From a33c255419eb539901b8d07852e1c4b189b615df Mon Sep 17 00:00:00 2001 From: Melanie Plageman Date: Sat, 30 Dec 2023 16:59:27 -0500 Subject: [PATCH v5a 3/7] Confine vacuum skip logic to lazy_scan_skip In preparation for vacuum to use the streaming read interface (and eventually AIO), refactor vacuum's logic for skipping blocks such that it is entirely confined to lazy_scan_skip(). This turns lazy_scan_skip() and the skip state in LVRelState it uses into an iterator which yields blocks to lazy_scan_heap(). Such a structure is conducive to an async interface. While we are at it, rename lazy_scan_skip() to heap_vac_scan_get_next_block(), which now more accurately describes it. By always calling heap_vac_scan_get_next_block() -- instead of only when we have reached the next unskippable block, we no longer need the skipping_current_range variable. lazy_scan_heap() no longer needs to manage the skipped range -- checking if we reached the end in order to then call heap_vac_scan_get_next_block(). And heap_vac_scan_get_next_block() can derive the visibility status of a block from whether or not we are in a skippable range -- that is, whether or not the next_block is equal to the next unskippable block. --- src/backend/access/heap/vacuumlazy.c | 243 ++++++++++++++------------- 1 file changed, 126 insertions(+), 117 deletions(-) diff --git a/src/backend/access/heap/vacuumlazy.c b/src/backend/access/heap/vacuumlazy.c index 0ddb986bc03..99d160335e1 100644 --- a/src/backend/access/heap/vacuumlazy.c +++ b/src/backend/access/heap/vacuumlazy.c @@ -206,8 +206,8 @@ typedef struct LVRelState int64 missed_dead_tuples; /* # removable, but not removed */ /* - * Parameters maintained by lazy_scan_skip() to manage skipping ranges of - * pages greater than SKIP_PAGES_THRESHOLD. + * Parameters maintained by heap_vac_scan_get_next_block() to manage + * skipping ranges of pages greater than SKIP_PAGES_THRESHOLD. */ struct { @@ -232,7 +232,9 @@ typedef struct LVSavedErrInfo /* non-export function prototypes */ static void lazy_scan_heap(LVRelState *vacrel); -static void lazy_scan_skip(LVRelState *vacrel, BlockNumber next_block); +static bool heap_vac_scan_get_next_block(LVRelState *vacrel, BlockNumber next_block, + BlockNumber *blkno, + bool *all_visible_according_to_vm); static bool lazy_scan_new_or_empty(LVRelState *vacrel, Buffer buf, BlockNumber blkno, Page page, bool sharelock); @@ -814,8 +816,11 @@ static void lazy_scan_heap(LVRelState *vacrel) { BlockNumber rel_pages = vacrel->rel_pages, - blkno, next_fsm_block_to_vacuum = 0; + bool all_visible_according_to_vm; + + /* relies on InvalidBlockNumber overflowing to 0 */ + BlockNumber blkno = InvalidBlockNumber; VacDeadItems *dead_items = vacrel->dead_items; const int initprog_index[] = { PROGRESS_VACUUM_PHASE, @@ -830,40 +835,17 @@ lazy_scan_heap(LVRelState *vacrel) initprog_val[2] = dead_items->max_items; pgstat_progress_update_multi_param(3, initprog_index, initprog_val); + vacrel->skip.next_unskippable_block = InvalidBlockNumber; vacrel->skip.vmbuffer = InvalidBuffer; - /* Set up an initial range of skippable blocks using the visibility map */ - lazy_scan_skip(vacrel, 0); - for (blkno = 0; blkno < rel_pages; blkno++) + + while (heap_vac_scan_get_next_block(vacrel, blkno + 1, + &blkno, &all_visible_according_to_vm)) { Buffer buf; Page page; - bool all_visible_according_to_vm; bool has_lpdead_items; bool got_cleanup_lock = false; - if (blkno == vacrel->skip.next_unskippable_block) - { - /* - * Can't skip this page safely. Must scan the page. But - * determine the next skippable range after the page first. - */ - all_visible_according_to_vm = vacrel->skip.next_unskippable_allvis; - lazy_scan_skip(vacrel, blkno + 1); - - Assert(vacrel->skip.next_unskippable_block >= blkno + 1); - } - else - { - /* Last page always scanned (may need to set nonempty_pages) */ - Assert(blkno < rel_pages - 1); - - if (vacrel->skip.skipping_current_range) - continue; - - /* Current range is too small to skip -- just scan the page */ - all_visible_according_to_vm = true; - } - vacrel->scanned_pages++; /* Report as block scanned, update error traceback information */ @@ -1083,20 +1065,14 @@ lazy_scan_heap(LVRelState *vacrel) } /* - * lazy_scan_skip() -- set up range of skippable blocks using visibility map. - * - * lazy_scan_heap() calls here every time it needs to set up a new range of - * blocks to skip via the visibility map. Caller passes next_block, the next - * block in line. The parameters of the skipped range are recorded in skip. - * vacrel is an in/out parameter here; vacuum options