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Improve the naming of Parallel Hash Join phases.

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* Commit 3048898e dropped -ING from PHJ wait event names.  Update the
  corresponding barrier phases names to match.

* Rename the "DONE" phases to "FREE".  That's symmetrical with
  "ALLOCATE", and names the activity that actually happens in that phase
  (as we do for the other phases) rather than a state.  The bug fixed by
  commit 8d578b9b might have been more obvious with this name.

* Rename the batch/bucket growth barriers' "ALLOCATE" phases to
  "REALLOCATE", a better description of what they do.

* Update the high level comments about phases to highlight phases
  are executed by a single process with an asterisk (mostly memory
  management phases).

No behavior change, as this is just improving internal identifiers.  The
only user-visible sign of this is that a couple of wait events' display
names change from "...Allocate" to "...Reallocate" in pg_stat_activity,
to stay in sync with the internal names.

Reviewed-by: Melanie Plageman <melanieplageman@gmail.com>
Discussion: https://postgr.es/m/CA%2BhUKG%2BMDpwF2Eo2LAvzd%3DpOh81wUTsrwU1uAwR-v6OGBB6%2B7g%40mail.gmail.com
This commit is contained in:
Thomas Munro
2023-03-23 12:39:43 +13:00
parent 11470f544e
commit 8fba928fd7
6 changed files with 119 additions and 118 deletions
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90
src/backend/executor/nodeHashjoin.c
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@@ -39,27 +39,30 @@
*
* One barrier called build_barrier is used to coordinate the hashing phases.
* The phase is represented by an integer which begins at zero and increments
* one by one, but in the code it is referred to by symbolic names as follows:
* one by one, but in the code it is referred to by symbolic names as follows.
* An asterisk indicates a phase that is performed by a single arbitrarily
* chosen process.
*
* PHJ_BUILD_ELECTING -- initial state
* PHJ_BUILD_ALLOCATING -- one sets up the batches and table 0
* PHJ_BUILD_HASHING_INNER -- all hash the inner rel
* PHJ_BUILD_HASHING_OUTER -- (multi-batch only) all hash the outer
* PHJ_BUILD_RUNNING -- building done, probing can begin
* PHJ_BUILD_DONE -- all work complete, one frees batches
* PHJ_BUILD_ELECT -- initial state
* PHJ_BUILD_ALLOCATE* -- one sets up the batches and table 0
* PHJ_BUILD_HASH_INNER -- all hash the inner rel
* PHJ_BUILD_HASH_OUTER -- (multi-batch only) all hash the outer
* PHJ_BUILD_RUN -- building done, probing can begin
* PHJ_BUILD_FREE* -- all work complete, one frees batches
*
* While in the phase PHJ_BUILD_HASHING_INNER a separate pair of barriers may
* While in the phase PHJ_BUILD_HASH_INNER a separate pair of barriers may
* be used repeatedly as required to coordinate expansions in the number of
* batches or buckets. Their phases are as follows:
*
* PHJ_GROW_BATCHES_ELECTING -- initial state
* PHJ_GROW_BATCHES_ALLOCATING -- one allocates new batches
* PHJ_GROW_BATCHES_REPARTITIONING -- all repartition
* PHJ_GROW_BATCHES_FINISHING -- one cleans up, detects skew
* PHJ_GROW_BATCHES_ELECT -- initial state
* PHJ_GROW_BATCHES_REALLOCATE* -- one allocates new batches
* PHJ_GROW_BATCHES_REPARTITION -- all repartition
* PHJ_GROW_BATCHES_DECIDE* -- one detects skew and cleans up
* PHJ_GROW_BATCHES_FINISH -- finished one growth cycle
*
* PHJ_GROW_BUCKETS_ELECTING -- initial state
* PHJ_GROW_BUCKETS_ALLOCATING -- one allocates new buckets
* PHJ_GROW_BUCKETS_REINSERTING -- all insert tuples
