.) - Routines to obtain memory from the OS also reside here. - Alternatively this can be done entirely by the macro GET_MEM - defined in gc_priv.h. Routines to disable and reenable signals - also reside here if they are need by the macros DISABLE_SIGNALS - and ENABLE_SIGNALS defined in gc_priv.h. - In a multithreaded environment, the macros LOCK and UNLOCK - in gc_priv.h will need to be suitably redefined. - The incremental collector requires page dirty information, which - is acquired through routines defined in os_dep.c. Unless directed - otherwise by gcconfig.h, these are implemented as stubs that simply - treat all pages as dirty. (This of course makes the incremental - collector much less useful.) - -4. dyn_load.c - This provides a routine that allows the collector to scan data - segments associated with dynamic libraries. Often it is not - necessary to provide this routine unless user-written dynamic - libraries are used. - - For a different version of UN*X or different machines using the -Motorola 68000, Vax, SPARC, 80386, NS 32000, PC/RT, or MIPS architecture, -it should frequently suffice to change definitions in gcconfig.h. - - -THE C INTERFACE TO THE ALLOCATOR - - The following routines are intended to be directly called by the user. -Note that usually only GC_malloc is necessary. GC_clear_roots and GC_add_roots -calls may be required if the collector has to trace from nonstandard places -(e.g. from dynamic library data areas on a machine on which the -collector doesn't already understand them.) On some machines, it may -be desirable to set GC_stacktop to a good approximation of the stack base. -(This enhances code portability on HP PA machines, since there is no -good way for the collector to compute this value.) Client code may include -"gc.h", which defines all of the following, plus many others. - -1) GC_malloc(nbytes) - - allocate an object of size nbytes. Unlike malloc, the object is - cleared before being returned to the user. Gc_malloc will - invoke the garbage collector when it determines this to be appropriate. - GC_malloc may return 0 if it is unable to acquire sufficient - space from the operating system. This is the most probable - consequence of running out of space. Other possible consequences - are that a function call will fail due to lack of stack space, - or that the collector will fail in other ways because it cannot - maintain its internal data structures, or that a crucial system - process will fail and take down the machine. Most of these - possibilities are independent of the malloc implementation. - -2) GC_malloc_atomic(nbytes) - - allocate an object of size nbytes that is guaranteed not to contain any - pointers. The returned object is not guaranteed to be cleared. - (Can always be replaced by GC_malloc, but results in faster collection - times. The collector will probably run faster if large character - arrays, etc. are allocated with GC_malloc_atomic than if they are - statically allocated.) - -3) GC_realloc(object, new_size) - - change the size of object to be new_size. Returns a pointer to the - new object, which may, or may not, be the same as the pointer to - the old object. The new object is taken to be atomic iff the old one - was. If the new object is composite and larger than the original object, - then the newly added bytes are cleared (we hope). This is very likely - to allocate a new object, unless MERGE_SIZES is defined in gc_priv.h. - Even then, it is likely to recycle the old object only if the object - is grown in small additive increments (which, we claim, is generally bad - coding practice.) - -4) GC_free(object) - - explicitly deallocate an object returned by GC_malloc or - GC_malloc_atomic. Not necessary, but can be used to minimize - collections if performance is critical. Probably a performance - loss for very small objects (<= 8 bytes). - -5) GC_expand_hp(bytes) - - Explicitly increase the heap size. (This is normally done automatically - if a garbage collection failed to GC_reclaim enough memory. Explicit - calls to GC_expand_hp may prevent unnecessarily frequent collections at - program startup.) - -6) GC_malloc_ignore_off_page(bytes) - - identical to GC_malloc, but the client promises to keep a pointer to - the somewhere within the first 256 bytes of the object while it is - live. (This pointer should nortmally be declared volatile to prevent - interference from compiler optimizations.) This is the recommended - way to allocate anything that is likely to be larger than 100Kbytes - or so. (GC_malloc may result in failure to reclaim such objects.) - -7) GC_set_warn_proc(proc) - - Can be used to redirect warnings from the collector. Such warnings - should be rare, and should not be ignored during code development. - -8) GC_enable_incremental() - - Enables generational and incremental collection. Useful for large - heaps on machines that provide access to page dirty information. - Some dirty bit implementations may interfere with debugging - (by catching address faults) and place restrictions on heap arguments - to system calls (since write faults inside a system call may not be - handled well). - -9) Several routines to allow for registration of finalization code. - User supplied finalization code may be invoked when an object becomes - unreachable. To call (*f)(obj, x) when obj becomes inaccessible, use - GC_register_finalizer(obj, f, x, 0, 0); - For more