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Convert pg_restore's ready_list to a priority queue.
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Presently, parallel restores spend a lot of time sorting this list
so that we pick the largest items first.  With many tables, this
sorting can become a significant bottleneck.  There are a couple of
reports from the field about this, and it is easy to reproduce.

This commit improves the performance of parallel pg_restore with
many tables by converting its ready_list to a priority queue, i.e.,
a binary heap.  We will first try to run the highest priority item,
but if it cannot be chosen due to the lock heuristic, we'll do a
sequential scan through the heap nodes until we find one that is
runnable.  This means that we might end up picking an item with a
much lower priority.  However, we expect that we will typically be
able to pick one of the first few items, which should usually have
a relatively high priority.

Suggested-by: Tom Lane
Tested-by: Pierre Ducroquet
Reviewed-by: Tom Lane
Discussion: https://postgr.es/m/3612876.1689443232%40sss.pgh.pa.us
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@nathan-bossart
nathan-bossart committed Sep 19, 2023
1 parent 1f99886 commit 9bfd44b
Showing 1 changed file with 58 additions and 140 deletions.
198 changes: 58 additions & 140 deletions src/bin/pg_dump/pg_backup_archiver.c
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Expand Up @@ -34,6 +34,7 @@
#include "compress_io.h"
#include "dumputils.h"
#include "fe_utils/string_utils.h"
#include "lib/binaryheap.h"
#include "lib/stringinfo.h"
#include "libpq/libpq-fs.h"
#include "parallel.h"
Expand All @@ -44,24 +45,6 @@
#define TEXT_DUMP_HEADER "--\n-- PostgreSQL database dump\n--\n\n"
#define TEXT_DUMPALL_HEADER "--\n-- PostgreSQL database cluster dump\n--\n\n"

/*
* State for tracking TocEntrys that are ready to process during a parallel
* restore. (This used to be a list, and we still call it that, though now
* it's really an array so that we can apply qsort to it.)
*
* tes[] is sized large enough that we can't overrun it.
* The valid entries are indexed first_te .. last_te inclusive.
* We periodically sort the array to bring larger-by-dataLength entries to
* the front; "sorted" is true if the valid entries are known sorted.
*/
typedef struct _parallelReadyList
{
TocEntry **tes; /* Ready-to-dump TocEntrys */
int first_te; /* index of first valid entry in tes[] */
int last_te; /* index of last valid entry in tes[] */
bool sorted; /* are valid entries currently sorted? */
} ParallelReadyList;


static ArchiveHandle *_allocAH(const char *FileSpec, const ArchiveFormat fmt,
const pg_compress_specification compression_spec,
Expand Down Expand Up @@ -111,16 +94,12 @@ static void restore_toc_entries_postfork(ArchiveHandle *AH,
static void pending_list_header_init(TocEntry *l);
static void pending_list_append(TocEntry *l, TocEntry *te);
static void pending_list_remove(TocEntry *te);
static void ready_list_init(ParallelReadyList *ready_list, int tocCount);
static void ready_list_free(ParallelReadyList *ready_list);
static void ready_list_insert(ParallelReadyList *ready_list, TocEntry *te);
static void ready_list_remove(ParallelReadyList *ready_list, int i);
static void ready_list_sort(ParallelReadyList *ready_list);
static int TocEntrySizeCompare(const void *p1, const void *p2);
static void move_to_ready_list(TocEntry *pending_list,
ParallelReadyList *ready_list,
static int TocEntrySizeCompareQsort(const void *p1, const void *p2);
static int TocEntrySizeCompareBinaryheap(void *p1, void *p2, void *arg);
static void move_to_ready_heap(TocEntry *pending_list,
binaryheap *ready_heap,
RestorePass pass);
static TocEntry *pop_next_work_item(ParallelReadyList *ready_list,
static TocEntry *pop_next_work_item(binaryheap *ready_heap,
ParallelState *pstate);
static void mark_dump_job_done(ArchiveHandle *AH,
TocEntry *te,
Expand All @@ -135,7 +114,7 @@ static bool has_lock_conflicts(TocEntry *te1, TocEntry *te2);
static void repoint_table_dependencies(ArchiveHandle *AH);
static void identify_locking_dependencies(ArchiveHandle *AH, TocEntry *te);
static void reduce_dependencies(ArchiveHandle *AH, TocEntry *te,
ParallelReadyList *ready_list);
binaryheap *ready_heap);
static void mark_create_done(ArchiveHandle *AH, TocEntry *te);
static void inhibit_data_for_failed_table(ArchiveHandle *AH, TocEntry *te);

