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Cleanup

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Justin Ethier 2016-01-03 22:17:47 -05:00
parent ac7fec70a0
commit 4d7cd020cc
2 changed files with 91 additions and 176 deletions
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87
gc-notes.txt
View file

@ -6,13 +6,6 @@ Phase 5 (gc-dev5) - Require pthreads library, stand cyclone back up using new GC
Phase 6 (gc-dev6) - Multiple mutators (application threads)
Phase 7 (TBD) - Sharing of variables between threads (ideally without limitation, but that might not be realistic)
Mutex TODO:
- add mutex type, and associated functions from SRFI-18
when allocating a mutex, probably should do it on thread since by definition these are
shared among multiple threads
- may be able to free mutex using mutex_destroy from within gc_sweep.
unfortunately it requires type checking each object before free, which is not ideal
TODO:
- multiple mutators, and threading functions/types. probably want this on a new branch, when ready
part of this is implementing the beginnings of srfi-18, to create multiple threads, sync them, etc
@ -119,3 +112,83 @@ DONE:
this feature might end up being gc-dev7 (possibly the final phase)
ORIGINAL notes migrated here from gc.c:
/*
Rough plan for how to implement new GC algorithm. We need to do this in
phases in order to have any hope of getting everything working. Let's prove
the algorithm out, then extend support to multiple mutators if everything
looks good.
PHASE 1 - separation of mutator and collector into separate threads
need to syncronize access (preferably via atomics) for anything shared between the
collector and mutator threads.
can cooperate be part of a minor gc? in that case, the
marking could be done as part of allocation
but then what exactly does that mean, to mark gray? because
objects moved to the heap will be set to mark color at that
point (until collector thread finishes). but would want
objects on the heap referenced by them to be traced, so
I suppose that is the purpose of the gray, to indicate
those still need to be traced. but need to think this through,
do we need the markbuffer and last read/write? do those make
sense with mta approach (assume so)???
ONLY CONCERN - what happens if an object on the stack
has a reference to an object on the heap that is collected?
but how would this happen? collector marks global roots before
telling mutators to go to async, and once mutators go async
any allocations will not be collected. also once collectors go
async they have a chance to markgray, which will include the write
barrier. so given that, is it still possible for an old heap ref to
sneak into a stack object during the async phase?
more questions on above point:
- figure out how/if after cooperation/async, can a stack object pick
up a reference to a heap object that will be collected during that GC cycle?
need to be able to prevent this somehow...
- need to figure out real world use case(s) where this could happen, to try and
figure out how to address this problem
from my understanding of the paper, the write barrier prevents this. consider, at the
start of async, the mutator's roots, global roots, and anything on the write barrier
have been marked. any new objects will be allocated as marked. that way, anything the
mutator could later access is either marked or will be after tracing. the only exception
is if the mutator changes a reference such that tracing will no longer find an object.
but the write barrier prevents this - during tracing a heap update causes the old
object to be marked as well. so it will eventually be traced, and there should be no
dangling objects after GC completes.
PHASE 2 - multi-threaded mutator (IE, more than one stack thread):
- how does the collector handle stack objects that reference objects from
another thread's stack?
* minor GC will only relocate that thread's objects, so another thread's would not
be moved. however, if another thread references one of the GC'd thread's
stack objects, it will now get a forwarding pointer. even worse, what if the
other thread is blocked and the reference becomes corrupt due to the stack
longjmp? there are major issues with one thread referencing another thread's
objects.
* had considered adding a stack bit to the object header. if we do this and
initialize it during object creation, a thread could in theory detect
if an object belongs to another thread. but it might be expensive because
a read barrier would have to be used to check the object's stack bit and
address (to see if it is on this heap).
* alternatively, how would one thread pick up a reference to another one's
objects? are there any ways to detect these events and deal with them?
it might be possible to detect such a case and allocate the object on the heap,
replacing it with a fwd pointer. unfortunately that means we need a read
barrier (ick) to handle forwarding pointers in arbitrary places
* but does that mean we need a fwd pointer to be live for awhile? do we need
a read barrier to get this to work? obviously we want to avoid a read barrier
at all costs.
- what are the real costs of allowing forwarding pointers to exist outside of just
minor GC? assume each runtime primitive would need to be updated to handle the
case where the obj is a fwd pointer - is it just a matter of each function
detecting this and (possibly) calling itself again with the 'real' address?
obviously that makes the runtime slower due to more checks, but maybe it is
not *so* bad?
*/

178
gc.c
View file

@ -26,7 +26,6 @@
// variables shared between threads
static int gc_color_mark = 1; // Black, is swapped during GC
static int gc_color_clear = 3; // White, is swapped during GC
//static const int gc_color_grey = 4; // TODO: appears unused, clean up
// unfortunately this had to be split up; const colors are located in types.h
static int gc_status_col = STATUS_SYNC1;
@ -403,11 +402,6 @@ void *gc_try_alloc(gc_heap *h, size_t size, char *obj, gc_thread_data *thd)
return NULL;
}
//TODO: need a heap lock.
