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GC bridge: Include non-bridged objects in the exported object graph
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Introduces a mechanism for non-bridged SCCs which lay between bridged
SCCs in the SCC forest to be reported to the bridge client. In this
version, such non-bridged SCCs are always reported. The result is that
most GCs get a little slower but GCs in which the "double fan" shape
appears (many bridged objects link to one C# object which links to
many bridged objects) become massively faster.

Because before clients were allowed to assume exported SCCs were
always bridged, SGEN_BRIDGE_VERSION has been incremented.
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@xmcclure
xmcclure committed Aug 19, 2016
1 parent ca48b5e commit db60c62
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Showing 3 changed files with 60 additions and 26 deletions.
7 changes: 3 additions & 4 deletions docs/sources/mono-api-gc.html
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Expand Up @@ -40,9 +40,8 @@ <h3>SGen Bridge</h3>
<p>The output of the SCC analysis is passed to the
`cross_references()` callback. It is expected to set the
`is_alive` flag on those strongly connected components that it
wishes to be kept alive. Only bridged objects will be
reported to the callback, i.e., non-bridged objects are
removed from the callback graph.
wishes to be kept alive. The value of `is_alive` will be
ignored on any SCCs which lack bridges.

<p>In monodroid each bridged object has a corresponding Java
mirror object. In the bridge callback it reifies the Mono
Expand All @@ -63,7 +62,7 @@ <h3>SGen Bridge</h3>

<div class="mapi-header">
enum {
SGEN_BRIDGE_VERSION = 4
SGEN_BRIDGE_VERSION = 5
};

typedef enum {
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6 changes: 3 additions & 3 deletions mono/metadata/sgen-bridge.h
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Expand Up @@ -17,8 +17,8 @@
* When SGen is done marking, it puts together a list of all dead bridged
* objects. This is passed to the bridge processor, which does an analysis to
* simplify the graph: It replaces strongly-connected components with single
* nodes, and then removes any nodes corresponding to components which do not
* contain bridged objects.
* nodes, and may remove nodes corresponding to components which do not contain
* bridged objects.
*
* The output of the SCC analysis is passed to the client's `cross_references()`
* callback. This consists of 2 arrays, an array of SCCs (MonoGCBridgeSCC),
Expand Down Expand Up @@ -54,7 +54,7 @@
MONO_BEGIN_DECLS

enum {
SGEN_BRIDGE_VERSION = 4
SGEN_BRIDGE_VERSION = 5
};

typedef enum {
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73 changes: 54 additions & 19 deletions mono/metadata/sgen-tarjan-bridge.c
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Original file line number Diff line number Diff line change
Expand Up @@ -122,7 +122,15 @@ typedef struct _ScanData {
unsigned obj_state : 2;
} ScanData;

// Should color be made visible to client even though it has no bridges?
static inline gboolean bridgeless_color_is_heavy (ColorData *data) {
return TRUE;
}

// Should color be made visible to client?
static inline gboolean color_visible_to_client (ColorData *data) {
return dyn_array_ptr_size (&data->bridges) || bridgeless_color_is_heavy (data);
}

// Stacks of ScanData objects used for tarjan algorithm.
// The Tarjan algorithm is normally defined recursively; here scan_stack simulates the call stack of a recursive algorithm,
Expand Down Expand Up @@ -159,6 +167,7 @@ static ObjectBucket *root_object_bucket;
static ObjectBucket *cur_object_bucket;
static int object_data_count;

// Arenas to allocate ScanData from
static ObjectBucket*
new_object_bucket (void)
{
Expand Down Expand Up @@ -211,7 +220,7 @@ free_object_buckets (void)
//ColorData buckets
#define NUM_COLOR_ENTRIES ((BUCKET_SIZE - SIZEOF_VOID_P * 2) / sizeof (ColorData))

// Same as ObjectBucket except NUM_COLOR_ENTRIES and NUM_SCAN_ENTRIES differ
// Arenas for ColorDatas, same as ObjectBucket except items-per-bucket differs
typedef struct _ColorBucket ColorBucket;
struct _ColorBucket {
ColorBucket *next;
Expand Down Expand Up @@ -358,7 +367,9 @@ typedef struct {
} HashEntry;

