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Commit663fc32

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Eliminate O(N^2) behavior in parallel restore with many blobs.
With hundreds of thousands of TOC entries, the repeated searches inreduce_dependencies() become the dominant cost. Get rid of that searchingby constructing reverse-dependency lists, which we can do in O(N) timeduring the fix_dependencies() preprocessing. I chose to store the reversedependencies as DumpId arrays for consistency with the forward-dependencyrepresentation, and keep the previously-transient tocsByDumpId[] arrayaround to locate actual TOC entry structs quickly from dump IDs.While this fixes the slow case reported by Vlad Arkhipov, there is stilla potential for O(N^2) behavior with sufficiently many tables:fix_dependencies itself, as well as mark_create_done andinhibit_data_for_failed_table, are doing repeated searches to deal withtable-to-table-data dependencies. Possibly this work could be extendedto deal with that, although the latter two functions are also used innon-parallel restore where we currently don't run fix_dependencies.Another TODO is that we fail to parallelize restore of multiple blobsat 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 issuesas 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.
1 parent9975c68 commit663fc32

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2 files changed

+76
-42
lines changed

2 files changed

+76
-42
lines changed

‎src/bin/pg_dump/pg_backup_archiver.c

Lines changed: 74 additions & 42 deletions
Original file line numberDiff line numberDiff line change
@@ -79,6 +79,10 @@ const char *progname;
7979

8080
staticconstchar*modulename=gettext_noop("archiver");
8181

82+
/* index array created by fix_dependencies -- only used in parallel restore */
83+
staticTocEntry**tocsByDumpId;/* index by dumpId - 1 */
84+
staticDumpIdmaxDumpId;/* length of above array */
85+
8286

8387
staticArchiveHandle*_allocAH(constchar*FileSpec,constArchiveFormatfmt,
8488
constintcompression,ArchiveModemode);
@@ -134,9 +138,7 @@ static void fix_dependencies(ArchiveHandle *AH);
134138
staticboolhas_lock_conflicts(TocEntry*te1,TocEntry*te2);
135139
staticvoidrepoint_table_dependencies(ArchiveHandle*AH,
136140
DumpIdtableId,DumpIdtableDataId);
137-
staticvoididentify_locking_dependencies(TocEntry*te,
138-
TocEntry**tocsByDumpId,
139-
DumpIdmaxDumpId);
141+
staticvoididentify_locking_dependencies(TocEntry*te);
140142
staticvoidreduce_dependencies(ArchiveHandle*AH,TocEntry*te,
141143
TocEntry*ready_list);
142144
staticvoidmark_create_done(ArchiveHandle*AH,TocEntry*te);
@@ -3743,7 +3745,12 @@ mark_work_done(ArchiveHandle *AH, TocEntry *ready_list,
37433745
/*
37443746
* Process the dependency information into a form useful for parallel restore.
37453747
*
3746-
* We set up depCount fields that are the number of as-yet-unprocessed
3748+
* This function takes care of fixing up some missing or badly designed
3749+
* dependencies, and then prepares subsidiary data structures that will be
3750+
* used in the main parallel-restore logic, including:
3751+
* 1. We build the tocsByDumpId[] index array.
3752+
* 2. We build the revDeps[] arrays of incoming dependency dumpIds.
3753+
* 3. We set up depCount fields that are the number of as-yet-unprocessed
37473754
* dependencies for each TOC entry.
37483755
*
37493756
* We also identify locking dependencies so that we can avoid trying to
@@ -3752,29 +3759,29 @@ mark_work_done(ArchiveHandle *AH, TocEntry *ready_list,
37523759
staticvoid
37533760
fix_dependencies(ArchiveHandle*AH)
37543761
{
3755-
TocEntry**tocsByDumpId;
37563762
TocEntry*te;
3757-
DumpIdmaxDumpId;
37583763
inti;
37593764