//lock during - alloc, sweep? but now sweep becomes a stop the world...
// maybe only lock during each individual operation, not for a whole
// sweep or alloc
void *gc_alloc(gc_heap *h, size_t size, char *obj, gc_thread_data *thd, int *heap_grown)
{
void *result = NULL;
@ -678,95 +672,9 @@ void vpbuffer_free(void **buf)
{
free(buf);
}
// void gc_init()
// {
// }
// END heap definitions
/*
Rough plan for how to implement new GC algorithm. We need to do this in
phases in order to have any hope of getting everything working. Let's prove
the algorithm out, then extend support to multiple mutators if everything
looks good.
PHASE 1 - separation of mutator and collector into separate threads
need to syncronize access (preferably via atomics) for anything shared between the
collector and mutator threads.
can cooperate be part of a minor gc? in that case, the
marking could be done as part of allocation
but then what exactly does that mean, to mark gray? because
objects moved to the heap will be set to mark color at that
point (until collector thread finishes). but would want
objects on the heap referenced by them to be traced, so
I suppose that is the purpose of the gray, to indicate
those still need to be traced. but need to think this through,
do we need the markbuffer and last read/write? do those make
sense with mta approach (assume so)???
ONLY CONCERN - what happens if an object on the stack
has a reference to an object on the heap that is collected?
but how would this happen? collector marks global roots before
telling mutators to go to async, and once mutators go async
any allocations will not be collected. also once collectors go
async they have a chance to markgray, which will include the write
barrier. so given that, is it still possible for an old heap ref to
sneak into a stack object during the async phase?
more questions on above point:
- figure out how/if after cooperation/async, can a stack object pick
up a reference to a heap object that will be collected during that GC cycle?
need to be able to prevent this somehow...
- need to figure out real world use case(s) where this could happen, to try and
figure out how to address this problem
from my understanding of the paper, the write barrier prevents this. consider, at the
start of async, the mutator's roots, global roots, and anything on the write barrier
have been marked. any new objects will be allocated as marked. that way, anything the
mutator could later access is either marked or will be after tracing. the only exception
is if the mutator changes a reference such that tracing will no longer find an object.
but the write barrier prevents this - during tracing a heap update causes the old
object to be marked as well. so it will eventually be traced, and there should be no
dangling objects after GC completes.
PHASE 2 - multi-threaded mutator (IE, more than one stack thread):
- how does the collector handle stack objects that reference objects from
another thread's stack?
* minor GC will only relocate that thread's objects, so another thread's would not
be moved. however, if another thread references one of the GC'd thread's
stack objects, it will now get a forwarding pointer. even worse, what if the
other thread is blocked and the reference becomes corrupt due to the stack
longjmp? there are major issues with one thread referencing another thread's
objects.
* had considered adding a stack bit to the object header. if we do this and
initialize it during object creation, a thread could in theory detect
if an object belongs to another thread. but it might be expensive because
a read barrier would have to be used to check the object's stack bit and
address (to see if it is on this heap).
* alternatively, how would one thread pick up a reference to another one's
objects? are there any ways to detect these events and deal with them?
it might be possible to detect such a case and allocate the object on the heap,
replacing it with a fwd pointer. unfortunately that means we need a read
barrier (ick) to handle forwarding pointers in arbitrary places
* but does that mean we need a fwd pointer to be live for awhile? do we need
a read barrier to get this to work? obviously we want to avoid a read barrier
at all costs.
- what are the real costs of allowing forwarding pointers to exist outside of just
minor GC? assume each runtime primitive would need to be updated to handle the
case where the obj is a fwd pointer - is it just a matter of each function
detecting this and (possibly) calling itself again with the 'real' address?
obviously that makes the runtime slower due to more checks, but maybe it is
not *so* bad?
*/
// tri-color GC section, WIP
// Tri-color GC section
/////////////////////////////////////////////
// GC functions called by the Mutator threads
@ -784,29 +692,24 @@ int gc_is_stack_obj(gc_thread_data *thd, object obj)
}
/**
Write barrier for updates to heap-allocated objects
Plans:
The key for this barrier is to identify stack objects that contain
heap references, so they can be marked to avoid collection.
* Write barrier for updates to heap-allocated objects
* The key for this barrier is to identify stack objects that contain
* heap references, so they can be marked to avoid collection.