* PHJ_GROW_BUCKETS_ELECT -- initial state
* PHJ_GROW_BUCKETS_REALLOCATE* -- one allocates new buckets
* PHJ_GROW_BUCKETS_REINSERT -- all insert tuples
*
* If the planner got the number of batches and buckets right, those won't be
* necessary, but on the other hand we might finish up needing to expand the
@@ -67,27 +70,27 @@
* within our memory budget and load factor target. For that reason it's a
* separate pair of barriers using circular phases.
*
* The PHJ_BUILD_HASHING_OUTER phase is required only for multi-batch joins,
* The PHJ_BUILD_HASH_OUTER phase is required only for multi-batch joins,
* because we need to divide the outer relation into batches up front in order
* to be able to process batches entirely independently. In contrast, the
* parallel-oblivious algorithm simply throws tuples 'forward' to 'later'
* batches whenever it encounters them while scanning and probing, which it
* can do because it processes batches in serial order.
*
* Once PHJ_BUILD_RUNNING is reached, backends then split up and process
* Once PHJ_BUILD_RUN is reached, backends then split up and process
* different batches, or gang up and work together on probing batches if there
* aren't enough to go around. For each batch there is a separate barrier
* with the following phases:
*
* PHJ_BATCH_ELECTING -- initial state
* PHJ_BATCH_ALLOCATING -- one allocates buckets
* PHJ_BATCH_LOADING -- all load the hash table from disk
* PHJ_BATCH_PROBING -- all probe
* PHJ_BATCH_DONE -- end
* PHJ_BATCH_ELECT -- initial state
* PHJ_BATCH_ALLOCATE* -- one allocates buckets
* PHJ_BATCH_LOAD -- all load the hash table from disk
* PHJ_BATCH_PROBE -- all probe
* PHJ_BATCH_FREE* -- one frees memory
*
* Batch 0 is a special case, because it starts out in phase
* PHJ_BATCH_PROBING; populating batch 0's hash table is done during
* PHJ_BUILD_HASHING_INNER so we can skip loading.
* PHJ_BATCH_PROBE; populating batch 0's hash table is done during
* PHJ_BUILD_HASH_INNER so we can skip loading.
*
* Initially we try to plan for a single-batch hash join using the combined
* hash_mem of all participants to create a large shared hash table. If that
@@ -99,8 +102,8 @@
* finished. Practically, that means that we never emit a tuple while attached
* to a barrier, unless the barrier has reached a phase that means that no
* process will wait on it again. We emit tuples while attached to the build
* barrier in phase PHJ_BUILD_RUNNING, and to a per-batch barrier in phase
* PHJ_BATCH_PROBING. These are advanced to PHJ_BUILD_DONE and PHJ_BATCH_DONE
* barrier in phase PHJ_BUILD_RUN, and to a per-batch barrier in phase
* PHJ_BATCH_PROBE. These are advanced to PHJ_BUILD_FREE and PHJ_BATCH_FREE
* respectively without waiting, using BarrierArriveAndDetach(). The last to
* detach receives a different return value so that it knows that it's safe to
* clean up. Any straggler process that attaches after that phase is reached
@@ -306,13 +309,12 @@ ExecHashJoinImpl(PlanState *pstate, bool parallel)
if (parallel)
{
/*
* Advance the build barrier to PHJ_BUILD_RUNNING
* before proceeding so we can negotiate resource
* cleanup.
* Advance the build barrier to PHJ_BUILD_RUN before
* proceeding so we can negotiate resource cleanup.
*/
Barrier *build_barrier = &parallel_state->build_barrier;
while (BarrierPhase(build_barrier) < PHJ_BUILD_RUNNING)
while (BarrierPhase(build_barrier) < PHJ_BUILD_RUN)
BarrierArriveAndWait(build_barrier, 0);
}
return NULL;
@@ -336,10 +338,10 @@ ExecHashJoinImpl(PlanState *pstate, bool parallel)
Barrier *build_barrier;
build_barrier = &parallel_state->build_barrier;
