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Eliminate O(N^2) behavior in parallel restore with many blobs.

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With hundreds of thousands of TOC entries, the repeated searches in
reduce_dependencies() become the dominant cost.  Get rid of that searching
by constructing reverse-dependency lists, which we can do in O(N) time
during the fix_dependencies() preprocessing.  I chose to store the reverse
dependencies as DumpId arrays for consistency with the forward-dependency
representation, and keep the previously-transient tocsByDumpId[] array
around to locate actual TOC entry structs quickly from dump IDs.

While this fixes the slow case reported by Vlad Arkhipov, there is still
a potential for O(N^2) behavior with sufficiently many tables:
fix_dependencies itself, as well as mark_create_done and
inhibit_data_for_failed_table, are doing repeated searches to deal with
table-to-table-data dependencies.  Possibly this work could be extended
to deal with that, although the latter two functions are also used in
non-parallel restore where we currently don't run fix_dependencies.

Another TODO is that we fail to parallelize restore of multiple blobs
at all.  This appears to require changes in the archive format to fix.

Back-patch to 9.0 where the problem was reported.  8.4 has potential issues
as well; but since it doesn't create a separate TOC entry for each blob,
it's at much less risk of having enough TOC entries to cause real problems.
This commit is contained in:
Tom Lane 2010-12-09 13:03:11 -05:00
parent 9975c683b1
commit 663fc32e26
2 changed files with 76 additions and 42 deletions
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116
src/bin/pg_dump/pg_backup_archiver.c
View File