Expand Down Expand Up @@ -2384,7 +2363,7 @@ WriteDataChunks(ArchiveHandle *AH, ParallelState *pstate)
}

if (ntes > 1)
qsort(tes, ntes, sizeof(TocEntry *), TocEntrySizeCompare);
qsort(tes, ntes, sizeof(TocEntry *), TocEntrySizeCompareQsort);

for (int i = 0; i < ntes; i++)
DispatchJobForTocEntry(AH, pstate, tes[i], ACT_DUMP,
Expand Down Expand Up @@ -3984,7 +3963,7 @@ restore_toc_entries_prefork(ArchiveHandle *AH, TocEntry *pending_list)

(void) restore_toc_entry(AH, next_work_item, false);

/* Reduce dependencies, but don't move anything to ready_list */
/* Reduce dependencies, but don't move anything to ready_heap */
reduce_dependencies(AH, next_work_item, NULL);
}
else
Expand Down Expand Up @@ -4027,24 +4006,26 @@ static void
restore_toc_entries_parallel(ArchiveHandle *AH, ParallelState *pstate,
TocEntry *pending_list)
{
ParallelReadyList ready_list;
binaryheap *ready_heap;
TocEntry *next_work_item;

pg_log_debug("entering restore_toc_entries_parallel");

/* Set up ready_list with enough room for all known TocEntrys */
ready_list_init(&ready_list, AH->tocCount);
/* Set up ready_heap with enough room for all known TocEntrys */
ready_heap = binaryheap_allocate(AH->tocCount,
TocEntrySizeCompareBinaryheap,
NULL);

/*
* The pending_list contains all items that we need to restore. Move all
* items that are available to process immediately into the ready_list.
* items that are available to process immediately into the ready_heap.
* After this setup, the pending list is everything that needs to be done
* but is blocked by one or more dependencies, while the ready list
* but is blocked by one or more dependencies, while the ready heap
* contains items that have no remaining dependencies and are OK to
* process in the current restore pass.
*/
AH->restorePass = RESTORE_PASS_MAIN;
move_to_ready_list(pending_list, &ready_list, AH->restorePass);
move_to_ready_heap(pending_list, ready_heap, AH->restorePass);

/*
* main parent loop
Expand All @@ -4058,7 +4039,7 @@ restore_toc_entries_parallel(ArchiveHandle *AH, ParallelState *pstate,
for (;;)
{
/* Look for an item ready to be dispatched to a worker */
next_work_item = pop_next_work_item(&ready_list, pstate);
next_work_item = pop_next_work_item(ready_heap, pstate);
if (next_work_item != NULL)
{
/* If not to be restored, don't waste time launching a worker */
Expand All @@ -4068,7 +4049,7 @@ restore_toc_entries_parallel(ArchiveHandle *AH, ParallelState *pstate,
next_work_item->dumpId,
next_work_item->desc, next_work_item->tag);
/* Update its dependencies as though we'd completed it */
reduce_dependencies(AH, next_work_item, &ready_list);
reduce_dependencies(AH, next_work_item, ready_heap);
/* Loop around to see if anything else can be dispatched */
continue;
}
Expand All @@ -4079,7 +4060,7 @@ restore_toc_entries_parallel(ArchiveHandle *AH, ParallelState *pstate,

