A skewed associative indexing policy. More...

#include <skewed_associative.hh>

Inheritance diagram for gem5::SkewedAssociative:

Public Types
typedef SkewedAssociativeParams	Params
	Convenience typedef.

Public Types inherited from gem5::BaseIndexingPolicy
typedef BaseIndexingPolicyParams	Params
	Convenience typedef.

Public Types inherited from gem5::SimObject
typedef SimObjectParams	Params

Public Member Functions
	SkewedAssociative (const Params &p)
	Construct and initialize this policy.

	~SkewedAssociative ()
	Destructor.

std::vector< ReplaceableEntry * >	getPossibleEntries (const Addr addr) const override
	Find all possible entries for insertion and replacement of an address.

Addr	regenerateAddr (const Addr tag, const ReplaceableEntry *entry) const override
	Regenerate an entry's address from its tag and assigned set and way.

Public Member Functions inherited from gem5::BaseIndexingPolicy
	BaseIndexingPolicy (const Params &p)
	Construct and initialize this policy.

	~BaseIndexingPolicy ()
	Destructor.

void	setEntry (ReplaceableEntry *entry, const uint64_t index)
	Associate a pointer to an entry to its physical counterpart.

ReplaceableEntry *	getEntry (const uint32_t set, const uint32_t way) const
	Get an entry based on its set and way.

virtual Addr	extractTag (const Addr addr) const
	Generate the tag from the given address.

Public Member Functions inherited from gem5::SimObject
const Params &	params () const

	SimObject (const Params &p)

virtual	~SimObject ()

virtual void	init ()
	init() is called after all C++ SimObjects have been created and all ports are connected.

virtual void	loadState (CheckpointIn &cp)
	loadState() is called on each SimObject when restoring from a checkpoint.

virtual void	initState ()
	initState() is called on each SimObject when not restoring from a checkpoint.

virtual void	regProbePoints ()
	Register probe points for this object.

virtual void	regProbeListeners ()
	Register probe listeners for this object.

ProbeManager *	getProbeManager ()
	Get the probe manager for this object.

virtual Port &	getPort (const std::string &if_name, PortID idx=InvalidPortID)
	Get a port with a given name and index.

virtual void	startup ()
	startup() is the final initialization call before simulation.

DrainState	drain () override
	Provide a default implementation of the drain interface for objects that don't need draining.

virtual void	memWriteback ()
	Write back dirty buffers to memory using functional writes.

virtual void	memInvalidate ()
	Invalidate the contents of memory buffers.

void	serialize (CheckpointOut &cp) const override
	Serialize an object.

void	unserialize (CheckpointIn &cp) override
	Unserialize an object.

Public Member Functions inherited from gem5::EventManager
EventQueue *	eventQueue () const

void	schedule (Event &event, Tick when)

void	deschedule (Event &event)

void	reschedule (Event &event, Tick when, bool always=false)

void	schedule (Event *event, Tick when)

void	deschedule (Event *event)

void	reschedule (Event *event, Tick when, bool always=false)

void	wakeupEventQueue (Tick when=(Tick) -1)
	This function is not needed by the usual gem5 event loop but may be necessary in derived EventQueues which host gem5 on other schedulers.

void	setCurTick (Tick newVal)

	EventManager (EventManager &em)
	Event manger manages events in the event queue.

	EventManager (EventManager *em)

	EventManager (EventQueue *eq)

Public Member Functions inherited from gem5::Serializable
	Serializable ()

virtual	~Serializable ()

void	serializeSection (CheckpointOut &cp, const char *name) const
	Serialize an object into a new section.

void	serializeSection (CheckpointOut &cp, const std::string &name) const

void	unserializeSection (CheckpointIn &cp, const char *name)
	Unserialize an a child object.

void	unserializeSection (CheckpointIn &cp, const std::string &name)

Public Member Functions inherited from gem5::Drainable
DrainState	drainState () const
	Return the current drain state of an object.

virtual void	notifyFork ()
	Notify a child process of a fork.

Public Member Functions inherited from gem5::statistics::Group
	Group (Group parent, const char name=nullptr)
	Construct a new statistics group.

virtual	~Group ()

virtual void	regStats ()
	Callback to set stat parameters.

virtual void	resetStats ()
	Callback to reset stats.

virtual void	preDumpStats ()
	Callback before stats are dumped.

void	addStat (statistics::Info *info)
	Register a stat with this group.

const std::map< std::string, Group * > &	getStatGroups () const
	Get all child groups associated with this object.

const std::vector< Info * > &	getStats () const
	Get all stats associated with this object.

void	addStatGroup (const char name, Group block)
	Add a stat block as a child of this block.

const Info *	resolveStat (std::string name) const
	Resolve a stat by its name within this group.

void	mergeStatGroup (Group *block)
	Merge the contents (stats & children) of a block to this block.

	Group ()=delete

	Group (const Group &)=delete

Group &	operator= (const Group &)=delete

Public Member Functions inherited from gem5::Named
	Named (const std::string &name_)

virtual	~Named ()=default

virtual std::string	name () const

Private Member Functions
Addr	hash (const Addr addr) const
	The hash function itself.