@ -79,6 +79,10 @@ const char *progname;
static const char *modulename = gettext_noop("archiver");
/* index array created by fix_dependencies -- only used in parallel restore */
static TocEntry **tocsByDumpId; /* index by dumpId - 1 */
static DumpId maxDumpId; /* length of above array */
static ArchiveHandle *_allocAH(const char *FileSpec, const ArchiveFormat fmt,
const int compression, ArchiveMode mode);
@ -134,9 +138,7 @@ static void fix_dependencies(ArchiveHandle *AH);
static bool has_lock_conflicts(TocEntry *te1, TocEntry *te2);
static void repoint_table_dependencies(ArchiveHandle *AH,
DumpId tableId, DumpId tableDataId);
static void identify_locking_dependencies(TocEntry *te,
TocEntry **tocsByDumpId,
DumpId maxDumpId);
static void identify_locking_dependencies(TocEntry *te);
static void reduce_dependencies(ArchiveHandle *AH, TocEntry *te,
TocEntry *ready_list);
static void mark_create_done(ArchiveHandle *AH, TocEntry *te);
@ -3743,7 +3745,12 @@ mark_work_done(ArchiveHandle *AH, TocEntry *ready_list,
/*
* Process the dependency information into a form useful for parallel restore.
*
* We set up depCount fields that are the number of as-yet-unprocessed
* This function takes care of fixing up some missing or badly designed
* dependencies, and then prepares subsidiary data structures that will be
* used in the main parallel-restore logic, including:
* 1. We build the tocsByDumpId[] index array.
* 2. We build the revDeps[] arrays of incoming dependency dumpIds.
* 3. We set up depCount fields that are the number of as-yet-unprocessed
* dependencies for each TOC entry.
*
* We also identify locking dependencies so that we can avoid trying to
@ -3752,22 +3759,20 @@ mark_work_done(ArchiveHandle *AH, TocEntry *ready_list,
static void
fix_dependencies(ArchiveHandle *AH)
{
TocEntry **tocsByDumpId;
TocEntry *te;
DumpId maxDumpId;
int i;
/*
* For some of the steps here, it is convenient to have an array that
* indexes the TOC entries by dump ID, rather than searching the TOC list
* repeatedly. Entries for dump IDs not present in the TOC will be NULL.
* It is convenient to have an array that indexes the TOC entries by dump
* ID, rather than searching the TOC list repeatedly. Entries for dump
* IDs not present in the TOC will be NULL.
*
* NOTE: because maxDumpId is just the highest dump ID defined in the
* archive, there might be dependencies for IDs > maxDumpId. All uses of
* this array must guard against out-of-range dependency numbers.
*
* Also, initialize the depCount fields, and make sure all the TOC items
* are marked as not being in any parallel-processing list.
* Also, initialize the depCount/revDeps/nRevDeps fields, and make sure
* the TOC items are marked as not being in any parallel-processing list.
*/
maxDumpId = AH->maxDumpId;
tocsByDumpId = (TocEntry **) calloc(maxDumpId, sizeof(TocEntry *));
@ -3775,6 +3780,8 @@ fix_dependencies(ArchiveHandle *AH)
{
tocsByDumpId[te->dumpId - 1] = te;
te->depCount = te->nDeps;
te->revDeps = NULL;
te->nRevDeps = 0;
te->par_prev = NULL;
te->par_next = NULL;
}
@ -3792,6 +3799,9 @@ fix_dependencies(ArchiveHandle *AH)
* TABLE, if possible. However, if the dependency isn't in the archive
* then just assume it was a TABLE; this is to cover cases where the table
* was suppressed but we have the data and some dependent post-data items.
*
* XXX this is O(N^2) if there are a lot of tables. We ought to fix
* pg_dump to produce correctly-linked dependencies in the first place.
*/
for (te = AH->toc->next; te != AH->toc; te = te->next)
{
@ -3838,8 +3848,14 @@ fix_dependencies(ArchiveHandle *AH)
}
/*
* It is possible that the dependencies list items that are not in the
* archive at all. Subtract such items from the depCounts.
* At this point we start to build the revDeps reverse-dependency arrays,
* so all changes of dependencies must be complete.
*/
/*
* Count the incoming dependencies for each item. Also, it is possible
* that the dependencies list items that are not in the archive at
* all. Subtract such items from the depCounts.
*/
for (te = AH->toc->next; te != AH->toc; te = te->next)
{
@ -3847,11 +3863,43 @@ fix_dependencies(ArchiveHandle *AH)
{
DumpId depid = te->dependencies[i];
if (depid > maxDumpId || tocsByDumpId[depid - 1] == NULL)
if (depid <= maxDumpId && tocsByDumpId[depid - 1] != NULL)
tocsByDumpId[depid - 1]->nRevDeps++;
else
te->depCount--;
}
}
/*
* Allocate space for revDeps[] arrays, and reset nRevDeps so we can
* use it as a counter below.
*/
for (te = AH->toc->next; te != AH->toc; te = te->next)
{
if (te->nRevDeps > 0)
te->revDeps = (DumpId *) malloc(te->nRevDeps * sizeof(DumpId));
te->nRevDeps = 0;
}
/*
* Build the revDeps[] arrays of incoming-dependency dumpIds. This
* had better agree with the loops above.
*/
for (te = AH->toc->next; te != AH->toc; te = te->next)
{
for (i = 0; i < te->nDeps; i++)
{
DumpId depid = te->dependencies[i];
if (depid <= maxDumpId && tocsByDumpId[depid - 1] != NULL)
{
TocEntry *otherte = tocsByDumpId[depid - 1];
otherte->revDeps[otherte->nRevDeps++] = te->dumpId;
}
}
}
/*
* Lastly, work out the locking dependencies.
*/
@ -3859,10 +3907,8 @@ fix_dependencies(ArchiveHandle *AH)
{
te->lockDeps = NULL;
te->nLockDeps = 0;
identify_locking_dependencies(te, tocsByDumpId, maxDumpId);
identify_locking_dependencies(te);
}
free(tocsByDumpId);
}
/*
@ -3896,13 +3942,9 @@ repoint_table_dependencies(ArchiveHandle *AH,
* Identify which objects we'll need exclusive lock on in order to restore
* the given TOC entry (*other* than the one identified by the TOC entry
* itself). Record their dump IDs in the entry's lockDeps[] array.
* tocsByDumpId[] is a convenience array (of size maxDumpId) to avoid
* searching the TOC for each dependency.
*/
static void
identify_locking_dependencies(TocEntry *te,
TocEntry **tocsByDumpId,
DumpId maxDumpId)
identify_locking_dependencies(TocEntry *te)
{
DumpId *lockids;
int nlockids;
@ -3956,31 +3998,21 @@ identify_locking_dependencies(TocEntry *te,
static void
reduce_dependencies(ArchiveHandle *AH, TocEntry *te, TocEntry *ready_list)
{
DumpId target = te->dumpId;
int i;
ahlog(AH, 2, "reducing dependencies for %d\n", target);
ahlog(AH, 2, "reducing dependencies for %d\n", te->dumpId);
/*
* We must examine all entries, not only the ones after the target item,
* because if the user used a -L switch then the original dependency-
* respecting order has been destroyed by SortTocFromFile.
*/
for (te = AH->toc->next; te != AH->toc; te = te->next)
for (i = 0; i < te->nRevDeps; i++)
{
for (i = 0; i < te->nDeps; i++)
TocEntry *otherte = tocsByDumpId[te->revDeps[i] - 1];
otherte->depCount--;
if (otherte->depCount == 0 && otherte->par_prev != NULL)
{
if (te->dependencies[i] == target)
{
te->depCount--;
if (te->depCount == 0 && te->par_prev != NULL)
{
/* It must be in the pending list, so remove it ... */
par_list_remove(te);
/* ... and add to ready_list */
par_list_append(ready_list, te);
}
}
/* It must be in the pending list, so remove it ... */
par_list_remove(otherte);
/* ... and add to ready_list */
par_list_append(ready_list, otherte);
}
}
}

2
src/bin/pg_dump/pg_backup_archiver.h
View File

@ -326,6 +326,8 @@ typedef struct _tocEntry
struct _tocEntry *par_next; /* these are NULL if not in either list */
bool created; /* set for DATA member if TABLE was created */
int depCount; /* number of dependencies not yet restored */
DumpId *revDeps; /* dumpIds of objects depending on this one */
int nRevDeps; /* number of such dependencies */
DumpId *lockDeps; /* dumpIds of objects this one needs lock on */
int nLockDeps; /* number of such dependencies */
} TocEntry;