Architecture of Computing Systems (Lecture Notes in Computer ...

Efficient Transaction Nesting in Hardware Transactional Memory 141
3.2 The Scheme of Conditional Partial Rollback
The ideal method to improve performance of closed nesting is: when a transaction
aborts due to a conflict with other transactions, it just rolls back to the beginning
of itself instead of the outermost transaction. However, this requires that each
nested transaction has its own data set, as in the nested LogTM, and will increase
complexities and costs of hardware significantly.
To improve the performance of closed nesting in reasonable hardware cost, we
propose a scheme which supports conditional partial rollback (CPR) of transactions.
The idea of the scheme is: instead of maintaining independent data set
for each nesting level, a global data set is used for all of the nested transactions,
while the access status of each nesting level to the data set is recorded. When an
inner level transaction aborts due to a conflict, it rolls back to the beginning of
itself if the data set it accessed has no overlap with other transactions, otherwise
it rolls back to the beginning of the outer-level transaction which accessed same
data with it, in the worst case, it rolls back to the beginning of the outermost
transaction.
In addition, the partial rollback can be further distinguished by read/write
accesses. Only when the conflicted transaction has written to the same data with
other transactions, it rolls back to the beginning of the outer-level transaction.
The reason is that the data updated by the outer-level transactions has been
overwritten by the conflicted transaction in the later, and cannot be restored
to the beginning status of conflicted transaction, in that case, a rollback to the
beginning of the outer-level transaction is needed. In other cases, the conflicted
transaction just rolls back to the beginning of itself despite it has accessed the
same data with other transactions.
The above CPR scheme can be described formally as following: suppose the
number of nesting level is n, the outermost transaction is T0, the innermost transaction
is Tn−1, and write set of each nested transaction are: D0,D1,...,Dn−1.
When transaction Tc aborts due to a conflict, it rolls back to the beginning
of transaction Tm, and:
m =min{∀i∀j[(Dc ∩ Di �= ∅) ∧ (Di ∩ Dj = ∅)]} . (1)
(i =0, 1, 2,...,c; j =0, 1, 2,...,i− 1)
According to the above expression, the transaction Tc will roll back to the transaction
that is the outermost among the transactions that have overlapped write
set with Tc. If there is no overlapped write set between Tc and others, it will just
roll back to the beginning of itself.
Fig. 1 gives an example of conditional partial rollback. According to the flattening
model for closed nesting, an inner transaction Tc rolls back to the outermost
transaction when conflict occurs, as shown in Fig. 1(a). For CPR scheme,
the transaction rolls back to the beginning of itself if there is no overlap between
its write set and others, as shown in Fig. 1(b); otherwise it rolls back to the outer
transaction which has overlapped write set with Tc , as shown in Fig. 1(c).