gem5
v21.1.0.2

The stack distance calculator is a passive object that merely observes the addresses pass to it. More...
#include <stack_dist_calc.hh>
Classes  
struct  Node 
Node which takes form of Leaf, INode or Root. More...  
Public Member Functions  
StackDistCalc (bool verify_stack=false)  
~StackDistCalc ()  
std::pair< uint64_t, bool >  calcStackDist (const Addr r_address, bool mark=false) 
Process the given address. More...  
std::pair< uint64_t, bool >  calcStackDistAndUpdate (const Addr r_address, bool addNewNode=true) 
Process the given address: More...  
Static Public Attributes  
static constexpr uint64_t  Infinity = std::numeric_limits<uint64_t>::max() 
A convenient way of refering to infinity. More...  
Private Types  
typedef std::map< uint64_t, Node * >  IndexNodeMap 
typedef std::map< Addr, uint64_t >  AddressIndexMap 
typedef std::vector< IndexNodeMap >  TreeType 
Private Member Functions  
uint64_t  getSum (Node *node, bool from_left, uint64_t sum_from_below, uint64_t stack_dist, uint64_t level) const 
Gets sum from the node upwards recursively till the root. More...  
uint64_t  getSumsLeavesToRoot (Node *node) const 
Gets the sum from the leaf node specified. More...  
uint64_t  updateSum (Node *node, bool from_left, uint64_t sum_from_below, uint64_t level, uint64_t stack_dist, bool discard_node) 
Updates the nodes upwards recursively till the root. More...  
uint64_t  updateSumsLeavesToRoot (Node *node, bool is_new_leaf) 
Updates the leaf nodes and nodes above. More...  
void  updateTree () 
updateTree is a tree balancing operation, which maintains the binary tree structure. More...  
void  sanityCheckTree (const Node *node, uint64_t level=0) const 
This method is used for verification purposes It recursively traverses upwards from the given node till the root to check if the ultimate parent node (rootnode) points to null. More...  
uint64_t  getIndex () const 
Return the counter for address accesses (unique and nonunique). More...  
uint64_t  getTreeDepth () const 
Query depth of the tree (tree[0] represents leaf layer while tree[treeDepth] represents the root layer, all layers in between contain intermediate nodes) More...  
void  printStack (int n=5) const 
Print the last n items on the stack. More...  
uint64_t  verifyStackDist (const Addr r_address, bool update_stack=false) 
This is an alternative implementation of the stackdistance in a naive way. More...  
Private Attributes  
uint64_t  index 
Internal counter for address accesses (unique and nonunique) This counter increments everytime the calcStackDist() method is called. More...  
TreeType  tree 
AddressIndexMap  aiMap 
std::vector< uint64_t >  nextIndex 
std::vector< uint64_t >  stack 
const bool  verifyStack 
The stack distance calculator is a passive object that merely observes the addresses pass to it.
It calculates stack distances of incoming addresses based on the partial sum hierarchy tree algorithm described by Alamasi et al. http://doi.acm.org/10.1145/773039.773043.
A tree structure is maintained and updated at each transaction (unique or nonunique). The tree is implemented as an STL vector with layers of the form <map> Each layer in this tree is an ordered map <uint64_t, Node*>. Nodes are structs which take form of leaf, intermediate and root nodes. For example, in a tree with 3 layers, tree[0][5] gives a leaf node pointer for key=5 tree[1][1] gives an intermediate node pointer for key=1 tree[2][0] gives the root node in the tree.
At every transaction a hashmap (aiMap) is looked up to check if the address was already encountered before. Based on this lookup a transaction can be termed as unique or nonunique.
In addition to the normal stack distance calculation, a feature to mark an old node in the tree is added. This is useful if it is required to see the reuse pattern. For example, BackInvalidates from a lower level (e.g. membus to L2), can be marked (isMarked flag of Node set to True). Then later if this same address is accessed (by L1), the value of the isMarked flag would be True. This would give some insight on how the BackInvalidates policy of the lower level affect the read/write accesses in an application.