Addr	dehash (const Addr addr) const
	Inverse of the hash function.

Addr	skew (const Addr addr, const uint32_t way) const
	Address skewing function selection.

Addr	deskew (const Addr addr, const uint32_t way) const
	Address deskewing function (inverse of the skew function) of the given way.

uint32_t	extractSet (const Addr addr, const uint32_t way) const
	Apply a skewing function to calculate address' set given a way.

Private Attributes
const int	NUM_SKEWING_FUNCTIONS = 8
	The number of skewing functions implemented.

const int	msbShift
	The amount to shift a set index to get its MSB.

Additional Inherited Members
Static Public Member Functions inherited from gem5::SimObject
static void	serializeAll (const std::string &cpt_dir)
	Create a checkpoint by serializing all SimObjects in the system.

static SimObject *	find (const char *name)
	Find the SimObject with the given name and return a pointer to it.

static void	setSimObjectResolver (SimObjectResolver *resolver)
	There is a single object name resolver, and it is only set when simulation is restoring from checkpoints.

static SimObjectResolver *	getSimObjectResolver ()
	There is a single object name resolver, and it is only set when simulation is restoring from checkpoints.

Static Public Member Functions inherited from gem5::Serializable
static const std::string &	currentSection ()
	Gets the fully-qualified name of the active section.

static void	generateCheckpointOut (const std::string &cpt_dir, std::ofstream &outstream)
	Generate a checkpoint file so that the serialization can be routed to it.

Protected Member Functions inherited from gem5::Drainable
	Drainable ()

virtual	~Drainable ()

virtual void	drainResume ()
	Resume execution after a successful drain.

void	signalDrainDone () const
	Signal that an object is drained.

Protected Attributes inherited from gem5::BaseIndexingPolicy
const unsigned	assoc
	The associativity.

const uint32_t	numSets
	The number of sets in the cache.

const int	setShift
	The amount to shift the address to get the set.

const unsigned	setMask
	Mask out all bits that aren't part of the set index.

std::vector< std::vector< ReplaceableEntry * > >	sets
	The cache sets.

const int	tagShift
	The amount to shift the address to get the tag.

Protected Attributes inherited from gem5::SimObject
const SimObjectParams &	_params
	Cached copy of the object parameters.

Protected Attributes inherited from gem5::EventManager
EventQueue *	eventq
	A pointer to this object's event queue.

Detailed Description

A skewed associative indexing policy.

See also: gem5 Memory System

The skewed indexing policy has a variable mapping based on a hash function, so a value x can be mapped to different sets, based on the way being used.

For example, let's assume address A maps to set 3 on way 0. It will likely have a different set for every other way. Visually, the possible locations of A are, for a table with 4 ways and 8 sets (arbitrarily chosen sets; these locations depend on A and the hashing function used): Way 0 1 2 3 Set _ _ _ _ 0 |_| |_| |X| |_| 1 |_| |_| |_| |X| 2 |_| |_| |_| |_| 3 |X| |_| |_| |_| 4 |_| |_| |_| |_| 5 |_| |X| |_| |_| 6 |_| |_| |_| |_| 7 |_| |_| |_| |_|

If provided with an associativity higher than the number of skewing functions, the skewing functions of the extra ways might be sub-optimal.

Definition at line 72 of file skewed_associative.hh.

Member Typedef Documentation

◆ Params

typedef SkewedAssociativeParams gem5::SkewedAssociative::Params

Convenience typedef.

Definition at line 143 of file skewed_associative.hh.

Constructor & Destructor Documentation

◆ SkewedAssociative()

gem5::SkewedAssociative::SkewedAssociative ( const Params & p )

Construct and initialize this policy.

Definition at line 44 of file skewed_associative.cc.

References gem5::BaseIndexingPolicy::assoc, fatal_if, msbShift, NUM_SKEWING_FUNCTIONS, gem5::BaseIndexingPolicy::numSets, panic_if, gem5::BaseIndexingPolicy::setShift, and warn_once.

◆ ~SkewedAssociative()

gem5::SkewedAssociative::~SkewedAssociative ( )

inline

Destructor.

Definition at line 153 of file skewed_associative.hh.

Member Function Documentation

◆ dehash()

Addr gem5::SkewedAssociative::dehash ( const Addr addr ) const

private

Inverse of the hash function.

See also: hash().

Parameters

addr	The address to be dehashed.
The	dehashed address.

Definition at line 75 of file skewed_associative.cc.

References gem5::X86ISA::addr, gem5::bits(), gem5::insertBits(), gem5::mbits(), and msbShift.

Referenced by deskew(), and skew().

◆ deskew()

Addr gem5::SkewedAssociative::deskew	(	const Addr	addr,
		const uint32_t	way ) const

private

Address deskewing function (inverse of the skew function) of the given way.

See also: skew()

Parameters

addr	Address to be deskewed. Should contain the set and tag bits.
way	The cache way, used to select a hash function.

