Annotation of OpenXM_contrib2/asir2000/gc/doc/README, Revision 1.2
1.1 noro 1: Copyright (c) 1988, 1989 Hans-J. Boehm, Alan J. Demers
2: Copyright (c) 1991-1996 by Xerox Corporation. All rights reserved.
3: Copyright (c) 1996-1999 by Silicon Graphics. All rights reserved.
4: Copyright (c) 1999-2001 by Hewlett-Packard Company. All rights reserved.
5:
6: The file linux_threads.c is also
7: Copyright (c) 1998 by Fergus Henderson. All rights reserved.
8:
9: The files Makefile.am, and configure.in are
10: Copyright (c) 2001 by Red Hat Inc. All rights reserved.
11:
12: Several files supporting GNU-style builds are copyrighted by the Free
13: Software Foundation, and carry a different license from that given
14: below.
15:
16: THIS MATERIAL IS PROVIDED AS IS, WITH ABSOLUTELY NO WARRANTY EXPRESSED
17: OR IMPLIED. ANY USE IS AT YOUR OWN RISK.
18:
19: Permission is hereby granted to use or copy this program
20: for any purpose, provided the above notices are retained on all copies.
21: Permission to modify the code and to distribute modified code is granted,
22: provided the above notices are retained, and a notice that the code was
23: modified is included with the above copyright notice.
24:
25: A few of the files needed to use the GNU-style build procedure come with
26: slightly different licenses, though they are all similar in spirit. A few
27: are GPL'ed, but with an exception that should cover all uses in the
28: collector. (If you are concerned about such things, I recommend you look
29: at the notice in config.guess or ltmain.sh.)
30:
1.2 ! noro 31: This is version 6.2alpha6 of a conservative garbage collector for C and C++.
1.1 noro 32:
33: You might find a more recent version of this at
34:
35: http://www.hpl.hp.com/personal/Hans_Boehm/gc
36:
37: OVERVIEW
38:
39: This is intended to be a general purpose, garbage collecting storage
40: allocator. The algorithms used are described in:
41:
42: Boehm, H., and M. Weiser, "Garbage Collection in an Uncooperative Environment",
43: Software Practice & Experience, September 1988, pp. 807-820.
44:
45: Boehm, H., A. Demers, and S. Shenker, "Mostly Parallel Garbage Collection",
46: Proceedings of the ACM SIGPLAN '91 Conference on Programming Language Design
47: and Implementation, SIGPLAN Notices 26, 6 (June 1991), pp. 157-164.
48:
49: Boehm, H., "Space Efficient Conservative Garbage Collection", Proceedings
50: of the ACM SIGPLAN '91 Conference on Programming Language Design and
51: Implementation, SIGPLAN Notices 28, 6 (June 1993), pp. 197-206.
52:
53: Boehm H., "Reducing Garbage Collector Cache Misses", Proceedings of the
54: 2000 International Symposium on Memory Management.
55:
56: Possible interactions between the collector and optimizing compilers are
57: discussed in
58:
59: Boehm, H., and D. Chase, "A Proposal for GC-safe C Compilation",
60: The Journal of C Language Translation 4, 2 (December 1992).
61:
62: and
63:
64: Boehm H., "Simple GC-safe Compilation", Proceedings
65: of the ACM SIGPLAN '96 Conference on Programming Language Design and
66: Implementation.
67:
68: (Some of these are also available from
69: http://www.hpl.hp.com/personal/Hans_Boehm/papers/, among other places.)
70:
71: Unlike the collector described in the second reference, this collector
72: operates either with the mutator stopped during the entire collection
73: (default) or incrementally during allocations. (The latter is supported
74: on only a few machines.) On the most common platforms, it can be built
75: with or without thread support. On a few platforms, it can take advantage
76: of a multiprocessor to speed up garbage collection.