There are two functions provided to interface with the calculator:
At every nonunique transaction the tree is traversed from the leaf at the returned index to the root, the old node is deleted from the tree, and the sums (to the right are collected) and decremented. The collected sum represets the stack distance of the found node. If this node was marked then a bool flag set to True is returned with the stack_distance. During this operation a node is discarded at the leaf layer always. Moreover during the traversal upwards using the updateSum() method, if an intermediate node is found with no children connected to it, then that is discarded too.
The return value of this function is a pair representing the stack_distance and the value of the marked flag.
At every unique transaction the stackdistance is returned as a constant representing INFINITY.
At every nonunique transaction the tree is traversed from the leaf at the returned index to the root, and the sums (to the right) are collected. The collected sum represets the stack distance of the found node.
This function does NOT Modify the stack. (No node is added or deleted). It is just used to mark a node already created and get its stack distance.
The return value of this function is a pair representing the stack distance and the value of the marked flag.
The table below depicts the usage of the Algorithm using the functions: pair<uint64_t Stack_dist, bool isMarked> calcStackDistAndUpdate (Addr r_address, bool addNewNode) pair<uint64_t Stack_dist, bool isMarked> calcStackDist (Addr r_address, bool mark)
 Function  Arguments Return Val Use For calcStackDistAndUpdater_address, TrueI/SD,False A,GD,GM calcStackDistAndUpdater_address,FalseSD,prevMarkD,GD,GM calcStackDist r_address,FalseSD,prevMark GD,GM calcStackDist r_address, TrueSD,prevMark GD,GM
(*A: Allocate an address in stack, if old entry present then it is deleted, *U: Delete oldaddress from stack, no new entry is added *GD: GetStack distance of an address, *GM: Get value of Mark flag, indicates if that address has been touched before, *I: stackdistance = infinity, *SD: Stack Distance *r_address: address to be added, *prevMark: value of isMarked flag of the Node)
Invalidates refer to a type of packet that removes something from a cache, either autonoumously (dueto cache's own replacement policy), or snoops from other caches which invalidate something inside our cache.
Usage  Function to use Typical Use  Add new entry calcStackDistAndUpdateRead/Write Allocate  Delete Old Entry calcStackDistAndUpdateWritebacks/Cleanevicts Dist.of Old entrycalcStackDist Cleanevicts/Invalidate
Node Balancing: The tree structure is maintained by an updateTree() operation called when an intermediate node is required. The update operation is roughly categorized as a root update or intermediate layer update. When number of leaf nodes grow over a power of 2 then a new layer is added at the top of the tree and a new root node is initialized. The old node at the lower layer is connected to this. In an intermediate node update operation a new intermediate node is added to the required layer.
Debugging: Debugging can be enabled by setting the verifyStack flag true. Debugging is implemented using a dummy stack that behaves in a naive way, using STL vectors (i.e each unique address is pushed on the top of an STL vector stack, and SD is returned as Infinity. If a non unique address is encountered then the previous entry in the STL vector is removed, all the entities above it are pushed down, and the address is pushed at the top of the stack).
A printStack(int numOfEntitiesToPrint) is provided to print top n entities in both (tree and STL based dummy stack).
Definition at line 174 of file stack_dist_calc.hh.