and information about the - * relation are read and vacrel->skippedallvis is set to ensure we don't - * advance relfrozenxid when we have skipped vacuuming all visible blocks. + * heap_vac_scan_get_next_block() -- get next block for vacuum to process * - * skip->vmbuffer will contain the block from the VM containing visibility - * information for the next unskippable heap block. We may end up needed a - * different block from the VM (if we decide not to skip a skippable block). - * This is okay; visibilitymap_pin() will take care of this while processing - * the block. + * lazy_scan_heap() calls here every time it needs to get the next block to + * prune and vacuum, using the visibility map, vacuum options, and various + * thresholds to skip blocks which do not need to be processed. Caller passes + * next_block, the next block in line. This block may end up being skipped. + * heap_vac_scan_get_next_block() sets blkno to next block that actually needs + * to be processed. * * A block is unskippable if it is not all visible according to the visibility * map. It is also unskippable if it is the last block in the relation, if the @@ -1106,14 +1082,25 @@ lazy_scan_heap(LVRelState *vacrel) * Even if a block is skippable, we may choose not to skip it if the range of * skippable blocks is too small (below SKIP_PAGES_THRESHOLD). As a * consequence, we must keep track of the next truly unskippable block and its - * visibility status along with whether or not we are skipping the current - * range of skippable blocks. This can be used to derive the next block - * lazy_scan_heap() must process and its visibility status. + * visibility status separate from the next block lazy_scan_heap() should + * process (and its visibility status). * * The block number and visibility status of the next unskippable block are set - * in skip->next_unskippable_block and next_unskippable_allvis. - * skip->skipping_current_range indicates to the caller whether or not it is - * processing a skippable (and thus all-visible) block. + * in vacrel->skip->next_unskippable_block and next_unskippable_allvis. + * + * The block number and visibility status of the next block to process are set + * in blkno and all_visible_according_to_vm. heap_vac_scan_get_next_block() + * returns false if there are no further blocks to process. + * + * vacrel is an in/out parameter here; vacuum options and information about the + * relation are read and vacrel->skippedallvis is set to ensure we don't + * advance relfrozenxid when we have skipped vacuuming all visible blocks. + * + * skip->vmbuffer will contain the block from the VM containing visibility + * information for the next unskippable heap block. We may end up needed a + * different block from the VM (if we decide not to skip a skippable block). + * This is okay; visibilitymap_pin() will take care of this while processing + * the block. * * Note: our opinion of which blocks can be skipped can go stale immediately. * It's okay if caller "misses" a page whose all-visible or all-frozen marking @@ -1123,91 +1110,113 @@ lazy_scan_heap(LVRelState *vacrel) * older XIDs/MXIDs. The vacrel->skippedallvis flag will be set here when the * choice to skip such a range is actually made, making everything safe.) */ -static void -lazy_scan_skip(LVRelState *vacrel, BlockNumber next_block) +static bool +heap_vac_scan_get_next_block(LVRelState *vacrel, BlockNumber next_block, + BlockNumber *blkno, bool *all_visible_according_to_vm) { - /* Use local variables for better optimized loop code */ - BlockNumber rel_pages = vacrel->rel_pages, - next_unskippable_block = next_block; - bool skipsallvis = false; - vacrel->skip.next_unskippable_allvis = true; - while (next_unskippable_block < rel_pages) + if (next_block >= vacrel->rel_pages) { - uint8 mapbits = visibilitymap_get_status(vacrel->rel, - next_unskippable_block, - &vacrel->skip.vmbuffer); + *blkno = InvalidBlockNumber; + return false; + } - if ((mapbits & VISIBILITYMAP_ALL_VISIBLE) == 0) + if (vacrel->skip.next_unskippable_block == InvalidBlockNumber || + next_block > vacrel->skip.next_unskippable_block) + { + /* Use local variables for better optimized loop code */ + BlockNumber rel_pages = vacrel->rel_pages; + BlockNumber next_unskippable_block = vacrel->skip.next_unskippable_block; + + while (++next_unskippable_block < rel_pages) { - Assert((mapbits & VISIBILITYMAP_ALL_FROZEN) == 0); - vacrel->skip.next_unskippable_allvis = false; - break; - } + uint8 mapbits = visibilitymap_get_status(vacrel->rel, + next_unskippable_block, + &vacrel->skip.vmbuffer); - /* - * Caller