77:
78: Many of the ideas underlying the collector have previously been explored
79: by others. Notably, some of the run-time systems developed at Xerox PARC
80: in the early 1980s conservatively scanned thread stacks to locate possible
81: pointers (cf. Paul Rovner, "On Adding Garbage Collection and Runtime Types
82: to a Strongly-Typed Statically Checked, Concurrent Language" Xerox PARC
83: CSL 84-7). Doug McIlroy wrote a simpler fully conservative collector that
84: was part of version 8 UNIX (tm), but appears to not have received
85: widespread use.
86:
87: Rudimentary tools for use of the collector as a leak detector are included
88: (see http://www.hpl.hp.com/personal/Hans_Boehm/gc/leak.html),
89: as is a fairly sophisticated string package "cord" that makes use of the
90: collector. (See doc/README.cords and H.-J. Boehm, R. Atkinson, and M. Plass,
91: "Ropes: An Alternative to Strings", Software Practice and Experience 25, 12
92: (December 1995), pp. 1315-1330. This is very similar to the "rope" package
93: in Xerox Cedar, or the "rope" package in the SGI STL or the g++ distribution.)
94:
95: Further collector documantation can be found at
96:
97: http://www.hpl.hp.com/personal/Hans_Boehm/gc
98:
99:
100: GENERAL DESCRIPTION
101:
102: This is a garbage collecting storage allocator that is intended to be
103: used as a plug-in replacement for C's malloc.
104:
105: Since the collector does not require pointers to be tagged, it does not
106: attempt to ensure that all inaccessible storage is reclaimed. However,
107: in our experience, it is typically more successful at reclaiming unused
108: memory than most C programs using explicit deallocation. Unlike manually
109: introduced leaks, the amount of unreclaimed memory typically stays
110: bounded.
111:
112: In the following, an "object" is defined to be a region of memory allocated
113: by the routines described below.
114:
115: Any objects not intended to be collected must be pointed to either
116: from other such accessible objects, or from the registers,
117: stack, data, or statically allocated bss segments. Pointers from
118: the stack or registers may point to anywhere inside an object.
119: The same is true for heap pointers if the collector is compiled with
120: ALL_INTERIOR_POINTERS defined, as is now the default.
121:
122: Compiling without ALL_INTERIOR_POINTERS may reduce accidental retention
123: of garbage objects, by requiring pointers from the heap to to the beginning
124: of an object. But this no longer appears to be a significant
125: issue for most programs.
126:
127: There are a number of routines which modify the pointer recognition
128: algorithm. GC_register_displacement allows certain interior pointers
129: to be recognized even if ALL_INTERIOR_POINTERS is nor defined.
130: GC_malloc_ignore_off_page allows some pointers into the middle of large objects
131: to be disregarded, greatly reducing the probablility of accidental
132: retention of large objects. For most purposes it seems best to compile
133: with ALL_INTERIOR_POINTERS and to use GC_malloc_ignore_off_page if
134: you get collector warnings from allocations of very large objects.
135: See README.debugging for details.
136:
137: WARNING: pointers inside memory allocated by the standard "malloc" are not
138: seen by the garbage collector. Thus objects pointed to only from such a
139: region may be prematurely deallocated. It is thus suggested that the
140: standard "malloc" be used only for memory regions, such as I/O buffers, that
141: are guaranteed not to contain pointers to garbage collectable memory.
142: Pointers in C language automatic, static, or register variables,
143: are correctly recognized. (Note that GC_malloc_uncollectable has semantics
144: similar to standard malloc, but allocates objects that are traced by the
145: collector.)
146:
147: WARNING: the collector does not always know how to find pointers in data
148: areas that are associated with dynamic libraries. This is easy to
149: remedy IF you know how to find those data areas on your operating
150: system (see GC_add_roots). Code for doing this under SunOS, IRIX 5.X and 6.X,
151: HP/UX, Alpha OSF/1, Linux, and win32 is included and used by default. (See
152: README.win32 for win32 details.) On other systems pointers from dynamic
153: library data areas may not be considered by the collector.