private 
Definition at line 182 of file stack_dist_calc.hh.

private 
Definition at line 179 of file stack_dist_calc.hh.

private 
Definition at line 183 of file stack_dist_calc.hh.
gem5::StackDistCalc::StackDistCalc  (  bool  verify_stack = false  ) 
Definition at line 49 of file stack_dist_calc.cc.
References nextIndex, gem5::StackDistCalc::Node::nodeIndex, and tree.
gem5::StackDistCalc::~StackDistCalc  (  ) 
std::pair< uint64_t, bool > gem5::StackDistCalc::calcStackDist  (  const Addr  r_address, 
bool  mark = false 

) 
Process the given address.
If Mark is true then set the mark flag of the leaf node. This function returns the stack distance of the incoming address and the previous status of the mark flag.
r_address  The current address to process 
mark  set the mark flag for the address. 
Definition at line 463 of file stack_dist_calc.cc.
References aiMap, getSumsLeavesToRoot(), Infinity, panic_if, printStack(), tree, verifyStack, and verifyStackDist().
std::pair< uint64_t, bool > gem5::StackDistCalc::calcStackDistAndUpdate  (  const Addr  r_address, 
bool  addNewNode = true 

) 
Process the given address:
r_address  The current address to process 
addNewNode  If true, a new node is added to the tree 
Definition at line 361 of file stack_dist_calc.cc.
References aiMap, index, Infinity, gem5::StackDistCalc::Node::isLeftNode, gem5::StackDistCalc::Node::isMarked, nextIndex, gem5::StackDistCalc::Node::nodeIndex, panic_if, gem5::StackDistCalc::Node::parent, printStack(), sanityCheckTree(), tree, updateSumsLeavesToRoot(), updateTree(), verifyStack, and verifyStackDist().
Referenced by gem5::StackDistProbe::handleRequest().

inlineprivate 
Return the counter for address accesses (unique and nonunique).
This is further used to dump stats at regular intervals.
Definition at line 273 of file stack_dist_calc.hh.
References index.

private 
Gets sum from the node upwards recursively till the root.
This function is called first by getSumsLeavesToRoot, and then recursively calls itself.
node  pointer to the node which is updated 
from_left  variable which says that the request arrived from the left 
sum_from_below  Sum of left and right children below 
level  level in the tree the calling node is located 
stack_dist  stack distance of the node below 
Definition at line 215 of file stack_dist_calc.cc.
References gem5::StackDistCalc::Node::isLeftNode, gem5::X86ISA::level, gem5::StackDistCalc::Node::parent, gem5::StackDistCalc::Node::sumLeft, and gem5::StackDistCalc::Node::sumRight.
Referenced by getSumsLeavesToRoot().

private 
Gets the sum from the leaf node specified.
This function is called by calcStackDist.
node  pointer to the node which is updated 
Definition at line 238 of file stack_dist_calc.cc.
References getSum(), gem5::StackDistCalc::Node::isLeftNode, and gem5::StackDistCalc::Node::parent.
Referenced by calcStackDist().

inlineprivate 
Query depth of the tree (tree[0] represents leaf layer while tree[treeDepth] represents the root layer, all layers in between contain intermediate nodes)
Definition at line 282 of file stack_dist_calc.hh.
References tree.
Referenced by printStack(), sanityCheckTree(), and updateTree().

private 
Print the last n items on the stack.
This method prints top n entries in the tree based implementation as well as dummy stack.
n  Number of entries to print 
Definition at line 567 of file stack_dist_calc.cc.
References gem5::ArmISA::a, aiMap, gem5::X86ISA::count, DPRINTF, getTreeDepth(), gem5::ArmISA::n, gem5::StackDistCalc::Node::nodeIndex, stack, tree, and verifyStack.
Referenced by calcStackDist(), and calcStackDistAndUpdate().

private 
This method is used for verification purposes It recursively traverses upwards from the given node till the root to check if the ultimate parent node (rootnode) points to null.
node  pointer to the node whose sanity is being checked 
level  the level at which this node is located in the tree 
Definition at line 516 of file stack_dist_calc.cc.
References getTreeDepth(), gem5::ArmISA::i, gem5::X86ISA::level, gem5::StackDistCalc::Node::nodeIndex, panic_if, and gem5::StackDistCalc::Node::parent.
Referenced by calcStackDistAndUpdate().