37603765
/*
3761-
*For some of the steps here, itis convenient to have an array that
3762-
*indexes the TOC entries by dumpID, rather than searching the TOC list
3763-
*repeatedly.Entries for dumpIDs not present in the TOC will be NULL.
3766+
*Itis convenient to have an array that indexes the TOC entries by dump
3767+
* ID, rather than searching the TOC list repeatedly. Entries for dump
3768+
* IDs not present in the TOC will be NULL.
37643769
*
37653770
* NOTE: because maxDumpId is just the highest dump ID defined in the
37663771
* archive, there might be dependencies for IDs > maxDumpId. All uses of
37673772
* this array must guard against out-of-range dependency numbers.
37683773
*
3769-
* Also, initialize the depCount fields, and make sure all the TOC items
3770-
* are marked as not being in any parallel-processing list.
3774+
* Also, initialize the depCount/revDeps/nRevDeps fields, and make sure
3775+
*the TOC itemsare marked as not being in any parallel-processing list.
37713776
*/
37723777
maxDumpId=AH->maxDumpId;
37733778
tocsByDumpId= (TocEntry**)calloc(maxDumpId,sizeof(TocEntry*));
37743779
for (te=AH->toc->next;te!=AH->toc;te=te->next)
37753780
{
37763781
tocsByDumpId[te->dumpId-1]=te;
37773782
te->depCount=te->nDeps;
3783+
te->revDeps=NULL;
3784+
te->nRevDeps=0;
37783785
te->par_prev=NULL;
37793786
te->par_next=NULL;
37803787
}
@@ -3792,6 +3799,9 @@ fix_dependencies(ArchiveHandle *AH)
37923799
* TABLE, if possible.However, if the dependency isn't in the archive
37933800
* then just assume it was a TABLE; this is to cover cases where the table
37943801
* was suppressed but we have the data and some dependent post-data items.
3802+
*
3803+
* XXX this is O(N^2) if there are a lot of tables. We ought to fix
3804+
* pg_dump to produce correctly-linked dependencies in the first place.
37953805
*/
37963806
for (te=AH->toc->next;te!=AH->toc;te=te->next)
37973807
{
@@ -3838,31 +3848,67 @@ fix_dependencies(ArchiveHandle *AH)
38383848
}
38393849

38403850
/*
3841-
* It is possible that the dependencies list items that are not in the
3842-
* archive at all.Subtract such items from the depCounts.
3851+
* At this point we start to build the revDeps reverse-dependency arrays,
3852+
* so all changes of dependencies must be complete.
3853+
*/
3854+
3855+
/*
3856+
* Count the incoming dependencies for each item. Also, it is possible
3857+
* that the dependencies list items that are not in the archive at
3858+
* all. Subtract such items from the depCounts.
38433859
*/
38443860
for (te=AH->toc->next;te!=AH->toc;te=te->next)
38453861
{
38463862
for (i=0;i<te->nDeps;i++)
38473863
{
38483864
DumpIddepid=te->dependencies[i];
38493865

3850-
if (depid>maxDumpId||tocsByDumpId[depid-1]==NULL)
3866+
if (depid <=maxDumpId&&tocsByDumpId[depid-1]!=NULL)
3867+
tocsByDumpId[depid-1]->nRevDeps++;
3868+
else
38513869
te->depCount--;
38523870
}
38533871
}
38543872

3873+
/*
3874+
* Allocate space for revDeps[] arrays, and reset nRevDeps so we can
3875+
* use it as a counter below.
3876+
*/
3877+
for (te=AH->toc->next;te!=AH->toc;te=te->next)
3878+
{
3879+
if (te->nRevDeps>0)
3880+
te->revDeps= (DumpId*)malloc(te->nRevDeps*sizeof(DumpId));
3881+
te->nRevDeps=0;
3882+
}
3883+
3884+
/*
3885+
* Build the revDeps[] arrays of incoming-dependency dumpIds. This
3886+
* had better agree with the loops above.
3887+
*/
3888+
for (te=AH->toc->next;te!=AH->toc;te=te->next)
3889+
{
3890+
for (i=0;i<te->nDeps;i++)
3891+
{
3892+
DumpIddepid=te->dependencies[i];
3893+
3894+
if (depid <=maxDumpId&&tocsByDumpId[depid-1]!=NULL)
3895+
{
3896+
TocEntry*otherte=tocsByDumpId[depid-1];
3897+
3898+
otherte->revDeps[otherte->nRevDeps++]=te->dumpId;
3899+
}
3900+
}
3901+
}
3902+
38553903
/*
38563904
* Lastly, work out the locking dependencies.
38573905
*/
38583906
for (te=AH->toc->next;te!=AH->toc;te=te->next)
38593907
{
38603908
te->lockDeps=NULL;
38613909
te->nLockDeps=0;
3862-
identify_locking_dependencies(te,tocsByDumpId,maxDumpId);
3910+
identify_locking_dependencies(te);
38633911
}
3864-
3865-
free(tocsByDumpId);
38663912
}
38673913