154: If you're writing a program that depends on the collector scanning
155: dynamic library data areas, it may be a good idea to include at least
156: one call to GC_is_visible() to ensure that those areas are visible
157: to the collector.
158:
159: Note that the garbage collector does not need to be informed of shared
160: read-only data. However if the shared library mechanism can introduce
161: discontiguous data areas that may contain pointers, then the collector does
162: need to be informed.
163:
164: Signal processing for most signals may be deferred during collection,
165: and during uninterruptible parts of the allocation process.
166: Like standard ANSI C mallocs, by default it is unsafe to invoke
167: malloc (and other GC routines) from a signal handler while another
168: malloc call may be in progress. Removing -DNO_SIGNALS from Makefile
169: attempts to remedy that. But that may not be reliable with a compiler that
170: substantially reorders memory operations inside GC_malloc.
171:
172: The allocator/collector can also be configured for thread-safe operation.
173: (Full signal safety can also be achieved, but only at the cost of two system
174: calls per malloc, which is usually unacceptable.)
175: WARNING: the collector does not guarantee to scan thread-local storage
176: (e.g. of the kind accessed with pthread_getspecific()). The collector
177: does scan thread stacks, though, so generally the best solution is to
178: ensure that any pointers stored in thread-local storage are also
179: stored on the thread's stack for the duration of their lifetime.
180: (This is arguably a longstanding bug, but it hasn't been fixed yet.)
181:
182: INSTALLATION AND PORTABILITY
183:
184: As distributed, the macro SILENT is defined in Makefile.
185: In the event of problems, this can be removed to obtain a moderate
186: amount of descriptive output for each collection.
187: (The given statistics exhibit a few peculiarities.
188: Things don't appear to add up for a variety of reasons, most notably
189: fragmentation losses. These are probably much more significant for the
190: contrived program "test.c" than for your application.)
191:
192: Note that typing "make test" will automatically build the collector
193: and then run setjmp_test and gctest. Setjmp_test will give you information
194: about configuring the collector, which is useful primarily if you have
195: a machine that's not already supported. Gctest is a somewhat superficial
196: test of collector functionality. Failure is indicated by a core dump or
197: a message to the effect that the collector is broken. Gctest takes about
198: 35 seconds to run on a SPARCstation 2. It may use up to 8 MB of memory. (The
199: multi-threaded version will use more. 64-bit versions may use more.)
200: "Make test" will also, as its last step, attempt to build and test the
201: "cord" string library. This will fail without an ANSI C compiler, but
202: the garbage collector itself should still be usable.
203:
204: The Makefile will generate a library gc.a which you should link against.
205: Typing "make cords" will add the cord library to gc.a.
206: Note that this requires an ANSI C compiler.
207:
208: It is suggested that if you need to replace a piece of the collector
209: (e.g. GC_mark_rts.c) you simply list your version ahead of gc.a on the
210: ld command line, rather than replacing the one in gc.a. (This will
211: generate numerous warnings under some versions of AIX, but it still
212: works.)
213:
214: All include files that need to be used by clients will be put in the
215: include subdirectory. (Normally this is just gc.h. "Make cords" adds
216: "cord.h" and "ec.h".)
217:
218: The collector currently is designed to run essentially unmodified on
219: machines that use a flat 32-bit or 64-bit address space.
220: That includes the vast majority of Workstations and X86 (X >= 3) PCs.
221: (The list here was deleted because it was getting too long and constantly
222: out of date.)
223: It does NOT run under plain 16-bit DOS or Windows 3.X. There are however
224: various packages (e.g. win32s, djgpp) that allow flat 32-bit address
225: applications to run under those systemsif the have at least an 80386 processor,
226: and several of those are compatible with the collector.
227:
228: In a few cases (Amiga, OS/2, Win32, MacOS) a separate makefile
229: or equivalent is supplied. Many of these have separate README.system
230: files.