sophisticated uses, and for finalization ordering issues, - see gc.h. - - The global variable GC_free_space_divisor may be adjusted up from its -default value of 4 to use less space and more collection time, or down for -the opposite effect. Setting it to 1 or 0 will effectively disable collections -and cause all allocations to simply grow the heap. - - The variable GC_non_gc_bytes, which is normally 0, may be changed to reflect -the amount of memory allocated by the above routines that should not be -considered as a candidate for collection. Careless use may, of course, result -in excessive memory consumption. - - Some additional tuning is possible through the parameters defined -near the top of gc_priv.h. - - If only GC_malloc is intended to be used, it might be appropriate to define: - -#define malloc(n) GC_malloc(n) -#define calloc(m,n) GC_malloc((m)*(n)) - - For small pieces of VERY allocation intensive code, gc_inl.h -includes some allocation macros that may be used in place of GC_malloc -and friends. - - All externally visible names in the garbage collector start with "GC_". -To avoid name conflicts, client code should avoid this prefix, except when -accessing garbage collector routines or variables. - - There are provisions for allocation with explicit type information. -This is rarely necessary. Details can be found in gc_typed.h. - -THE C++ INTERFACE TO THE ALLOCATOR: - - The Ellis-Hull C++ interface to the collector is included in -the collector distribution. If you intend to use this, type -"make c++" after the initial build of the collector is complete. -See gc_cpp.h for the definition of the interface. This interface -tries to approximate the Ellis-Detlefs C++ garbage collection -proposal without compiler changes. - -Cautions: -1. Arrays allocated without new placement syntax are -allocated as uncollectable objects. They are traced by the -collector, but will not be reclaimed. - -2. Failure to use "make c++" in combination with (1) will -result in arrays allocated using the default new operator. -This is likely to result in disaster without linker warnings. - -3. If your compiler supports an overloaded new[] operator, -then gc_cpp.cc and gc_cpp.h should be suitably modified. - -4. Many current C++ compilers have deficiencies that -break some of the functionality. See the comments in gc_cpp.h -for suggested workarounds. - -USE AS LEAK DETECTOR: - - The collector may be used to track down leaks in C programs that are -intended to run with malloc/free (e.g. code with extreme real-time or -portability constraints). To do so define FIND_LEAK in Makefile -This will cause the collector to invoke the report_leak -routine defined near the top of reclaim.c whenever an inaccessible -object is found that has not been explicitly freed. Such objects will -also be automatically reclaimed. - Productive use of this facility normally involves redefining report_leak -to do something more intelligent. This typically requires annotating -objects with additional information (e.g. creation time stack trace) that -identifies their origin. Such code is typically not very portable, and is -not included here, except on SPARC machines. - If all objects are allocated with GC_DEBUG_MALLOC (see next section), -then the default version of report_leak will report the source file -and line number at which the leaked object was allocated. This may -sometimes be sufficient. (On SPARC/SUNOS4 machines, it will also report -a cryptic stack trace. This can often be turned into a sympolic stack -trace by invoking program "foo" with "callprocs foo". Callprocs is -a short shell script that invokes adb to expand program counter values -to symbolic addresses. It was largely supplied by Scott Schwartz.) - Note that the debugging facilities described in the next section can -sometimes be slightly LESS effective in leak finding mode, since in -leak finding mode, GC_debug_free actually results in reuse of the object. -(Otherwise the object is simply marked invalid.) Also note that the test -program is not designed to run meaningfully in FIND_LEAK mode. -Use "make gc.a" to build the collector. - -DEBUGGING FACILITIES: - - The routines GC_debug_malloc, GC_debug_malloc_atomic, GC_debug_realloc, -and GC_debug_free provide an alternate interface to the collector, which -provides some help with memory overwrite errors, and the like. -Objects allocated in this way are annotated with additional -information. Some of this information is checked during garbage -collections, and detected inconsistencies are reported to stderr. - - Simple cases of writing past the end of an allocated object should -be caught if the object is explicitly deallocated, or if the -collector is invoked while the object is live. The first deallocation -of an object will clear the debugging info associated with an -object, so accidentally repeated calls to GC_debug_free will report the -deallocation of an object without debugging information. Out of -memory errors will be reported to stderr, in addition to returning -NIL. - - GC_debug_malloc checking during garbage collection is enabled -with the first call to GC_debug_malloc. This will result in some -slowdown during collections. If frequent heap checks are desired, -this can be achieved by explicitly invoking GC_gcollect, e.g. from -the debugger. - - GC_debug_malloc allocated objects should not be passed to GC_realloc -or GC_free, and