NAME

Software Transactional Memory Frontend

ABSTRACT

Describes interfaces for Software Transactional Memory within Parrot.

VERSION

$Revision: 20926 $

SYNOPSIS

DESCRIPTION

Software transactional memory provides a transaction-like interface to shared data. This document describes an interface through which Parrot's STM implementation can be used from PIR and PASM. Functionality necessary to start, commit, abort, and retry transactions and to wrap PMCs for transactional access is supplied.

TODO: Deal with issues like exceptions in the middle of transactions. (Presumably can be dealt with on top of what's already here)

DEFINITIONS

A transaction is a sequence of a reads and writes from STM-controlled variables that appears, as far reads and writes to those variables are concerned, to happen either all at once or not at all. Transactions may, in actuality, need to be retried to perform that effect and the STM implementation makes no effort to ensure that each attempt does not have other side effects -- this functionality, if provided, is the responsibility of the language implementation. The effects of a transaction that has committed will be seen by all future transactions, and no transaction will see the effects of an aborted transaction.

A transaction is valid only if an attempt to commit it might succeed. (Thus, an invalid transaction is guaranteed to fail to commit. A valid transaction will commit in the absence of activity in other threads.)

Transactions can be nested. Nested transactions may be called inner transactions below and the transactions enclosing them outer transactions. If an inner transaction commits, its effects are still not seen globally until the corresponding outer transaction commits.

IMPLEMENTATION

Ops

Ops that require a transaction to be open will throw an exception if there is no transaction in progress.

stm_start

Starts a transaction.

stm_validate INVALID

Jumps to the label INVALID if the current transaction is not valid.

stm_commit RETRY

Commits the most recently opened transaction. Jumps to RETRY if the commit fails. (If the outer transaction is invalid, this should succeed always.)

stm_wait INVALID_OUTER

Aborts the current transaction, then waits for some value it read to have changed. If the outer transaction is invalid after waiting or after the inner transaction is aborted, jumps to INVALID_OUTER. If nested transactions are allowed, then either this variant should be used or the simpler stm_wait should be followed by an explicit stm_validate.

(This check is needed because the inner transaction may never succeed because the outer transaction constrains it to not see new values.)

stm_abort

Aborts the most recently opened transaction. Always succeeds (if there is such a transaction).

stm_depth INT

Returns how deeply nested in transactions the current thread is. The number of transactions active in the current thread is assigned to the integer register (0 if the current thread is outside any transaction).

STMVar PMC

The STMVar type wraps a variable managed by the transactional system. Unlike STMRef (below), it does not attempt to make the transactions transparent.

STMVars are safe to pass between threads.

Constructors

new 'STMVar'

Creates a new variable. Its initial value will be the NULL PMC.

new 'STMVar', Px

If Px is an STMRef, creates an STMVar instance referring to the same variable as the STMRef. Otherwise, creates a new variable whose initial value Px.

Methods

get_read

Returns some copy of the stored PMC for reading only. The PMC is not (guaranteed to be) protected from modification, but modifying it should be considered dangerous.

get_update

Returns some copy of the stored PMC suitable for modification. The value of the PMC at the time the transaction commits will be stored if the transaction commits. The PMC should not be used after the transaction commits.

set(PMC)

Sets the value of the stored PMC to the supplied PMC. This is safe to call if there is no active transaction.

STMRef PMC

The STMRef type is like Ref (that is, its default implementation of everything delegates) but is a reference to a variable managed by STM. Unlike, STMVar it provides transparent access to the underlying value. STMRef assumes that operations that do not ordinarily result in a write (e.g. get_keyed) will not result in a write. If this is not so, the below methods must be used. STMRefs can be passed between threads safely.

STMRefs are safe to pass between threads.

Constructors

new Px, 'STMRef'

Creates a new transactionally managed variable which initially contains a clone of Px.

new Px, 'STMRef', Py

If Py is an STMVar, creates an STMRef referring the same variable as Py. Otherwise, creates a new STM-managed variable which initially contains a clone of Py.

Reference operations

setref ref, Px

Changes the wrapped PMC without invoking the PMC's set, etc. methods.

deref ref, Px

Produces a copy of the value stored. The state of this copy as of the end of transaction will be reflected when the transaction commits. This copy should not be used after the transaction commits.

STMLog PMC

The STMLog PMC allows a transaction log to be saved in memory and then replayed later. Currently, only replaying for the purpose of then using stm_wait is supported.

Constructors

new Px, 'STMLog'

Copies the current transaction into Px. If there is no active transaction, Px holds an empty transaction.

Methods

replay()

Replays the saved transaction log into the current transaction. It is an error to use this outside of a transaction. Replaying may invalidate the current transaction.

cpanm Parrot::Op

perl -MCPAN -e shell
install Parrot::Op