Hi,
As a result of numerous class unloading discussions, we
I've hacked vtable marking proposals into GC_CC directly, and measured
their impact on the performance. I've attached the two patches
corresponding to "vtable marks" and "indirect marks".
Benchmark: dacapo-2006-10 hsqldb
Machine: IBM Thinkpad T41p, Pentium M 1700 MHz (1 core), 1 Gb
Windows XP SP2, MSVC 7.0, release build
Benchmark arguments:
java -verbose:gc -jar c:/work/dacapo/dacapo-2006-10.jar -s default -n
3 hsqldb
Benchmarks results:
no vtable marks: ===== DaCapo hsqldb PASSED in 6168 msec =====
vtable marks: ===== DaCapo hsqldb PASSED in 6218 msec =====
(0.8% slowdown)
indirect marks: ===== DaCapo hsqldb PASSED in 6409 msec =====
(3.9% slowdown)
Garbage collection times:
(garbage collection times were collected for the whole dacapo run,
including warmup benchmark runs).
no vtable marks:
COMPACT avg 614.375 +/- 117.537 = 4915.000 / 8, min 50.000, max 911.000
COPY avg 255.000 +/- 39.325 = 2040.000 / 8, min 90.000, max 490.000
FORCED avg 189.333 +/- 7.589 = 2840.000 / 15, min 140.000, max 240.000
vtable marks:
COMPACT avg 615.500 +/- 119.544 = 4924.000 / 8, min 40.000, max 931.000
COPY avg 260.000 +/- 27.839 = 2340.000 / 9, min 160.000, max 460.000
FORCED avg 186.667 +/- 7.411 = 2800.000 / 15, min 140.000, max 240.000
indirect marks:
COMPACT avg 619.375 +/- 123.104 = 4955.000 / 8, min 30.000, max 941.000
COPY avg 265.000 +/- 38.868 = 2120.000 / 8, min 110.000, max 500.000
FORCED avg 194.000 +/- 8.095 = 2910.000 / 15, min 150.000, max 250.000
Resume: as was predicted, adding unconditional write to object scanning
does not have much impact on the garbage collection time. However,
overall impact is visible on benchmark level.
Regarding the false sharing wnen writing vtable marks,
the benchmarking should be run on a multiprocessor machine and with a
parallel GC.
diff --git vm/gc_cc/src/collect_copy.cpp vm/gc_cc/src/collect_copy.cpp
index a3b6a96..a4663fc 100644
--- vm/gc_cc/src/collect_copy.cpp
+++ vm/gc_cc/src/collect_copy.cpp
@@ -168,6 +168,7 @@ static bool gc_copy_process_reference(Sl
// move the object?
#define pos ((unsigned char*) obj)
Partial_Reveal_VTable *vtable = ah_to_vtable(vt);
+ vtable->mark = 1;
GC_VTable_Info *gcvt = vtable->get_gcvt();
if (pos >= heap.compaction_region_start() && pos <
heap.compaction_region_end()) {
diff --git vm/gc_cc/src/collect_forced.cpp vm/gc_cc/src/collect_forced.cpp
index 072f21e..92bf167 100644
--- vm/gc_cc/src/collect_forced.cpp
+++ vm/gc_cc/src/collect_forced.cpp
@@ -64,6 +64,7 @@ static void forced_process_reference(Par
obj->obj_info() = (info & ~MARK_BITS) | heap_mark_phase;
Partial_Reveal_VTable *vtable = obj->vtable();
+ vtable->mark = 1;
GC_VTable_Info *gcvt = vtable->get_gcvt();
if (gcvt->is_array()) { // is array
diff --git vm/gc_cc/src/collect_slide_compact.cpp
vm/gc_cc/src/collect_slide_compact.cpp
index e5b4f54..985b94e 100644
--- vm/gc_cc/src/collect_slide_compact.cpp
+++ vm/gc_cc/src/collect_slide_compact.cpp
@@ -454,6 +454,7 @@ static void slide_process_object(Partial
assert(obj->vt() & ~RESCAN_BIT); // has vt
Partial_Reveal_VTable *vtable = ah_to_vtable(vt & ~RESCAN_BIT);
+ vtable->mark = 1;
GC_VTable_Info *gcvt = vtable->get_gcvt();
// process slots
diff --git vm/gc_cc/src/gc_types.h vm/gc_cc/src/gc_types.h
index 1ac4236..849aaf0 100644
--- vm/gc_cc/src/gc_types.h
+++ vm/gc_cc/src/gc_types.h
@@ -152,6 +152,9 @@ typedef struct Partial_Reveal_VTable {
private:
GC_VTable_Info *gcvt;
public:
+ /// pointer to the class reachability mark,
+ /// used for class unloading
+ size_t mark;
void set_gcvt(struct GC_VTable_Info *new_gcvt) { gcvt = new_gcvt; }
struct GC_VTable_Info *get_gcvt() { return gcvt; }
diff --git vm/vmcore/include/vtable.h vm/vmcore/include/vtable.h
index a1fc8b4..eb08687 100644
--- vm/vmcore/include/vtable.h
+++ vm/vmcore/include/vtable.h
@@ -53,6 +53,7 @@ typedef struct Intfc_Table {
typedef struct VTable {
Byte _gc_private_information[GC_BYTES_IN_VTABLE];
+ size_t mark;
Class* clss;
// See the masks in vm_for_gc.h.