231:
1.2 ! noro 232: Dynamic libraries are completely supported only under SunOS/Solaris,
1.1 noro 233: (and even that support is not functional on the last Sun 3 release),
1.2 ! noro 234: Linux, FreeBSD, NetBSD, IRIX 5&6, HP/UX, Win32 (not Win32S) and OSF/1
! 235: on DEC AXP machines plus perhaps a few others listed near the top
! 236: of dyn_load.c. On other machines we recommend that you do one of
! 237: the following:
1.1 noro 238:
239: 1) Add dynamic library support (and send us the code).
240: 2) Use static versions of the libraries.
241: 3) Arrange for dynamic libraries to use the standard malloc.
242: This is still dangerous if the library stores a pointer to a
243: garbage collected object. But nearly all standard interfaces
244: prohibit this, because they deal correctly with pointers
245: to stack allocated objects. (Strtok is an exception. Don't
246: use it.)
247:
248: In all cases we assume that pointer alignment is consistent with that
249: enforced by the standard C compilers. If you use a nonstandard compiler
250: you may have to adjust the alignment parameters defined in gc_priv.h.
1.2 ! noro 251: Note that this may also be an issue with packed records/structs, if those
! 252: enforce less alignment for pointers.
1.1 noro 253:
254: A port to a machine that is not byte addressed, or does not use 32 bit
255: or 64 bit addresses will require a major effort. A port to plain MSDOS
256: or win16 is hard.
257:
258: For machines not already mentioned, or for nonstandard compilers, the
259: following are likely to require change:
260:
261: 1. The parameters in gcconfig.h.
262: The parameters that will usually require adjustment are
263: STACKBOTTOM, ALIGNMENT and DATASTART. Setjmp_test
264: prints its guesses of the first two.
265: DATASTART should be an expression for computing the
266: address of the beginning of the data segment. This can often be
267: &etext. But some memory management units require that there be
268: some unmapped space between the text and the data segment. Thus
269: it may be more complicated. On UNIX systems, this is rarely
270: documented. But the adb "$m" command may be helpful. (Note
271: that DATASTART will usually be a function of &etext. Thus a
272: single experiment is usually insufficient.)
273: STACKBOTTOM is used to initialize GC_stackbottom, which
274: should be a sufficient approximation to the coldest stack address.
275: On some machines, it is difficult to obtain such a value that is
276: valid across a variety of MMUs, OS releases, etc. A number of
277: alternatives exist for using the collector in spite of this. See the
278: discussion in gcconfig.h immediately preceding the various
279: definitions of STACKBOTTOM.
280:
281: 2. mach_dep.c.
282: The most important routine here is one to mark from registers.
283: The distributed file includes a generic hack (based on setjmp) that
284: happens to work on many machines, and may work on yours. Try
285: compiling and running setjmp_t.c to see whether it has a chance of
286: working. (This is not correct C, so don't blame your compiler if it
287: doesn't work. Based on limited experience, register window machines
288: are likely to cause trouble. If your version of setjmp claims that
289: all accessible variables, including registers, have the value they
290: had at the time of the longjmp, it also will not work. Vanilla 4.2 BSD
291: on Vaxen makes such a claim. SunOS does not.)
292: If your compiler does not allow in-line assembly code, or if you prefer
293: not to use such a facility, mach_dep.c may be replaced by a .s file
294: (as we did for the MIPS machine and the PC/RT).
295: At this point enough architectures are supported by mach_dep.c
296: that you will rarely need to do more than adjust for assembler
297: syntax.
298:
299: 3. os_dep.c (and gc_priv.h).
300: Several kinds of operating system dependent routines reside here.
301: Many are optional. Several are invoked only through corresponding
302: macros in gc_priv.h, which may also be redefined as appropriate.
303: The routine GC_register_data_segments is crucial. It registers static
304: data areas that must be traversed by the collector. (User calls to
305: GC_add_roots may sometimes be used for similar effect.)