must scan the last page to determine whether it has tuples - * (caller must have the opportunity to set vacrel->nonempty_pages). - * This rule avoids having lazy_truncate_heap() take access-exclusive - * lock on rel to attempt a truncation that fails anyway, just because - * there are tuples on the last page (it is likely that there will be - * tuples on other nearby pages as well, but those can be skipped). - * - * Implement this by always treating the last block as unsafe to skip. - */ - if (next_unskippable_block == rel_pages - 1) - break; + vacrel->skip.next_unskippable_allvis = mapbits & VISIBILITYMAP_ALL_VISIBLE; - /* DISABLE_PAGE_SKIPPING makes all skipping unsafe */ - if (!vacrel->skipwithvm) - { - /* Caller shouldn't rely on all_visible_according_to_vm */ - vacrel->skip.next_unskippable_allvis = false; - break; - } + if (!vacrel->skip.next_unskippable_allvis) + { + Assert((mapbits & VISIBILITYMAP_ALL_FROZEN) == 0); + break; + } - /* - * Aggressive VACUUM caller can't skip pages just because they are - * all-visible. They may still skip all-frozen pages, which can't - * contain XIDs < OldestXmin (XIDs that aren't already frozen by now). - */ - if ((mapbits & VISIBILITYMAP_ALL_FROZEN) == 0) - { - if (vacrel->aggressive) + /* + * Caller must scan the last page to determine whether it has + * tuples (caller must have the opportunity to set + * vacrel->nonempty_pages). This rule avoids having + * lazy_truncate_heap() take access-exclusive lock on rel to + * attempt a truncation that fails anyway, just because there are + * tuples on the last page (it is likely that there will be tuples + * on other nearby pages as well, but those can be skipped). + * + * Implement this by always treating the last block as unsafe to + * skip. + */ + if (next_unskippable_block == rel_pages - 1) break; + /* DISABLE_PAGE_SKIPPING makes all skipping unsafe */ + if (!vacrel->skipwithvm) + { + /* Caller shouldn't rely on all_visible_according_to_vm */ + vacrel->skip.next_unskippable_allvis = false; + break; + } + /* - * All-visible block is safe to skip in non-aggressive case. But - * remember that the final range contains such a block for later. + * Aggressive VACUUM caller can't skip pages just because they are + * all-visible. They may still skip all-frozen pages, which can't + * contain XIDs < OldestXmin (XIDs that aren't already frozen by + * now). */ - skipsallvis = true; + if ((mapbits & VISIBILITYMAP_ALL_FROZEN) == 0) + { + if (vacrel->aggressive) + break; + + /* + * All-visible block is safe to skip in non-aggressive case. + * But remember that the final range contains such a block for + * later. + */ + skipsallvis = true; + } + + vacuum_delay_point(); } - vacuum_delay_point(); - next_unskippable_block++; - } + vacrel->skip.next_unskippable_block = next_unskippable_block; - vacrel->skip.next_unskippable_block = next_unskippable_block; + /* + * We only skip a range with at least SKIP_PAGES_THRESHOLD consecutive + * pages. Since we're reading sequentially, the OS should be doing + * readahead for us, so there's no gain in skipping a page now and + * then. Skipping such a range might even discourage sequential + * detection. + * + * This test also enables more frequent relfrozenxid advancement + * during non-aggressive VACUUMs. If the range has any all-visible + * pages then skipping makes updating relfrozenxid unsafe, which is a + * real downside. + */ + if (vacrel->skip.next_unskippable_block - next_block >= SKIP_PAGES_THRESHOLD) + { + next_block = vacrel->skip.next_unskippable_block; + if (skipsallvis) + vacrel->skippedallvis = true; + } + } - /* - * We only skip a range with at least SKIP_PAGES_THRESHOLD consecutive - * pages. Since we're reading sequentially, the OS should be doing - * readahead for us, so there's no gain in skipping a page now and then. - * Skipping such a range might even discourage sequential detection. - * - * This test also enables more frequent relfrozenxid advancement during - * non-aggressive VACUUMs. If the range has any all-visible pages then - * skipping makes updating relfrozenxid unsafe, which is a real downside. - */ - if (vacrel->skip.next_unskippable_block - next_block < SKIP_PAGES_THRESHOLD) - vacrel->skip.skipping_current_range = false; + if (next_block == vacrel->skip.next_unskippable_block) + *all_visible_according_to_vm = vacrel->skip.next_unskippable_allvis; else - { - vacrel->skip.skipping_current_range = true; - if (skipsallvis) - vacrel->skippedallvis = true; - } + *all_visible_according_to_vm = true; + + *blkno = next_block; + return true; } /* -- 2.40.1