diff --git vm/gc_cc/src/collect_copy.cpp vm/gc_cc/src/collect_copy.cpp
index a3b6a96..c2caac2 100644
--- vm/gc_cc/src/collect_copy.cpp
+++ vm/gc_cc/src/collect_copy.cpp
@@ -168,6 +168,7 @@ static bool gc_copy_process_reference(Sl
// move the object?
#define pos ((unsigned char*) obj)
Partial_Reveal_VTable *vtable = ah_to_vtable(vt);
+ *vtable->mark = 1;
GC_VTable_Info *gcvt = vtable->get_gcvt();
if (pos >= heap.compaction_region_start() && pos <
heap.compaction_region_end()) {
diff --git vm/gc_cc/src/collect_forced.cpp vm/gc_cc/src/collect_forced.cpp
index 072f21e..7e4de43 100644
--- vm/gc_cc/src/collect_forced.cpp
+++ vm/gc_cc/src/collect_forced.cpp
@@ -64,6 +64,7 @@ static void forced_process_reference(Par
obj->obj_info() = (info & ~MARK_BITS) | heap_mark_phase;
Partial_Reveal_VTable *vtable = obj->vtable();
+ *vtable->mark = 1;
GC_VTable_Info *gcvt = vtable->get_gcvt();
if (gcvt->is_array()) { // is array
diff --git vm/gc_cc/src/collect_slide_compact.cpp
vm/gc_cc/src/collect_slide_compact.cpp
index e5b4f54..4a3ee9c 100644
--- vm/gc_cc/src/collect_slide_compact.cpp
+++ vm/gc_cc/src/collect_slide_compact.cpp
@@ -454,6 +454,7 @@ static void slide_process_object(Partial
assert(obj->vt() & ~RESCAN_BIT); // has vt
Partial_Reveal_VTable *vtable = ah_to_vtable(vt & ~RESCAN_BIT);
+ *vtable->mark = 1;
GC_VTable_Info *gcvt = vtable->get_gcvt();
// process slots
diff --git vm/gc_cc/src/gc_types.h vm/gc_cc/src/gc_types.h
index 1ac4236..da9a48c 100644
--- vm/gc_cc/src/gc_types.h
+++ vm/gc_cc/src/gc_types.h
@@ -152,6 +152,9 @@ typedef struct Partial_Reveal_VTable {
private:
GC_VTable_Info *gcvt;
public:
+ /// pointer to the class reachability mark,
+ /// used for class unloading
+ size_t *mark;
void set_gcvt(struct GC_VTable_Info *new_gcvt) { gcvt = new_gcvt; }
struct GC_VTable_Info *get_gcvt() { return gcvt; }
diff --git vm/vmcore/include/Class.h vm/vmcore/include/Class.h
index 7194edb..a6c198c 100644
--- vm/vmcore/include/Class.h
+++ vm/vmcore/include/Class.h
@@ -772,6 +772,8 @@ enum AccessAndPropertiesFlags {
* calling the verifier, preparing, resolving and initializing the class.*/
struct Class {
+ /// mark used for the class unloading
+ size_t mark;
private:
typedef struct {
union {