306: Routines to obtain memory from the OS also reside here.
307: Alternatively this can be done entirely by the macro GET_MEM
308: defined in gc_priv.h. Routines to disable and reenable signals
309: also reside here if they are need by the macros DISABLE_SIGNALS
310: and ENABLE_SIGNALS defined in gc_priv.h.
311: In a multithreaded environment, the macros LOCK and UNLOCK
312: in gc_priv.h will need to be suitably redefined.
313: The incremental collector requires page dirty information, which
314: is acquired through routines defined in os_dep.c. Unless directed
315: otherwise by gcconfig.h, these are implemented as stubs that simply
316: treat all pages as dirty. (This of course makes the incremental
317: collector much less useful.)
318:
319: 4. dyn_load.c
320: This provides a routine that allows the collector to scan data
321: segments associated with dynamic libraries. Often it is not
322: necessary to provide this routine unless user-written dynamic
323: libraries are used.
324:
325: For a different version of UN*X or different machines using the
326: Motorola 68000, Vax, SPARC, 80386, NS 32000, PC/RT, or MIPS architecture,
327: it should frequently suffice to change definitions in gcconfig.h.
328:
329:
330: THE C INTERFACE TO THE ALLOCATOR
331:
332: The following routines are intended to be directly called by the user.
333: Note that usually only GC_malloc is necessary. GC_clear_roots and GC_add_roots
334: calls may be required if the collector has to trace from nonstandard places
335: (e.g. from dynamic library data areas on a machine on which the
336: collector doesn't already understand them.) On some machines, it may
337: be desirable to set GC_stacktop to a good approximation of the stack base.
338: (This enhances code portability on HP PA machines, since there is no
339: good way for the collector to compute this value.) Client code may include
340: "gc.h", which defines all of the following, plus many others.
341:
342: 1) GC_malloc(nbytes)
343: - allocate an object of size nbytes. Unlike malloc, the object is
344: cleared before being returned to the user. Gc_malloc will
345: invoke the garbage collector when it determines this to be appropriate.
346: GC_malloc may return 0 if it is unable to acquire sufficient
347: space from the operating system. This is the most probable
348: consequence of running out of space. Other possible consequences
349: are that a function call will fail due to lack of stack space,
350: or that the collector will fail in other ways because it cannot
351: maintain its internal data structures, or that a crucial system
352: process will fail and take down the machine. Most of these
353: possibilities are independent of the malloc implementation.
354:
355: 2) GC_malloc_atomic(nbytes)
356: - allocate an object of size nbytes that is guaranteed not to contain any
357: pointers. The returned object is not guaranteed to be cleared.
358: (Can always be replaced by GC_malloc, but results in faster collection
359: times. The collector will probably run faster if large character
360: arrays, etc. are allocated with GC_malloc_atomic than if they are
361: statically allocated.)
362:
363: 3) GC_realloc(object, new_size)
364: - change the size of object to be new_size. Returns a pointer to the
365: new object, which may, or may not, be the same as the pointer to
366: the old object. The new object is taken to be atomic iff the old one
367: was. If the new object is composite and larger than the original object,
368: then the newly added bytes are cleared (we hope). This is very likely
369: to allocate a new object, unless MERGE_SIZES is defined in gc_priv.h.
370: Even then, it is likely to recycle the old object only if the object
371: is grown in small additive increments (which, we claim, is generally bad
372: coding practice.)
373:
374: 4) GC_free(object)
375: - explicitly deallocate an object returned by GC_malloc or
376: GC_malloc_atomic. Not necessary, but can be used to minimize
377: collections if performance is critical. Probably a performance
378: loss for very small objects (<= 8 bytes).
379:
380: 5) GC_expand_hp(bytes)
381: - Explicitly increase the heap size. (This is normally done automatically
382: if a garbage collection failed to GC_reclaim enough memory. Explicit
383: calls to GC_expand_hp may prevent unnecessarily frequent collections at
384: program startup.)