/* Dispatch to some worker */
DispatchJobForTocEntry(AH, pstate, next_work_item, ACT_RESTORE,
mark_restore_job_done, &ready_list);
mark_restore_job_done, ready_heap);
}
else if (IsEveryWorkerIdle(pstate))
{
Expand All @@ -4093,7 +4074,7 @@ restore_toc_entries_parallel(ArchiveHandle *AH, ParallelState *pstate,
/* Advance to next restore pass */
AH->restorePass++;
/* That probably allows some stuff to be made ready */
move_to_ready_list(pending_list, &ready_list, AH->restorePass);
move_to_ready_heap(pending_list, ready_heap, AH->restorePass);
/* Loop around to see if anything's now ready */
continue;
}
Expand Down Expand Up @@ -4122,10 +4103,10 @@ restore_toc_entries_parallel(ArchiveHandle *AH, ParallelState *pstate,
next_work_item ? WFW_ONE_IDLE : WFW_GOT_STATUS);
}

/* There should now be nothing in ready_list. */
Assert(ready_list.first_te > ready_list.last_te);
/* There should now be nothing in ready_heap. */
Assert(binaryheap_empty(ready_heap));

ready_list_free(&ready_list);
binaryheap_free(ready_heap);

pg_log_info("finished main parallel loop");
}
Expand Down Expand Up @@ -4225,80 +4206,9 @@ pending_list_remove(TocEntry *te)
}


/*
* Initialize the ready_list with enough room for up to tocCount entries.
*/
static void
ready_list_init(ParallelReadyList *ready_list, int tocCount)
{
ready_list->tes = (TocEntry **)
pg_malloc(tocCount * sizeof(TocEntry *));
ready_list->first_te = 0;
ready_list->last_te = -1;
ready_list->sorted = false;
}

/*
* Free storage for a ready_list.
*/
static void
ready_list_free(ParallelReadyList *ready_list)
{
pg_free(ready_list->tes);
}

/* Add te to the ready_list */
static void
ready_list_insert(ParallelReadyList *ready_list, TocEntry *te)
{
ready_list->tes[++ready_list->last_te] = te;
/* List is (probably) not sorted anymore. */
ready_list->sorted = false;
}

/* Remove the i'th entry in the ready_list */
static void
ready_list_remove(ParallelReadyList *ready_list, int i)
{
int f = ready_list->first_te;

Assert(i >= f && i <= ready_list->last_te);

/*
* In the typical case where the item to be removed is the first ready
* entry, we need only increment first_te to remove it. Otherwise, move
* the entries before it to compact the list. (This preserves sortedness,
* if any.) We could alternatively move the entries after i, but there
* are typically many more of those.
*/
if (i > f)
{
TocEntry **first_te_ptr = &ready_list->tes[f];

memmove(first_te_ptr + 1, first_te_ptr, (i - f) * sizeof(TocEntry *));
}
ready_list->first_te++;
}

/* Sort the ready_list into the desired order */
static void
ready_list_sort(ParallelReadyList *ready_list)
{
if (!ready_list->sorted)
{
int n = ready_list->last_te - ready_list->first_te + 1;

if (n > 1)
qsort(ready_list->tes + ready_list->first_te, n,
sizeof(TocEntry *),
TocEntrySizeCompare);
ready_list->sorted = true;
}
}

/* qsort comparator for sorting TocEntries by dataLength */
static int
TocEntrySizeCompare(const void *p1, const void *p2)
TocEntrySizeCompareQsort(const void *p1, const void *p2)
{
const TocEntry *te1 = *(const TocEntry *const *) p1;
const TocEntry *te2 = *(const TocEntry *const *) p2;
Expand All @@ -4318,17 +4228,25 @@ TocEntrySizeCompare(const void *p1, const void *p2)
return 0;
}

/* binaryheap comparator for sorting TocEntries by dataLength */
static int
TocEntrySizeCompareBinaryheap(void *p1, void *p2, void *arg)
{
/* return opposite of qsort comparator for max-heap */
return -TocEntrySizeCompareQsort(&p1, &p2);
}


/*
* Move all immediately-ready items from pending_list to ready_list.
* Move all immediately-ready items from pending_list to ready_heap.
*
* Items are considered ready if they have no remaining dependencies and
* they belong in the current restore pass. (See also reduce_dependencies,
* which applies the same logic one-at-a-time.)
*/
static void
move_to_ready_list(TocEntry *pending_list,
ParallelReadyList *ready_list,
move_to_ready_heap(TocEntry *pending_list,
binaryheap *ready_heap,
RestorePass pass)
{
TocEntry *te;
Expand All @@ -4344,38 +4262,38 @@ move_to_ready_list(TocEntry *pending_list,
{
/* Remove it from pending_list ... */
pending_list_remove(te);
/* ... and add to ready_list */
ready_list_insert(ready_list, te);
/* ... and add to ready_heap */
binaryheap_add(ready_heap, te);
}
}
}

