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diff --git a/boehm-gc/doc/README b/boehm-gc/doc/README new file mode 100644 index 000000000..a19cb3021 --- /dev/null +++ b/boehm-gc/doc/README @@ -0,0 +1,622 @@ +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) 1999-2004 Hewlett-Packard Development Company, L.P. + +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 6.6 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/Solaris, +(and even that support is not functional on the last Sun 3 release), +Linux, FreeBSD, NetBSD, IRIX 5&6, HP/UX, Win32 (not Win32S) and OSF/1 +on DEC AXP machines plus perhaps a few others listed near the top +of dyn_load.c. 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. +Note that this may also be an issue with packed records/structs, if those +enforce less alignment for pointers. + + 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). + |