385:
386: 6) GC_malloc_ignore_off_page(bytes)
387: - identical to GC_malloc, but the client promises to keep a pointer to
388: the somewhere within the first 256 bytes of the object while it is
389: live. (This pointer should nortmally be declared volatile to prevent
390: interference from compiler optimizations.) This is the recommended
391: way to allocate anything that is likely to be larger than 100Kbytes
392: or so. (GC_malloc may result in failure to reclaim such objects.)
393:
394: 7) GC_set_warn_proc(proc)
395: - Can be used to redirect warnings from the collector. Such warnings
396: should be rare, and should not be ignored during code development.
397:
398: 8) GC_enable_incremental()
399: - Enables generational and incremental collection. Useful for large
400: heaps on machines that provide access to page dirty information.
401: Some dirty bit implementations may interfere with debugging
402: (by catching address faults) and place restrictions on heap arguments
403: to system calls (since write faults inside a system call may not be
404: handled well).
405:
406: 9) Several routines to allow for registration of finalization code.
407: User supplied finalization code may be invoked when an object becomes
408: unreachable. To call (*f)(obj, x) when obj becomes inaccessible, use
409: GC_register_finalizer(obj, f, x, 0, 0);
410: For more sophisticated uses, and for finalization ordering issues,
411: see gc.h.
412:
413: The global variable GC_free_space_divisor may be adjusted up from its
414: default value of 4 to use less space and more collection time, or down for
415: the opposite effect. Setting it to 1 or 0 will effectively disable collections
416: and cause all allocations to simply grow the heap.
417:
418: The variable GC_non_gc_bytes, which is normally 0, may be changed to reflect
419: the amount of memory allocated by the above routines that should not be
420: considered as a candidate for collection. Careless use may, of course, result
421: in excessive memory consumption.
422:
423: Some additional tuning is possible through the parameters defined
424: near the top of gc_priv.h.
425:
426: If only GC_malloc is intended to be used, it might be appropriate to define:
427:
428: #define malloc(n) GC_malloc(n)
429: #define calloc(m,n) GC_malloc((m)*(n))
430:
431: For small pieces of VERY allocation intensive code, gc_inl.h
432: includes some allocation macros that may be used in place of GC_malloc
433: and friends.
434:
435: All externally visible names in the garbage collector start with "GC_".
436: To avoid name conflicts, client code should avoid this prefix, except when
437: accessing garbage collector routines or variables.
438:
439: There are provisions for allocation with explicit type information.
440: This is rarely necessary. Details can be found in gc_typed.h.
441:
442: THE C++ INTERFACE TO THE ALLOCATOR:
443:
444: The Ellis-Hull C++ interface to the collector is included in
445: the collector distribution. If you intend to use this, type
446: "make c++" after the initial build of the collector is complete.
447: See gc_cpp.h for the definition of the interface. This interface
448: tries to approximate the Ellis-Detlefs C++ garbage collection
449: proposal without compiler changes.
450:
451: Cautions:
452: 1. Arrays allocated without new placement syntax are
453: allocated as uncollectable objects. They are traced by the
454: collector, but will not be reclaimed.
455:
456: 2. Failure to use "make c++" in combination with (1) will
457: result in arrays allocated using the default new operator.
458: This is likely to result in disaster without linker warnings.
459:
460: 3. If your compiler supports an overloaded new[] operator,
461: then gc_cpp.cc and gc_cpp.h should be suitably modified.
462:
463: 4. Many current C++ compilers have deficiencies that
464: break some of the functionality. See the comments in gc_cpp.h
465: for suggested workarounds.
466:
467: USE AS LEAK DETECTOR:
468:
469: The collector may be used to track down leaks in C programs that are
470: intended to run with malloc/free (e.g. code with extreme real-time or
471: portability constraints). To do so define FIND_LEAK in Makefile
472: This will cause the collector to invoke the report_leak
473: routine defined near the top of reclaim.c whenever an inaccessible
474: object is found that has not been explicitly freed. Such objects will
475: also be automatically reclaimed.