/*
* Find the next work item (if any) that is capable of being run now,
* and remove it from the ready_list.
* and remove it from the ready_heap.
*
* Returns the item, or NULL if nothing is runnable.
*
* To qualify, the item must have no remaining dependencies
* and no requirements for locks that are incompatible with
* items currently running. Items in the ready_list are known to have
* items currently running. Items in the ready_heap are known to have
* no remaining dependencies, but we have to check for lock conflicts.
*/
static TocEntry *
pop_next_work_item(ParallelReadyList *ready_list,
pop_next_work_item(binaryheap *ready_heap,
ParallelState *pstate)
{
/*
* Sort the ready_list so that we'll tackle larger jobs first.
*/
ready_list_sort(ready_list);

/*
* Search the ready_list until we find a suitable item.
* Search the ready_heap until we find a suitable item. Note that we do a
* sequential scan through the heap nodes, so even though we will first
* try to choose the highest-priority item, we might end up picking
* something with a much lower priority. However, we expect that we will
* typically be able to pick one of the first few items, which should
* usually have a relatively high priority.
*/
for (int i = ready_list->first_te; i <= ready_list->last_te; i++)
for (int i = 0; i < binaryheap_size(ready_heap); i++)
{
TocEntry *te = ready_list->tes[i];
TocEntry *te = (TocEntry *) binaryheap_get_node(ready_heap, i);
bool conflicts = false;

/*
Expand All @@ -4401,7 +4319,7 @@ pop_next_work_item(ParallelReadyList *ready_list,
continue;

/* passed all tests, so this item can run */
ready_list_remove(ready_list, i);
binaryheap_remove_node(ready_heap, i);
return te;
}

Expand Down Expand Up @@ -4447,7 +4365,7 @@ mark_restore_job_done(ArchiveHandle *AH,
int status,
void *callback_data)
{
ParallelReadyList *ready_list = (ParallelReadyList *) callback_data;
binaryheap *ready_heap = (binaryheap *) callback_data;

pg_log_info("finished item %d %s %s",
te->dumpId, te->desc, te->tag);
Expand All @@ -4465,7 +4383,7 @@ mark_restore_job_done(ArchiveHandle *AH,
pg_fatal("worker process failed: exit code %d",
status);

reduce_dependencies(AH, te, ready_list);
reduce_dependencies(AH, te, ready_heap);
}


Expand Down Expand Up @@ -4708,11 +4626,11 @@ identify_locking_dependencies(ArchiveHandle *AH, TocEntry *te)
/*
* Remove the specified TOC entry from the depCounts of items that depend on
* it, thereby possibly making them ready-to-run. Any pending item that
* becomes ready should be moved to the ready_list, if that's provided.
* becomes ready should be moved to the ready_heap, if that's provided.
*/
static void
reduce_dependencies(ArchiveHandle *AH, TocEntry *te,
ParallelReadyList *ready_list)
binaryheap *ready_heap)
{
int i;

Expand All @@ -4730,18 +4648,18 @@ reduce_dependencies(ArchiveHandle *AH, TocEntry *te,
* the current restore pass, and it is currently a member of the
* pending list (that check is needed to prevent double restore in
* some cases where a list-file forces out-of-order restoring).
* However, if ready_list == NULL then caller doesn't want any list
* However, if ready_heap == NULL then caller doesn't want any list
* memberships changed.
*/
if (otherte->depCount == 0 &&
_tocEntryRestorePass(otherte) == AH->restorePass &&
otherte->pending_prev != NULL &&
ready_list != NULL)
ready_heap != NULL)
{
/* Remove it from pending list ... */
pending_list_remove(otherte);
/* ... and add to ready_list */
ready_list_insert(ready_list, otherte);
/* ... and add to ready_heap */
binaryheap_add(ready_heap, otherte);
}
}
}
Expand Down

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