/*
We tried 2/32, 2/128, 4/32, 4/128, 6/128 and 8/128.
The merge cache maps an ordered list of ColorDatas [the color_merge_array] to a single ColorData.
About cache bucket tuning: We tried 2/32, 2/128, 4/32, 4/128, 6/128 and 8/128.
The performance cost between 4/128 and 8/128 is so small since cache movement happens completely in the same cacheline,
making the extra space pretty much free.
Expand Down Expand Up @@ -409,8 +420,9 @@ match_colors (DynPtrArray *a, DynPtrArray *b)
return TRUE;
}

static int cache_hits, cache_misses;
static int cache_hits, cache_misses, cache_abstains;

// The cache contains only non-bridged colors.
static ColorData*
find_in_cache (int *insert_index)
{
Expand All @@ -419,8 +431,10 @@ find_in_cache (int *insert_index)

size = dyn_array_ptr_size (&color_merge_array);
/* Cache checking is very ineficient with a lot of elements*/
if (size > 3)
if (size > 3) {
++cache_abstains;
return NULL;
}

hash = 0;
for (i = 0 ; i < size; ++i)
Expand Down Expand Up @@ -448,13 +462,15 @@ find_in_cache (int *insert_index)
return NULL;
}

// A color is needed for an SCC. If the SCC has bridges, the color MUST be newly allocated.
// If the SCC lacks bridges, the allocator MAY use the cache to merge it with an existing one.
static ColorData*
new_color (gboolean force_new)
new_color (gboolean has_bridges)
{
int i = -1;
ColorData *cd;
/* XXX Try to find an equal one and return it */
if (!force_new) {
if (!has_bridges) {
cd = find_in_cache (&i);
if (cd)
return cd;
Expand Down Expand Up @@ -606,16 +622,24 @@ compute_low (ScanData *data)
#include "sgen/sgen-scan-object.h"
}

// A non-bridged object needs a single color describing the current merge array.
static ColorData*
reduce_color (void)
{
ColorData *color = NULL;
int size = dyn_array_ptr_size (&color_merge_array);

// Merge array is empty-- this SCC points to no bridged colors.
// This SCC can be ignored completely.
if (size == 0)
color = NULL;

// Merge array has one item-- this SCC points to a single bridged color.
// This SCC can be forwarded to the pointed-to color.
else if (size == 1) {
color = (ColorData *)dyn_array_ptr_get (&color_merge_array, 0);

// This SCC gets to talk to the color allocator.
} else
color = new_color (FALSE);

Expand Down Expand Up @@ -889,7 +913,7 @@ gather_xrefs (ColorData *color)
if (src->visited)
continue;
src->visited = TRUE;
if (dyn_array_ptr_size (&src->bridges))
if (color_visible_to_client (src))
dyn_array_ptr_add (&color_merge_array, src);
else
gather_xrefs (src);
Expand All @@ -905,7 +929,7 @@ reset_xrefs (ColorData *color)
if (!src->visited)
continue;
src->visited = FALSE;
if (!dyn_array_ptr_size (&src->bridges))
if (!color_visible_to_client (src))
reset_xrefs (src);
}
}
Expand Down Expand Up @@ -934,15 +958,26 @@ processing_build_callback_data (int generation)
printf ("number of SCCs %d\n", num_colors_with_bridges);
#endif

// Count the number of SCCs visible to the client
int num_sccs = 0;
for (cur = root_color_bucket; cur; cur = cur->next) {
ColorData *cd;
for (cd = &cur->data [0]; cd < cur->next_data; ++cd) {
if (color_visible_to_client (cd))
num_sccs++;
}
}

/* This is a straightforward translation from colors to the bridge callback format. */
api_sccs = (MonoGCBridgeSCC **)sgen_alloc_internal_dynamic (sizeof (MonoGCBridgeSCC*) * num_colors_with_bridges, INTERNAL_MEM_BRIDGE_DATA, TRUE);
api_sccs = (MonoGCBridgeSCC **)sgen_alloc_internal_dynamic (sizeof (MonoGCBridgeSCC*) * num_sccs, INTERNAL_MEM_BRIDGE_DATA, TRUE);
api_index = xref_count = 0;