476: Productive use of this facility normally involves redefining report_leak
477: to do something more intelligent. This typically requires annotating
478: objects with additional information (e.g. creation time stack trace) that
479: identifies their origin. Such code is typically not very portable, and is
480: not included here, except on SPARC machines.
481: If all objects are allocated with GC_DEBUG_MALLOC (see next section),
482: then the default version of report_leak will report the source file
483: and line number at which the leaked object was allocated. This may
484: sometimes be sufficient. (On SPARC/SUNOS4 machines, it will also report
485: a cryptic stack trace. This can often be turned into a sympolic stack
486: trace by invoking program "foo" with "callprocs foo". Callprocs is
487: a short shell script that invokes adb to expand program counter values
488: to symbolic addresses. It was largely supplied by Scott Schwartz.)
489: Note that the debugging facilities described in the next section can
490: sometimes be slightly LESS effective in leak finding mode, since in
491: leak finding mode, GC_debug_free actually results in reuse of the object.
492: (Otherwise the object is simply marked invalid.) Also note that the test
493: program is not designed to run meaningfully in FIND_LEAK mode.
494: Use "make gc.a" to build the collector.
495:
496: DEBUGGING FACILITIES:
497:
498: The routines GC_debug_malloc, GC_debug_malloc_atomic, GC_debug_realloc,
499: and GC_debug_free provide an alternate interface to the collector, which
500: provides some help with memory overwrite errors, and the like.
501: Objects allocated in this way are annotated with additional
502: information. Some of this information is checked during garbage
503: collections, and detected inconsistencies are reported to stderr.
504:
505: Simple cases of writing past the end of an allocated object should
506: be caught if the object is explicitly deallocated, or if the
507: collector is invoked while the object is live. The first deallocation
508: of an object will clear the debugging info associated with an
509: object, so accidentally repeated calls to GC_debug_free will report the
510: deallocation of an object without debugging information. Out of
511: memory errors will be reported to stderr, in addition to returning
512: NIL.
513:
514: GC_debug_malloc checking during garbage collection is enabled
515: with the first call to GC_debug_malloc. This will result in some
516: slowdown during collections. If frequent heap checks are desired,
517: this can be achieved by explicitly invoking GC_gcollect, e.g. from
518: the debugger.
519:
520: GC_debug_malloc allocated objects should not be passed to GC_realloc
521: or GC_free, and conversely. It is however acceptable to allocate only
522: some objects with GC_debug_malloc, and to use GC_malloc for other objects,
523: provided the two pools are kept distinct. In this case, there is a very
524: low probablility that GC_malloc allocated objects may be misidentified as
525: having been overwritten. This should happen with probability at most
526: one in 2**32. This probability is zero if GC_debug_malloc is never called.
527:
528: GC_debug_malloc, GC_malloc_atomic, and GC_debug_realloc take two
529: additional trailing arguments, a string and an integer. These are not
530: interpreted by the allocator. They are stored in the object (the string is
531: not copied). If an error involving the object is detected, they are printed.
532:
533: The macros GC_MALLOC, GC_MALLOC_ATOMIC, GC_REALLOC, GC_FREE, and
534: GC_REGISTER_FINALIZER are also provided. These require the same arguments
535: as the corresponding (nondebugging) routines. If gc.h is included
536: with GC_DEBUG defined, they call the debugging versions of these
537: functions, passing the current file name and line number as the two
538: extra arguments, where appropriate. If gc.h is included without GC_DEBUG
539: defined, then all these macros will instead be defined to their nondebugging
540: equivalents. (GC_REGISTER_FINALIZER is necessary, since pointers to
541: objects with debugging information are really pointers to a displacement
542: of 16 bytes form the object beginning, and some translation is necessary
543: when finalization routines are invoked. For details, about what's stored
544: in the header, see the definition of the type oh in debug_malloc.c)
545:
546: INCREMENTAL/GENERATIONAL COLLECTION:
547:
548: The collector normally interrupts client code for the duration of
549: a garbage collection mark phase. This may be unacceptable if interactive
550: response is needed for programs with large heaps. The collector
551: can also run in a "generational" mode, in which it usually attempts to
552: collect only objects allocated since the last garbage collection.
