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Optimizes memory usage of AliasCache (#432)
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Changes the storage datastructure in AliasCache to use less memory.

The new structure uses 16-bit indexes instead of pointers, and a
sorted array of 16-bit indexes instead of red-black trees for the
lookup methods.
The LRU double-linked list is kept.

With this change the memory cost goes from 88 bytes per entry to
16 bytes per entry. The compiled code size is also smaller.

===

* Changes the storage datastructure in AliasCache to use less memory.

The new structure uses 16-bit indexes instead of pointers, and a
sorted array of 16-bit indexes instead of red-black trees for the
lookup methods.
The LRU double-linked list is kept.

With this change the memory cost goes from 88 bytes per entry to
16 bytes per entry. The compiled code size is also smaller.

* fix whitespaces

* Add more comments.

* fix comment typos.
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@balazsracz
balazsracz authored Sep 17, 2020
1 parent 7ec055a commit e1b0c2d
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Showing 4 changed files with 342 additions and 163 deletions.
159 changes: 79 additions & 80 deletions src/openlcb/AliasCache.cxx
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Original file line number Diff line number Diff line change
Expand Up @@ -46,15 +46,15 @@ void AliasCache::clear()
{
idMap.clear();
aliasMap.clear();
oldest = nullptr;
newest = nullptr;
freeList = nullptr;
oldest.idx_ = NONE_ENTRY;
newest.idx_ = NONE_ENTRY;
freeList.idx_ = NONE_ENTRY;
/* initialize the freeList */
for (size_t i = 0; i < entries; ++i)
{
pool[i].prev = NULL;
pool[i].next = freeList;
freeList = pool + i;
pool[i].newer_.idx_ = NONE_ENTRY;
pool[i].older_ = freeList;
freeList.idx_ = i;
}
}

Expand All @@ -69,78 +69,78 @@ void AliasCache::add(NodeID id, NodeAlias alias)

Metadata *insert;

AliasMap::Iterator it = aliasMap.find(alias);
auto it = aliasMap.find(alias);
if (it != aliasMap.end())
{
/* we already have a mapping for this alias, so lets remove it */
insert = (*it).second;
remove(alias);
insert = it->deref(this);
remove(insert->alias_);

if (removeCallback)
{
/* tell the interface layer that we removed this mapping */
(*removeCallback)(insert->id, insert->alias, context);
(*removeCallback)(insert->get_node_id(), insert->alias_, context);
}
}

if (freeList)
if (!freeList.empty())
{
/* found an empty slot */
insert = freeList;
freeList = insert->next;
insert = freeList.deref(this);
freeList = insert->older_;
}
else
{
HASSERT(oldest != NULL && newest != NULL);
HASSERT(!oldest.empty() && !newest.empty());

/* kick out the oldest mapping and re-link the oldest endpoint */
insert = oldest;
if (oldest->newer)
insert = oldest.deref(this);
auto second = insert->newer_;
if (!second.empty())
{
oldest->newer->older = NULL;
second.deref(this)->older_.idx_ = NONE_ENTRY;
}
if (insert == newest)
if (insert == newest.deref(this))
{
newest = NULL;
newest.idx_ = NONE_ENTRY;
}
oldest = oldest->newer;
oldest = second;

aliasMap.erase(insert->alias);
idMap.erase(insert->id);
aliasMap.erase(aliasMap.find(insert->alias_));
idMap.erase(idMap.find(insert->get_node_id()));

if (removeCallback)
{
/* tell the interface layer that we removed this mapping */
(*removeCallback)(insert->id, insert->alias, context);
(*removeCallback)(insert->get_node_id(), insert->alias_, context);
}
}

insert->timestamp = OSTime::get_monotonic();
insert->id = id;
insert->alias = alias;

aliasMap[alias] = insert;
idMap[id] = insert;
insert->set_node_id(id);
insert->alias_ = alias;

PoolIdx n;
n.idx_ = insert - pool;
aliasMap.insert(PoolIdx(n));
idMap.insert(PoolIdx(n));