// Convert visible SCCs, along with their bridged object list, to MonoGCBridgeSCCs in the client's SCC list
for (cur = root_color_bucket; cur; cur = cur->next) {
ColorData *cd;
for (cd = &cur->data [0]; cd < cur->next_data; ++cd) {
int bridges = dyn_array_ptr_size (&cd->bridges);
if (!bridges)
if (!(bridges || bridgeless_color_is_heavy (cd)))
continue;

api_sccs [api_index] = (MonoGCBridgeSCC *)sgen_alloc_internal_dynamic (sizeof (MonoGCBridgeSCC) + sizeof (MonoObject*) * bridges, INTERNAL_MEM_BRIDGE_DATA, TRUE);
Expand All @@ -959,11 +994,11 @@ processing_build_callback_data (int generation)

scc_setup_time = step_timer (&curtime);

// Eliminate non-visible SCCs from the SCC list and redistribute xrefs
for (cur = root_color_bucket; cur; cur = cur->next) {
ColorData *cd;
for (cd = &cur->data [0]; cd < cur->next_data; ++cd) {
int bridges = dyn_array_ptr_size (&cd->bridges);
if (!bridges)
if (!color_visible_to_client (cd))
continue;

dyn_array_ptr_empty (&color_merge_array);
Expand All @@ -981,18 +1016,18 @@ processing_build_callback_data (int generation)
dump_color_table (" after xref pass", TRUE);
#endif

// Write out xrefs array
api_xrefs = (MonoGCBridgeXRef *)sgen_alloc_internal_dynamic (sizeof (MonoGCBridgeXRef) * xref_count, INTERNAL_MEM_BRIDGE_DATA, TRUE);
api_index = 0;
for (cur = root_color_bucket; cur; cur = cur->next) {
ColorData *src;
for (src = &cur->data [0]; src < cur->next_data; ++src) {
int bridges = dyn_array_ptr_size (&src->bridges);
if (!bridges)
if (!color_visible_to_client (src))
continue;

for (j = 0; j < dyn_array_ptr_size (&src->other_colors); ++j) {
ColorData *dest = (ColorData *)dyn_array_ptr_get (&src->other_colors, j);
g_assert (dyn_array_ptr_size (&dest->bridges)); /* We flattened the color graph, so this must never happen. */
g_assert (color_visible_to_client (dest)); /* Supposedly we already eliminated all xrefs to non-visible objects. */

api_xrefs [api_index].src_scc_index = src->api_index;
api_xrefs [api_index].dst_scc_index = dest->api_index;
Expand All @@ -1011,7 +1046,7 @@ processing_build_callback_data (int generation)
#endif

//FIXME move half of the cleanup to before the bridge callback?
bridge_processor->num_sccs = num_colors_with_bridges;
bridge_processor->num_sccs = num_sccs;
bridge_processor->api_sccs = api_sccs;
bridge_processor->num_xrefs = xref_count;
bridge_processor->api_xrefs = api_xrefs;
Expand All @@ -1033,18 +1068,18 @@ processing_after_callback (int generation)

cleanup_time = step_timer (&curtime);

mono_trace (G_LOG_LEVEL_INFO, MONO_TRACE_GC, "GC_TAR_BRIDGE bridges %d objects %d colors %d ignored %d sccs %d xref %d cache %d/%d setup %.2fms tarjan %.2fms scc-setup %.2fms gather-xref %.2fms xref-setup %.2fms cleanup %.2fms",
mono_trace (G_LOG_LEVEL_INFO, MONO_TRACE_GC, "GC_TAR_BRIDGE bridges %d objects %d colors %d ignored %d sccs %d xref %d cache %d/%d/%d setup %.2fms tarjan %.2fms scc-setup %.2fms gather-xref %.2fms xref-setup %.2fms cleanup %.2fms",
bridge_count, object_count, color_count,
ignored_objects, scc_count, xref_count,
cache_hits, cache_misses,
cache_hits, cache_misses, cache_abstains,
setup_time / 10000.0f,
tarjan_time / 10000.0f,
scc_setup_time / 10000.0f,
gather_xref_time / 10000.0f,
xref_setup_time / 10000.0f,
cleanup_time / 10000.0f);

cache_hits = cache_misses = 0;
cache_hits = cache_misses = cache_abstains = 0;
ignored_objects = 0;
}

Expand Down

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