conversely. It is however acceptable to allocate only -some objects with GC_debug_malloc, and to use GC_malloc for other objects, -provided the two pools are kept distinct. In this case, there is a very -low probablility that GC_malloc allocated objects may be misidentified as -having been overwritten. This should happen with probability at most -one in 2**32. This probability is zero if GC_debug_malloc is never called. - - GC_debug_malloc, GC_malloc_atomic, and GC_debug_realloc take two -additional trailing arguments, a string and an integer. These are not -interpreted by the allocator. They are stored in the object (the string is -not copied). If an error involving the object is detected, they are printed. - - The macros GC_MALLOC, GC_MALLOC_ATOMIC, GC_REALLOC, GC_FREE, and -GC_REGISTER_FINALIZER are also provided. These require the same arguments -as the corresponding (nondebugging) routines. If gc.h is included -with GC_DEBUG defined, they call the debugging versions of these -functions, passing the current file name and line number as the two -extra arguments, where appropriate. If gc.h is included without GC_DEBUG -defined, then all these macros will instead be defined to their nondebugging -equivalents. (GC_REGISTER_FINALIZER is necessary, since pointers to -objects with debugging information are really pointers to a displacement -of 16 bytes form the object beginning, and some translation is necessary -when finalization routines are invoked. For details, about what's stored -in the header, see the definition of the type oh in debug_malloc.c) - -INCREMENTAL/GENERATIONAL COLLECTION: - -The collector normally interrupts client code for the duration of -a garbage collection mark phase. This may be unacceptable if interactive -response is needed for programs with large heaps. The collector -can also run in a "generational" mode, in which it usually attempts to -collect only objects allocated since the last garbage collection. -Furthermore, in this mode, garbage collections run mostly incrementally, -with a small amount of work performed in response to each of a large number of -GC_malloc requests. - -This mode is enabled by a call to GC_enable_incremental(). - -Incremental and generational collection is effective in reducing -pause times only if the collector has some way to tell which objects -or pages have been recently modified. The collector uses two sources -of information: - -1. Information provided by the VM system. This may be provided in -one of several forms. Under Solaris 2.X (and potentially under other -similar systems) information on dirty pages can be read from the -/proc file system. Under other systems (currently SunOS4.X) it is -possible to write-protect the heap, and catch the resulting faults. -On these systems we require that system calls writing to the heap -(other than read) be handled specially by client code. -See os_dep.c for details. - -2. Information supplied by the programmer. We define "stubborn" -objects to be objects that are rarely changed. Such an object -can be allocated (and enabled for writing) with GC_malloc_stubborn. -Once it has been initialized, the collector should be informed with -a call to GC_end_stubborn_change. Subsequent writes that store -pointers into the object must be preceded by a call to -GC_change_stubborn. - -This mechanism performs best for objects that are written only for -initialization, and such that only one stubborn object is writable -at once. It is typically not worth using for short-lived -objects. Stubborn objects are treated less efficiently than pointerfree -(atomic) objects. - -A rough rule of thumb is that, in the absence of VM information, garbage -collection pauses are proportional to the amount of pointerful storage -plus the amount of modified "stubborn" storage that is reachable during -the collection. - -Initial allocation of stubborn objects takes longer than allocation -of other objects, since other data structures need to be maintained. - -We recommend against random use of stubborn objects in client -code, since bugs caused by inappropriate writes to stubborn objects -are likely to be very infrequently observed and hard to trace. -However, their use may be appropriate in a few carefully written -library routines that do not make the objects themselves available -for writing by client code. - - -BUGS: - - Any memory that does not have a recognizable pointer to it will be -reclaimed. Exclusive-or'ing forward and backward links in a list -doesn't cut it. - Some C optimizers may lose the last undisguised pointer to a memory -object as a consequence of clever optimizations. This has almost -never been observed in practice. Send mail to boehm@acm.org -for suggestions on how to fix your compiler. - This is not a real-time collector. In the standard configuration, -percentage of time required for collection should be constant across -heap sizes. But collection pauses will increase for larger heaps. -(On SPARCstation 2s collection times will be on the order of 300 msecs -per MB of accessible memory that needs to be scanned. Your mileage -may vary.) The incremental/generational collection facility helps, -but is portable only if "stubborn" allocation is used. - Please address bug reports to boehm@acm.org. If you are -contemplating a major addition, you might also send mail to ask whether -it's already been done (or whether we tried and discarded it). - |