38683914
/*
@@ -3896,13 +3942,9 @@ repoint_table_dependencies(ArchiveHandle *AH,
38963942
* Identify which objects we'll need exclusive lock on in order to restore
38973943
* the given TOC entry (*other* than the one identified by the TOC entry
38983944
* itself). Record their dump IDs in the entry's lockDeps[] array.
3899-
* tocsByDumpId[] is a convenience array (of size maxDumpId) to avoid
3900-
* searching the TOC for each dependency.
39013945
*/
39023946
staticvoid
3903-
identify_locking_dependencies(TocEntry*te,
3904-
TocEntry**tocsByDumpId,
3905-
DumpIdmaxDumpId)
3947+
identify_locking_dependencies(TocEntry*te)
39063948
{
39073949
DumpId*lockids;
39083950
intnlockids;
@@ -3956,31 +3998,21 @@ identify_locking_dependencies(TocEntry *te,
39563998
staticvoid
39573999
reduce_dependencies(ArchiveHandle*AH,TocEntry*te,TocEntry*ready_list)
39584000
{
3959-
DumpIdtarget=te->dumpId;
39604001
inti;
39614002

3962-
ahlog(AH,2,"reducing dependencies for %d\n",target);
4003+
ahlog(AH,2,"reducing dependencies for %d\n",te->dumpId);
39634004

3964-
/*
3965-
* We must examine all entries, not only the ones after the target item,
3966-
* because if the user used a -L switch then the original dependency-
3967-
* respecting order has been destroyed by SortTocFromFile.
3968-
*/
3969-
for (te=AH->toc->next;te!=AH->toc;te=te->next)
4005+
for (i=0;i<te->nRevDeps;i++)
39704006
{
3971-
for (i=0;i<te->nDeps;i++)
4007+
TocEntry*otherte=tocsByDumpId[te->revDeps[i]-1];
4008+
4009+
otherte->depCount--;
4010+
if (otherte->depCount==0&&otherte->par_prev!=NULL)
39724011
{
3973-
if (te->dependencies[i]==target)
3974-
{
3975-
te->depCount--;
3976-
if (te->depCount==0&&te->par_prev!=NULL)
3977-
{
3978-
/* It must be in the pending list, so remove it ... */
3979-
par_list_remove(te);
3980-
/* ... and add to ready_list */
3981-
par_list_append(ready_list,te);
3982-
}
3983-
}
4012+
/* It must be in the pending list, so remove it ... */
4013+
par_list_remove(otherte);
4014+
/* ... and add to ready_list */
4015+
par_list_append(ready_list,otherte);
39844016
}
39854017
}
39864018
}

‎src/bin/pg_dump/pg_backup_archiver.h

Lines changed: 2 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -326,6 +326,8 @@ typedef struct _tocEntry
326326
struct_tocEntry*par_next;/* these are NULL if not in either list */
327327
boolcreated;/* set for DATA member if TABLE was created */
328328
intdepCount;/* number of dependencies not yet restored */
329+
DumpId*revDeps;/* dumpIds of objects depending on this one */
330+
intnRevDeps;/* number of such dependencies */
329331
DumpId*lockDeps;/* dumpIds of objects this one needs lock on */
330332
intnLockDeps;/* number of such dependencies */
331333
}TocEntry;

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