private 
Updates the nodes upwards recursively till the root.
This function is first called by updateSumsLeavesToRoot, and then it recursively calls itself.
node  pointer to the node which is updated 
from_left  variable which says that the request arrived from the left 
sum_from_below  Sum of left and right children below 
level  level in the tree the calling node is located 
stack_dist  stack distance of the node below 
discard_node  whether the calling node was discarded or not 
Definition at line 101 of file stack_dist_calc.cc.
References gem5::StackDistCalc::Node::discardLeft, gem5::StackDistCalc::Node::discardRight, gem5::StackDistCalc::Node::isLeftNode, gem5::X86ISA::level, gem5::StackDistCalc::Node::nodeIndex, panic_if, gem5::StackDistCalc::Node::parent, gem5::StackDistCalc::Node::sumLeft, gem5::StackDistCalc::Node::sumRight, tree, and verifyStack.
Referenced by updateSumsLeavesToRoot().

private 
Updates the leaf nodes and nodes above.
This function is called by the calcStackDistAndUpdate.
node  pointer to the node which is updated 
is_new_leaf  is true if this is a newly added node 
Definition at line 190 of file stack_dist_calc.cc.
References Infinity, gem5::StackDistCalc::Node::isLeftNode, gem5::X86ISA::level, gem5::StackDistCalc::Node::parent, gem5::StackDistCalc::Node::sumLeft, and updateSum().
Referenced by calcStackDistAndUpdate().

private 
updateTree is a tree balancing operation, which maintains the binary tree structure.
This method is called whenever index%2 == 0 (i.e. every alternate cycle) The two main operation are : OP1. Moving the root node one layer up if index counter crosses power of 2 OP2. Addition of intermediate nodes as and when required and linking them to their parents in the layer above.
Definition at line 252 of file stack_dist_calc.cc.
References gem5::StackDistCalc::Node::discardLeft, getTreeDepth(), gem5::ArmISA::i, index, gem5::StackDistCalc::Node::isLeftNode, gem5::isPowerOf2(), nextIndex, gem5::StackDistCalc::Node::nodeIndex, gem5::StackDistCalc::Node::parent, gem5::StackDistCalc::Node::sumLeft, and tree.
Referenced by calcStackDistAndUpdate().

private 
This is an alternative implementation of the stackdistance in a naive way.
It uses simple STL vector to represent the stack. It can be used in parallel for debugging purposes. It is 10x slower than the tree based implemenation.
r_address  The current address to process 
update_stack  Flag to indicate if stack should be updated 
Definition at line 536 of file stack_dist_calc.cc.
References gem5::ArmISA::a, Infinity, and stack.
Referenced by calcStackDist(), and calcStackDistAndUpdate().

private 
Definition at line 404 of file stack_dist_calc.hh.
Referenced by calcStackDist(), calcStackDistAndUpdate(), printStack(), and ~StackDistCalc().

private 
Internal counter for address accesses (unique and nonunique) This counter increments everytime the calcStackDist() method is called.
This counter is used as a key for the hash map at the leaf layer. Practically at every call to the calculator this counter is incremented and a new leaf node is added in the tree at the leaf layer using this counter value as the key.
Definition at line 398 of file stack_dist_calc.hh.
Referenced by calcStackDistAndUpdate(), getIndex(), and updateTree().

staticconstexpr 
A convenient way of refering to infinity.
Definition at line 315 of file stack_dist_calc.hh.
Referenced by calcStackDist(), calcStackDistAndUpdate(), gem5::StackDistProbe::handleRequest(), updateSumsLeavesToRoot(), and verifyStackDist().

private 
Definition at line 408 of file stack_dist_calc.hh.
Referenced by calcStackDistAndUpdate(), StackDistCalc(), updateTree(), and ~StackDistCalc().

private 
Definition at line 411 of file stack_dist_calc.hh.
Referenced by printStack(), verifyStackDist(), and ~StackDistCalc().

private 
Definition at line 401 of file stack_dist_calc.hh.
Referenced by calcStackDist(), calcStackDistAndUpdate(), getTreeDepth(), printStack(), StackDistCalc(), updateSum(), updateTree(), and ~StackDistCalc().

private 
Definition at line 414 of file stack_dist_calc.hh.
Referenced by calcStackDist(), calcStackDistAndUpdate(), printStack(), and updateSum().