553: Furthermore, in this mode, garbage collections run mostly incrementally,
554: with a small amount of work performed in response to each of a large number of
555: GC_malloc requests.
556:
557: This mode is enabled by a call to GC_enable_incremental().
558:
559: Incremental and generational collection is effective in reducing
560: pause times only if the collector has some way to tell which objects
561: or pages have been recently modified. The collector uses two sources
562: of information:
563:
564: 1. Information provided by the VM system. This may be provided in
565: one of several forms. Under Solaris 2.X (and potentially under other
566: similar systems) information on dirty pages can be read from the
567: /proc file system. Under other systems (currently SunOS4.X) it is
568: possible to write-protect the heap, and catch the resulting faults.
569: On these systems we require that system calls writing to the heap
570: (other than read) be handled specially by client code.
571: See os_dep.c for details.
572:
573: 2. Information supplied by the programmer. We define "stubborn"
574: objects to be objects that are rarely changed. Such an object
575: can be allocated (and enabled for writing) with GC_malloc_stubborn.
576: Once it has been initialized, the collector should be informed with
577: a call to GC_end_stubborn_change. Subsequent writes that store
578: pointers into the object must be preceded by a call to
579: GC_change_stubborn.
580:
581: This mechanism performs best for objects that are written only for
582: initialization, and such that only one stubborn object is writable
583: at once. It is typically not worth using for short-lived
584: objects. Stubborn objects are treated less efficiently than pointerfree
585: (atomic) objects.
586:
587: A rough rule of thumb is that, in the absence of VM information, garbage
588: collection pauses are proportional to the amount of pointerful storage
589: plus the amount of modified "stubborn" storage that is reachable during
590: the collection.
591:
592: Initial allocation of stubborn objects takes longer than allocation
593: of other objects, since other data structures need to be maintained.
594:
595: We recommend against random use of stubborn objects in client
596: code, since bugs caused by inappropriate writes to stubborn objects
597: are likely to be very infrequently observed and hard to trace.
598: However, their use may be appropriate in a few carefully written
599: library routines that do not make the objects themselves available
600: for writing by client code.
601:
602:
603: BUGS:
604:
605: Any memory that does not have a recognizable pointer to it will be
606: reclaimed. Exclusive-or'ing forward and backward links in a list
607: doesn't cut it.
608: Some C optimizers may lose the last undisguised pointer to a memory
609: object as a consequence of clever optimizations. This has almost
610: never been observed in practice. Send mail to boehm@acm.org
611: for suggestions on how to fix your compiler.
612: This is not a real-time collector. In the standard configuration,
613: percentage of time required for collection should be constant across
614: heap sizes. But collection pauses will increase for larger heaps.
615: (On SPARCstation 2s collection times will be on the order of 300 msecs
616: per MB of accessible memory that needs to be scanned. Your mileage
617: may vary.) The incremental/generational collection facility helps,
618: but is portable only if "stubborn" allocation is used.
619: Please address bug reports to boehm@acm.org. If you are
620: contemplating a major addition, you might also send mail to ask whether
621: it's already been done (or whether we tried and discarded it).
622:
FreeBSD-CVSweb <freebsd-cvsweb@FreeBSD.org>
![[BACK]](/icons/cvsweb/back.gif){kind=icon}
Return to README CVS log ![[TXT]](/icons/cvsweb/text.gif){kind=icon}
![[DIR]](/icons/cvsweb/dir.gif){kind=icon}
Up to [local] / OpenXM_contrib2 / asir2000 / gc / doc