From 0151af40cd4e0321bc549cea9f0c631bce0303c5 Mon Sep 17 00:00:00 2001 From: Tom Lane Date: Tue, 2 Nov 2021 11:31:54 -0400 Subject: [PATCH] Avoid O(N^2) behavior in SyncPostCheckpoint(). As in commits 6301c3ada and e9d9ba2a4, avoid doing repetitive list_delete_first() operations, since that would be expensive when there are many files waiting to be unlinked. This is a slightly larger change than in those cases. We have to keep the list state valid for calls to AbsorbSyncRequests(), so it's necessary to invent a "canceled" field instead of immediately deleting PendingUnlinkEntry entries. Also, because we might not be able to process all the entries, we need a new list primitive list_delete_first_n(). list_delete_first_n() is almost list_copy_tail(), but it modifies the input List instead of making a new copy. I found a couple of existing uses of the latter that could profitably use the new function. (There might be more, but the other callers look like they probably shouldn't overwrite the input List.) As before, back-patch to v13. Discussion: https://postgr.es/m/CD2F0E7F-9822-45EC-A411-AE56F14DEA9F@amazon.com --- src/backend/nodes/list.c | 66 ++++++++++++++++++++++++++++ src/backend/optimizer/util/clauses.c | 2 +- src/backend/parser/parse_func.c | 2 +- src/backend/storage/sync/sync.c | 46 ++++++++++++++----- src/include/nodes/pg_list.h | 1 + 5 files changed, 103 insertions(+), 14 deletions(-) diff --git a/src/backend/nodes/list.c b/src/backend/nodes/list.c index 80fa8c84e49..bc2f2fcc025 100644 --- a/src/backend/nodes/list.c +++ b/src/backend/nodes/list.c @@ -891,6 +891,72 @@ list_delete_last(List *list) return list_truncate(list, list_length(list) - 1); } +/* + * Delete the first N cells of the list. + * + * The List is pfree'd if the request causes all cells to be deleted. + */ +List * +list_delete_first_n(List *list, int n) +{ + check_list_invariants(list); + + /* No-op request? */ + if (n <= 0) + return list; + + /* Delete whole list? */ + if (n >= list_length(list)) + { + list_free(list); + return NIL; + } + + /* + * Otherwise, we normally just collapse out the removed elements. But for + * debugging purposes, move the whole list contents someplace else. + * + * (Note that we *must* keep the contents in the same memory context.) + */ +#ifndef DEBUG_LIST_MEMORY_USAGE + memmove(&list->elements[0], &list->elements[n], + (list->length - n) * sizeof(ListCell)); + list->length -= n; +#else + { + ListCell *newelems; + int newmaxlen = list->length - n; + + newelems = (ListCell *) + MemoryContextAlloc(GetMemoryChunkContext(list), + newmaxlen * sizeof(ListCell)); + memcpy(newelems, &list->elements[n], newmaxlen * sizeof(ListCell)); + if (list->elements != list->initial_elements) + pfree(list->elements); + else + { + /* + * As in enlarge_list(), clear the initial_elements[] space and/or + * mark it inaccessible. + */ +#ifdef CLOBBER_FREED_MEMORY + wipe_mem(list->initial_elements, + list->max_length * sizeof(ListCell)); +#else + VALGRIND_MAKE_MEM_NOACCESS(list->initial_elements, + list->max_length * sizeof(ListCell)); +#endif + } + list->elements = newelems; + list->max_length = newmaxlen; + list->length = newmaxlen; + check_list_invariants(list); + } +#endif + + return list; +} + /* * Generate the union of two lists. This is calculated by copying * list1 via list_copy(), then adding to it all the members of list2 diff --git a/src/backend/optimizer/util/clauses.c b/src/backend/optimizer/util/clauses.c index 97346e87fe7..dc908eae436 100644 --- a/src/backend/optimizer/util/clauses.c +++ b/src/backend/optimizer/util/clauses.c @@ -4165,7 +4165,7 @@ add_function_defaults(List *args, HeapTuple func_tuple) if (ndelete < 0) elog(ERROR, "not enough default arguments"); if (ndelete > 0) - defaults = list_copy_tail(defaults, ndelete); + defaults = list_delete_first_n(defaults, ndelete); /* And form the combined argument list, not modifying the input list */ return list_concat_copy(args, defaults); diff --git a/src/backend/parser/parse_func.c