Returns: The deskewed address.

Definition at line 142 of file skewed_associative.cc.

References gem5::X86ISA::addr, gem5::bits(), dehash(), hash(), gem5::ArmISA::i, msbShift, NUM_SKEWING_FUNCTIONS, and panic.

Referenced by regenerateAddr().

◆ extractSet()

uint32_t gem5::SkewedAssociative::extractSet	(	const Addr	addr,
		const uint32_t	way ) const

private

Apply a skewing function to calculate address' set given a way.

Parameters

addr	The address to calculate the set for.
way	The way to get the set from.

Returns: The set index for given combination of address and way.

Definition at line 195 of file skewed_associative.cc.

References gem5::X86ISA::addr, gem5::BaseIndexingPolicy::setMask, gem5::BaseIndexingPolicy::setShift, and skew().

Referenced by getPossibleEntries().

◆ getPossibleEntries()

std::vector< ReplaceableEntry * > gem5::SkewedAssociative::getPossibleEntries ( const Addr addr ) const

overridevirtual

Find all possible entries for insertion and replacement of an address.

Should be called immediately before ReplacementPolicy's findVictim() not to break cache resizing.

Parameters

addr	The addr to a find possible entries for.

Returns: The possible entries.

Implements gem5::BaseIndexingPolicy.

Definition at line 210 of file skewed_associative.cc.

References gem5::X86ISA::addr, gem5::BaseIndexingPolicy::assoc, extractSet(), and gem5::BaseIndexingPolicy::sets.

◆ hash()

Addr gem5::SkewedAssociative::hash ( const Addr addr ) const

private

The hash function itself.

Uses the hash function H, as described in "Skewed-Associative Caches", from Seznec et al. (section 3.3): It applies an XOR to the MSB and LSB, shifts all bits one bit to the right, and set the result of the XOR as the new MSB.

This function is not bijective if the address has only 1 bit, as the MSB and LSB will be the same, and therefore the xor will always be 0.

Parameters

addr	The address to be hashed.
The	hashed address.

Definition at line 63 of file skewed_associative.cc.

References gem5::X86ISA::addr, gem5::bits(), gem5::insertBits(), and msbShift.

Referenced by deskew(), and skew().

◆ regenerateAddr()

Addr gem5::SkewedAssociative::regenerateAddr	(	const Addr	tag,
		const ReplaceableEntry *	entry ) const

overridevirtual

Regenerate an entry's address from its tag and assigned set and way.

Uses the inverse of the skewing function.

Parameters

tag	The tag bits.
entry	The entry.

Returns: the entry's address.

Implements gem5::BaseIndexingPolicy.

Definition at line 201 of file skewed_associative.cc.

References deskew(), gem5::ReplaceableEntry::getSet(), gem5::ReplaceableEntry::getWay(), msbShift, gem5::BaseIndexingPolicy::setMask, gem5::BaseIndexingPolicy::setShift, and gem5::BaseIndexingPolicy::tagShift.

◆ skew()

Addr gem5::SkewedAssociative::skew	(	const Addr	addr,
		const uint32_t	way ) const

private

Address skewing function selection.

It selects and applies one of the skewing functions functions based on the way provided.

Parameters

addr	Address to be skewed. Should contain the set and tag bits.
way	The cache way, used to select a hash function.

Returns: The skewed address.

Definition at line 90 of file skewed_associative.cc.

References gem5::X86ISA::addr, gem5::bits(), dehash(), hash(), gem5::ArmISA::i, msbShift, NUM_SKEWING_FUNCTIONS, and panic.

Referenced by extractSet().

Member Data Documentation

◆ msbShift

const int gem5::SkewedAssociative::msbShift

private

The amount to shift a set index to get its MSB.

Definition at line 86 of file skewed_associative.hh.

Referenced by dehash(), deskew(), hash(), regenerateAddr(), skew(), and SkewedAssociative().

◆ NUM_SKEWING_FUNCTIONS

const int gem5::SkewedAssociative::NUM_SKEWING_FUNCTIONS = 8

private

The number of skewing functions implemented.

Should be updated if more functions are added. If more than this number of skewing functions are needed (i.e., assoc > this value), we programatically generate new ones, which may be sub-optimal.

Definition at line 81 of file skewed_associative.hh.

Referenced by deskew(), skew(), and SkewedAssociative().

The documentation for this class was generated from the following files:

mem/cache/tags/indexing_policies/skewed_associative.hh
mem/cache/tags/indexing_policies/skewed_associative.cc

Public Types

Public Member Functions

Private Member Functions

Private Attributes

Additional Inherited Members

Detailed Description

Member Typedef Documentation

◆ Params

Constructor & Destructor Documentation

◆ SkewedAssociative()

◆ ~SkewedAssociative()

Member Function Documentation

◆ dehash()

◆ deskew()

◆ extractSet()

◆ getPossibleEntries()

◆ hash()

◆ regenerateAddr()

◆ skew()

Member Data Documentation

◆ msbShift

◆ NUM_SKEWING_FUNCTIONS