*/
void gc_mut_update(gc_thread_data *thd, object old_obj, object value)
{
int status = ck_pr_load_int(&gc_status_col),
stage = ck_pr_load_int(&gc_stage);
if (ck_pr_load_int(&(thd->gc_status)) != STATUS_ASYNC) {
//fprintf(stderr, "DEBUG - GC sync marking heap obj %p ", old_obj);
//Cyc_display(old_obj, stderr);
//fprintf(stderr, " and new value %p ", value);
//Cyc_display(value, stderr);
////fprintf(stderr, " for heap object ");
//fprintf(stderr, "\n");
pthread_mutex_lock(&(thd->lock));
gc_mark_gray(thd, old_obj);
// Check if value is on the heap. If so, mark gray right now,
// otherwise set it to be marked after moved to heap by next GC
if (gc_is_stack_obj(thd, value)) {
// Set object to be marked after moved to heap by next GC.
// This avoids having to recursively examine the stack now,
// which we have to do anyway during minor GC.
grayed(value) = 1;
} else {
// Value is on the heap, mark gray right now
gc_mark_gray(thd, value);
}
pthread_mutex_unlock(&(thd->lock));
@ -825,37 +728,8 @@ void gc_mut_update(gc_thread_data *thd, object old_obj, object value)
}
#endif
}
// TODO: concerned there may be an issue here with a stack object
// having a 'tree' of references that contains heap objects. these
// objects would be skipped and would never be grayed by the current
// code:
//
// the paper marks both the heap location being written to and the
// value being written. not sure it makes sense to mark the value
// as it will always be on the stack - issue is if any obj's it is
// referencing are on the heap. this is where that stack bit might
// come in handy.
//
// do we want to mark gray immediately during add mutator, or wait
// until minor GC? YES - I think for mutators we need to mark the
// object gray immediately. otherwise if we delay until GC, a sweep
// may have already finished up and freed such an obj that would
// otherwise not have been freed if we had waited.
//
// again, only potential issue seems to be if a stack obj could ref
// something else on the heap - can that happen? I think this can only
// happen if the heap obj it refs is linked to a root, because stack
// objs are so short lived??
//
// also we already know if objects are on the stack due to their color (RED).
// so can use this to not mark red values. otherwise probably do want
// to mark the 'y' as well (per paper) to prevent timing issues when we wait
//
// do have some concern though that mark_gray will stop when a stack obj
// is detected, and the code will not examine any refs held by the stack obj.
}
// TODO: still need to handle case where a mutator is blocked
void gc_mut_cooperate(gc_thread_data *thd, int buf_len)
{
int i, status_c, status_m;
@ -869,8 +743,8 @@ void gc_mut_cooperate(gc_thread_data *thd, int buf_len)
thd->pending_writes = 0;
pthread_mutex_unlock(&(thd->lock));
// TODO: I think below is thread safe, but this code is tricky.
// worst case should be that some work is done twice if there is
// I think below is thread safe, but this code is tricky.
// Worst case should be that some work is done twice if there is
// a race condition
//
// TODO: should use an atomic comparison here
@ -971,27 +845,6 @@ void gc_mark_gray2(gc_thread_data *thd, object obj)
}
}
// TODO: before doing this, may want to debug a bit and see what is
// being passed to gc_mut_update. see if those objects tend to have
// any heap refs. may need to add debug code to do that...
//
//
//// This is called from the heap write barrier. The issue here is that
//// this is not called during GC, so obj and some of its refs may be
//// on the stack. So scan all refs and mark the ones that are on the heap
//void gc_mark_gray_rec(gc_thread_data *thd, object obj)
//{
// int mark;
//
// if (is_object_type(obj)) {
// mark = mark(obj);
//
//// TODO: if we leave red as red and keep going, this could hang
//// if there is a cycle!!
// }&& mark(obj) == gc_color_clear) { // TODO: sync??
//
//}
void gc_collector_trace()
{
ck_array_iterator_t iterator;
@ -1003,14 +856,6 @@ void gc_collector_trace()
CK_ARRAY_FOREACH(&Cyc_mutators, &iterator, &m){
// TODO: ideally, want to use a lock-free data structure to prevent
// having to use a mutex here. see corresponding code in gc_mark_gray
//TODO: I think this locking is too coarse. observing immediate failures with the recent change to g_mark_gray locking and
//wonder if the problem is that this locking will prevent a batch of changes from being seen.
//you know, do we really need locking here? the last read/write can be made atomic, and any reads/writes to mark buffer can
//be made atomic as well. I think we may need a dirty flag to let the collector know something is happening when the mark buffer
//needs to be resized, but other than that it this good enough?
//on the other hand, a central issue with this collector is when can we be sure that we are existing tracing at the right time, and
//not too early? because an early exit here will surely mean that objects are incorrectly freed
pthread_mutex_lock(&(m->lock));
while (m->last_read < m->last_write) {
clean = 0;
@ -1237,9 +1082,6 @@ printf("DEBUG - collector is cooperating for blocked mutator\n");
/////////////////////////////////////////////
// GC Collection cycle
//TODO: create function to print globals, ideally want names and the mark flag.
//then call before/after tracing to see if we can catch a global not being marked.
//want to rule out an issue here, since we have seen globals that were corrupted (IE, appears they were collected)
void debug_dump_globals();
// Main collector function