Author: Armin Rigo <[email protected]>
Branch: extradoc
Changeset: r4724:9fde8cf27591
Date: 2012-08-25 23:17 +0200
http://bitbucket.org/pypy/extradoc/changeset/9fde8cf27591/
Log: in-progress
diff --git a/talk/stm2012/stmimpl.rst b/talk/stm2012/stmimpl.rst
--- a/talk/stm2012/stmimpl.rst
+++ b/talk/stm2012/stmimpl.rst
@@ -89,6 +89,7 @@
the transaction it has so far. The following data is transaction-local:
- start_time
+- is_inevitable
- global_to_local
- list_of_read_objects
- recent_reads_cache
@@ -99,6 +100,8 @@
the state at time ``start_time``. The global "time" is a single global
number that is atomically incremented whenever a transaction commits.
+``is_inevitable`` is a flag described later.
+
``global_to_local`` is a dictionary-like mapping of global objects to
their corresponding local objects.
@@ -499,7 +502,7 @@
def CommitTransaction():
AcquireLocks()
cur_time = global_cur_time
- while not CMPXCHG(&global_cur_time, cur_time, cur_time + 2):
+ while not CMPXCHG(&global_cur_time, cur_time, cur_time + 1):
cur_time = global_cur_time # try again
ValidateDuringCommit()
UpdateChainHeads(cur_time)
@@ -596,3 +599,41 @@
``LatestGlobalRevision``, in the loop. It was omitted here because this
is always a no-op (i.e. the CPUs always provide this effect for us), not
only on x86 but on all modern CPUs.
+
+
+Inevitable transactions
+------------------------------------
+
+A transaction is "inevitable" when it cannot abort any more. It occurs
+typically when the transaction tries to do I/O or a similar effect that
+we cannot roll back. Such effects are O.K., but they mean that we have
+to guarantee the transaction's eventual successful commit.
+
+The main restriction is that there can be only one inevitable
+transaction at a time. Right now the model doesn't allow any other
+transaction to start or commit when there is an inevitable transaction;
+this restriction could be lifted with additional work.
+
+For now, the hint that the system has currently got an inevitable
+transaction is given by the value stored in ``global_cur_time``:
+the largest positive number (equal to the ``INEVITABLE`` constant).
+
+``BecomeInevitable`` is called from the middle of a transaction to
+(attempt to) make the current transaction inevitable::
+
+ def BecomeInevitable():
+ cur_time = global_cur_time
+ while not CMPXCHG(&global_cur_time, cur_time, INEVITABLE):
+ cur_time = global_cur_time # try again
+ if start_time != cur_time:
+ ValidateForInevitable(cur_time)
+ is_inevitable = True
+
+ def ValidateForInevitable(t):
+ start_time = t
+ for R in list_of_read_objects:
+ if not (R->h_revision & 1):
+ global_cur_time = t # must restore the value
+ AbortTransaction()
+
+...
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