/* update the time based list */
insert->newer = NULL;
if (newest == NULL)
insert->newer_.idx_ = NONE_ENTRY;
if (newest.empty())
{
/* if newest == NULL, then oldest must also be NULL */
HASSERT(oldest == NULL);
HASSERT(oldest.empty());

insert->older = NULL;
oldest = insert;
insert->older_.idx_ = NONE_ENTRY;
oldest = n;
}
else
{
insert->older = newest;
newest->newer = insert;
insert->older_ = newest;
newest.deref(this)->newer_ = n;
}

newest = insert;

return;
newest = n;
}

/** Remove an alias from an alias cache. This method does not call the
Expand All @@ -150,33 +150,33 @@ void AliasCache::add(NodeID id, NodeAlias alias)
*/
void AliasCache::remove(NodeAlias alias)
{
AliasMap::Iterator it = aliasMap.find(alias);
auto it = aliasMap.find(alias);

if (it != aliasMap.end())
{
Metadata *metadata = (*it).second;
Metadata *metadata = it->deref(this);
aliasMap.erase(it);
idMap.erase(metadata->id);
if (metadata->newer)
idMap.erase(idMap.find(metadata->get_node_id()));

if (!metadata->newer_.empty())
{
metadata->newer->older = metadata->older;
metadata->newer_.deref(this)->older_ = metadata->older_;
}
if (metadata->older)
if (!metadata->older_.empty())
{
metadata->older->newer = metadata->newer;
metadata->older_.deref(this)->newer_ = metadata->newer_;
}
if (metadata == newest)
if (metadata == newest.deref(this))
{
newest = metadata->older;
newest = metadata->older_;
}
if (metadata == oldest)
if (metadata == oldest.deref(this))
{
oldest = metadata->newer;
oldest = metadata->newer_;
}
metadata->next = freeList;
freeList = metadata;

metadata->older_ = freeList;
freeList.idx_ = metadata - pool;
}

}
Expand All @@ -185,9 +185,9 @@ bool AliasCache::retrieve(unsigned entry, NodeID* node, NodeAlias* alias)
{
HASSERT(entry < size());
Metadata* md = pool + entry;
if (!md->alias) return false;
if (node) *node = md->id;
if (alias) *alias = md->alias;
if (!md->alias_) return false;
if (node) *node = md->get_node_id();
if (alias) *alias = md->alias_;
return true;
}

Expand All @@ -199,15 +199,15 @@ NodeAlias AliasCache::lookup(NodeID id)
{
HASSERT(id != 0);

IdMap::Iterator it = idMap.find(id);
auto it = idMap.find(id);

if (it != idMap.end())
{
Metadata *metadata = (*it).second;
Metadata *metadata = it->deref(this);

/* update timestamp */
touch(metadata);
return metadata->alias;
return metadata->alias_;
}

/* no match found */
Expand All @@ -222,15 +222,15 @@ NodeID AliasCache::lookup(NodeAlias alias)
{
HASSERT(alias != 0);

AliasMap::Iterator it = aliasMap.find(alias);
auto it = aliasMap.find(alias);

if (it != aliasMap.end())
{
Metadata *metadata = (*it).second;
Metadata *metadata = it->deref(this);

/* update timestamp */
touch(metadata);
return metadata->id;
return metadata->get_node_id();
}

/* no match found */
Expand All @@ -246,9 +246,10 @@ void AliasCache::for_each(void (*callback)(void*, NodeID, NodeAlias), void *cont
{
HASSERT(callback != NULL);

for (Metadata *metadata = newest; metadata != NULL; metadata = metadata->older)
for (PoolIdx idx = newest; !idx.empty(); idx = idx.deref(this)->older_)
{
(*callback)(context, metadata->id, metadata->alias);
Metadata *metadata = idx.deref(this);
(*callback)(context, metadata->get_node_id(), metadata->alias_);
}
}

Expand Down Expand Up @@ -277,25 +278,23 @@ NodeAlias AliasCache::generate()
*/
void AliasCache::touch(Metadata* metadata)
{
metadata->timestamp = OSTime::get_monotonic();

if (metadata != newest)
if (metadata != newest.deref(this))
{
if (metadata == oldest)
if (metadata == oldest.deref(this))
{
oldest = metadata->newer;
oldest->older = NULL;
oldest = metadata->newer_;
oldest.deref(this)->older_.idx_ = NONE_ENTRY;
}
else
{
/* we have someone older */
metadata->older->newer = metadata->newer;
metadata->older_.deref(this)->newer_ = metadata->newer_;
}
metadata->newer->older = metadata->older;
metadata->newer = NULL;
metadata->older = newest;
newest->newer = metadata;
newest = metadata;
metadata->newer_.deref(this)->older_ = metadata->older_;
metadata->newer_.idx_ = NONE_ENTRY;
metadata->older_ = newest;
newest.deref(this)->newer_.idx_ = metadata - pool;
newest.idx_ = metadata - pool;
}
}

Expand Down
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