b/src/backend/parser/parse_func.c index 9c3b6ad9166..c28fc7d0bdb 100644 --- a/src/backend/parser/parse_func.c +++ b/src/backend/parser/parse_func.c @@ -1679,7 +1679,7 @@ func_get_detail(List *funcname, ndelete = list_length(defaults) - best_candidate->ndargs; if (ndelete > 0) - defaults = list_copy_tail(defaults, ndelete); + defaults = list_delete_first_n(defaults, ndelete); *argdefaults = defaults; } } diff --git a/src/backend/storage/sync/sync.c b/src/backend/storage/sync/sync.c index 3ded2cdd716..4ce2e687d3c 100644 --- a/src/backend/storage/sync/sync.c +++ b/src/backend/storage/sync/sync.c @@ -66,6 +66,7 @@ typedef struct { FileTag tag; /* identifies handler and file */ CycleCtr cycle_ctr; /* checkpoint_cycle_ctr when request was made */ + bool canceled; /* true if request has been canceled */ } PendingUnlinkEntry; static HTAB *pendingOps = NULL; @@ -174,13 +175,18 @@ void SyncPostCheckpoint(void) { int absorb_counter; + ListCell *lc; absorb_counter = UNLINKS_PER_ABSORB; - while (pendingUnlinks != NIL) + foreach(lc, pendingUnlinks) { - PendingUnlinkEntry *entry = (PendingUnlinkEntry *) linitial(pendingUnlinks); + PendingUnlinkEntry *entry = (PendingUnlinkEntry *) lfirst(lc); char path[MAXPGPATH]; + /* Skip over any canceled entries */ + if (entry->canceled) + continue; + /* * New entries are appended to the end, so if the entry is new we've * reached the end of old entries. @@ -210,15 +216,13 @@ SyncPostCheckpoint(void) errmsg("could not remove file \"%s\": %m", path))); } - /* And remove the list entry */ - pendingUnlinks = list_delete_first(pendingUnlinks); - pfree(entry); + /* Mark the list entry as canceled, just in case */ + entry->canceled = true; /* * As in ProcessSyncRequests, we don't want to stop absorbing fsync * requests for a long time when there are many deletions to be done. - * We can safely call AbsorbSyncRequests() at this point in the loop - * (note it might try to delete list entries). + * We can safely call AbsorbSyncRequests() at this point in the loop. */ if (--absorb_counter <= 0) { @@ -226,6 +230,26 @@ SyncPostCheckpoint(void) absorb_counter = UNLINKS_PER_ABSORB; } } + + /* + * If we reached the end of the list, we can just remove the whole list + * (remembering to pfree all the PendingUnlinkEntry objects). Otherwise, + * we must keep the entries at or after "lc". + */ + if (lc == NULL) + { + list_free_deep(pendingUnlinks); + pendingUnlinks = NIL; + } + else + { + int ntodelete = list_cell_number(pendingUnlinks, lc); + + for (int i = 0; i < ntodelete; i++) + pfree(list_nth(pendingUnlinks, i)); + + pendingUnlinks = list_delete_first_n(pendingUnlinks, ntodelete); + } } /* @@ -465,17 +489,14 @@ RememberSyncRequest(const FileTag *ftag, SyncRequestType type) entry->canceled = true; } - /* Remove matching unlink requests */ + /* Cancel matching unlink requests */ foreach(cell, pendingUnlinks) { PendingUnlinkEntry *entry = (PendingUnlinkEntry *) lfirst(cell); if (entry->tag.handler == ftag->handler && syncsw[ftag->handler].sync_filetagmatches(ftag, &entry->tag)) - { - pendingUnlinks = foreach_delete_current(pendingUnlinks, cell); - pfree(entry); - } + entry->canceled = true; } } else if (type == SYNC_UNLINK_REQUEST) @@ -487,6 +508,7 @@ RememberSyncRequest(const FileTag *ftag, SyncRequestType type) entry = palloc(sizeof(PendingUnlinkEntry)); entry->tag = *ftag; entry->cycle_ctr = checkpoint_cycle_ctr; + entry->canceled = false; pendingUnlinks = lappend(pendingUnlinks, entry); diff --git a/src/include/nodes/pg_list.h b/src/include/nodes/pg_list.h index 54447bb5eb6..b30596812b7 100644 --- a/src/include/nodes/pg_list.h +++ b/src/include/nodes/pg_list.h @@ -560,6 +560,7 @@ extern List *list_delete_int(List *list, int datum); extern List *list_delete_oid(List *list, Oid datum); extern List *list_delete_first(List *list); extern List *list_delete_last(List *list); +extern List *list_delete_first_n(List *list, int n); extern List *list_delete_nth_cell(List *list, int n); extern List *list_delete_cell(List *list, ListCell *cell);