Assert(BarrierPhase(build_barrier) == PHJ_BUILD_HASHING_OUTER ||
BarrierPhase(build_barrier) == PHJ_BUILD_RUNNING ||
BarrierPhase(build_barrier) == PHJ_BUILD_DONE);
if (BarrierPhase(build_barrier) == PHJ_BUILD_HASHING_OUTER)
Assert(BarrierPhase(build_barrier) == PHJ_BUILD_HASH_OUTER ||
BarrierPhase(build_barrier) == PHJ_BUILD_RUN ||
BarrierPhase(build_barrier) == PHJ_BUILD_FREE);
if (BarrierPhase(build_barrier) == PHJ_BUILD_HASH_OUTER)
{
/*
* If multi-batch, we need to hash the outer relation
@@ -350,7 +352,7 @@ ExecHashJoinImpl(PlanState *pstate, bool parallel)
BarrierArriveAndWait(build_barrier,
WAIT_EVENT_HASH_BUILD_HASH_OUTER);
}
else if (BarrierPhase(build_barrier) == PHJ_BUILD_DONE)
else if (BarrierPhase(build_barrier) == PHJ_BUILD_FREE)
{
/*
* If we attached so late that the job is finished and
@@ -361,7 +363,7 @@ ExecHashJoinImpl(PlanState *pstate, bool parallel)
}
/* Each backend should now select a batch to work on. */
Assert(BarrierPhase(build_barrier) == PHJ_BUILD_RUNNING);
Assert(BarrierPhase(build_barrier) == PHJ_BUILD_RUN);
hashtable->curbatch = -1;
node->hj_JoinState = HJ_NEED_NEW_BATCH;
@@ -1153,7 +1155,7 @@ ExecParallelHashJoinNewBatch(HashJoinState *hjstate)
switch (BarrierAttach(batch_barrier))
{
case PHJ_BATCH_ELECTING:
case PHJ_BATCH_ELECT:
/* One backend allocates the hash table. */
if (BarrierArriveAndWait(batch_barrier,
@@ -1161,13 +1163,13 @@ ExecParallelHashJoinNewBatch(HashJoinState *hjstate)
ExecParallelHashTableAlloc(hashtable, batchno);
/* Fall through. */
case PHJ_BATCH_ALLOCATING:
case PHJ_BATCH_ALLOCATE:
/* Wait for allocation to complete. */
BarrierArriveAndWait(batch_barrier,
WAIT_EVENT_HASH_BATCH_ALLOCATE);
/* Fall through. */
case PHJ_BATCH_LOADING:
case PHJ_BATCH_LOAD:
/* Start (or join in) loading tuples. */
ExecParallelHashTableSetCurrentBatch(hashtable, batchno);
inner_tuples = hashtable->batches[batchno].inner_tuples;
@@ -1187,7 +1189,7 @@ ExecParallelHashJoinNewBatch(HashJoinState *hjstate)
WAIT_EVENT_HASH_BATCH_LOAD);
/* Fall through. */
case PHJ_BATCH_PROBING:
case PHJ_BATCH_PROBE:
/*
* This batch is ready to probe. Return control to
@@ -1197,13 +1199,13 @@ ExecParallelHashJoinNewBatch(HashJoinState *hjstate)
* this barrier again (or else a deadlock could occur).
* All attached participants must eventually call
* BarrierArriveAndDetach() so that the final phase
* PHJ_BATCH_DONE can be reached.
* PHJ_BATCH_FREE can be reached.
*/
ExecParallelHashTableSetCurrentBatch(hashtable, batchno);
sts_begin_parallel_scan(hashtable->batches[batchno].outer_tuples);
return true;
case PHJ_BATCH_DONE:
case PHJ_BATCH_FREE:
/*
* Already done. Detach and go around again (if any
@@ -1523,7 +1525,7 @@ ExecHashJoinReInitializeDSM(HashJoinState *state, ParallelContext *pcxt)
/*
* It would be possible to reuse the shared hash table in single-batch
* cases by resetting and then fast-forwarding build_barrier to
* PHJ_BUILD_DONE and batch 0's batch_barrier to PHJ_BATCH_PROBING, but
* PHJ_BUILD_FREE and batch 0's batch_barrier to PHJ_BATCH_PROBE, but
* currently shared hash tables are already freed by now (by the last
* participant to detach from the batch). We could consider keeping it
* around for single-batch joins. We'd also need to adjust
@@ -1542,7 +1544,7 @@ ExecHashJoinReInitializeDSM(HashJoinState *state, ParallelContext *pcxt)
/* Clear any shared batch files. */
SharedFileSetDeleteAll(&pstate->fileset);
/* Reset build_barrier to PHJ_BUILD_ELECTING so we can go around again. */
/* Reset build_barrier to PHJ_BUILD_ELECT so we can go around again. */
BarrierInit(&pstate->build_barrier, 0);
}