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Revert "Avoid the creation of the free space map for small heap relations".

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This feature was using a process local map to track the first few blocks
in the relation.  The map was reset each time we get the block with enough
freespace.  It was discussed that it would be better to track this map on
a per-relation basis in relcache and then invalidate the same whenever
vacuum frees up some space in the page or when FSM is created.  The new
design would be better both in terms of API design and performance.

List of commits reverted, in reverse chronological order:

06c8a5090e  Improve code comments in b0eaa4c51b.
13e8643bfc  During pg_upgrade, conditionally skip transfer of FSMs.
6f918159a9  Add more tests for FSM.
9c32e4c350  Clear the local map when not used.
29d108cdec  Update the documentation for FSM behavior..
08ecdfe7e5  Make FSM test portable.
b0eaa4c51b  Avoid creation of the free space map for small heap relations.

Discussion: https://postgr.es/m/20190416180452.3pm6uegx54iitbt5@alap3.anarazel.de
This commit is contained in:
Amit Kapila
2019-05-07 09:30:24 +05:30
parent af82f95abb
commit 7db0cde6b5
23 changed files with 107 additions and 686 deletions
Download Patch File Download Diff File Expand all files Collapse all files

38
src/backend/storage/freespace/README
View File

@@ -8,41 +8,7 @@ free space to hold a tuple to be stored; or to determine that no such page
exists and the relation must be extended by one page. As of PostgreSQL 8.4
each relation has its own, extensible free space map stored in a separate
"fork" of its relation. This eliminates the disadvantages of the former
fixed-size FSM. There are two exceptions:
1. Hash indexes never have a FSM.
2. For very small tables, a 3-page relation fork would be relatively large
and wasteful, so to save space we refrain from creating the FSM if the
heap has HEAP_FSM_CREATION_THRESHOLD pages or fewer.
To locate free space in the latter case, we simply try pages directly without
knowing ahead of time how much free space they have. To maintain good
performance, we create a local in-memory map of pages to try, and only mark
every other page as available. For example, in a 3-page heap, the local map
would look like:
ANAN
0123
Pages 0 and 2 are marked "available", and page 1 as "not available".
Page 3 is beyond the end of the relation, so is likewise marked "not
available". First we try page 2, and if that doesn't have sufficient free
space we try page 0 before giving up and extending the relation. There may
be some wasted free space on block 1, but if the relation extends to 4 pages:
NANA
0123
We not only have the new page 3 at our disposal, we can now check page 1
for free space as well.
Once the FSM is created for a heap we don't remove it even if somebody deletes
all the rows from the corresponding relation. We don't think it is a useful
optimization as it is quite likely that relation will again grow to the same
size.
FSM data structure
------------------
fixed-size FSM.
It is important to keep the map small so that it can be searched rapidly.
Therefore, we don't attempt to record the exact free space on a page.
@@ -226,3 +192,5 @@ TODO
----
- fastroot to avoid traversing upper nodes with just 1 child
- use a different system for tables that fit into one FSM page, with a
mechanism to switch to the real thing as it grows.

301
src/backend/storage/freespace/freespace.c
View File

@@ -76,14 +76,6 @@
#define FSM_ROOT_LEVEL (FSM_TREE_DEPTH - 1)
#define FSM_BOTTOM_LEVEL 0
/* Status codes for the local map. */
/* Either already tried, or beyond the end of the relation */
#define FSM_LOCAL_NOT_AVAIL 0x00
/* Available to try */
#define FSM_LOCAL_AVAIL 0x01
/*
* The internal FSM routines work on a logical addressing scheme. Each
* level of the tree can be thought of as a separately addressable file.
@@ -97,32 +89,6 @@ typedef struct
/* Address of the root page. */
static const FSMAddress FSM_ROOT_ADDRESS = {FSM_ROOT_LEVEL, 0};
/*
* For small relations, we don't create FSM to save space, instead we use
* local in-memory map of pages to try. To locate free space, we simply try
* pages directly without knowing ahead of time how much free space they have.
*
* Note that this map is used to the find the block with required free space
* for any given relation. We clear this map when we have found a block with
* enough free space, when we extend the relation, or on transaction abort.
* See src/backend/storage/freespace/README for further details.
*/
typedef struct
{
BlockNumber nblocks;
uint8 map[HEAP_FSM_CREATION_THRESHOLD];
} FSMLocalMap;
static FSMLocalMap fsm_local_map =
{
0,
{
FSM_LOCAL_NOT_AVAIL
}
};
#define FSM_LOCAL_MAP_EXISTS (fsm_local_map.nblocks > 0)
/* functions to navigate the tree */
static FSMAddress fsm_get_child(FSMAddress parent, uint16 slot);
static FSMAddress fsm_get_parent(FSMAddress child, uint16 *slot);
@@ -141,14 +107,10 @@ static Size fsm_space_cat_to_avail(uint8 cat);
/* workhorse functions for various operations */
static int fsm_set_and_search(Relation rel, FSMAddress addr, uint16 slot,
uint8 newValue, uint8 minValue);
static void fsm_local_set(Relation rel, BlockNumber cur_nblocks);
static BlockNumber fsm_search(Relation rel, uint8 min_cat);
static BlockNumber fsm_local_search(void);
static uint8 fsm_vacuum_page(Relation rel, FSMAddress addr,
BlockNumber start, BlockNumber end,
bool *eof);
static bool fsm_allow_writes(Relation rel, BlockNumber heapblk,
BlockNumber nblocks, BlockNumber *get_nblocks);
/******** Public API ********/
@@ -165,46 +127,13 @@ static bool fsm_allow_writes(Relation rel, BlockNumber heapblk,
* amount of free space available on that page and then try again (see
* RecordAndGetPageWithFreeSpace). If InvalidBlockNumber is returned,
* extend the relation.
*
* For very small heap relations that don't have a FSM, we try every other
* page before extending the relation. To keep track of which pages have
* been tried, initialize a local in-memory map of pages.
*/
BlockNumber
GetPageWithFreeSpace(Relation rel, Size spaceNeeded, bool check_fsm_only)
GetPageWithFreeSpace(Relation rel, Size spaceNeeded)
{
uint8 min_cat = fsm_space_needed_to_cat(spaceNeeded);
BlockNumber target_block,
nblocks;
/* First try the FSM, if it exists. */
target_block = fsm_search(rel, min_cat);
if (target_block == InvalidBlockNumber &&
(rel->rd_rel->relkind == RELKIND_RELATION ||
rel->rd_rel->relkind == RELKIND_TOASTVALUE) &&
!check_fsm_only)
{
nblocks = RelationGetNumberOfBlocks(rel);
if (nblocks > HEAP_FSM_CREATION_THRESHOLD)
{
/*
* If the FSM knows nothing of the rel, try the last page before
* we give up and extend. This avoids one-tuple-per-page syndrome
* during bootstrapping or in a recently-started system.
*/
target_block = nblocks - 1;
}
else if (nblocks > 0)
{
/* Initialize local map and get first candidate block. */
fsm_local_set(rel, nblocks);
target_block = fsm_local_search();
}
}
return target_block;
return fsm_search(rel, min_cat);
}
/*
@@ -215,47 +144,16 @@ GetPageWithFreeSpace(Relation rel, Size spaceNeeded, bool check_fsm_only)
* also some effort to return a page close to the old page; if there's a
* page with enough free space on the same FSM page where the old one page
* is located, it is preferred.
*
* For very small heap relations that don't have a FSM, we update the local
* map to indicate we have tried a page, and return the next page to try.
*/
BlockNumber
RecordAndGetPageWithFreeSpace(Relation rel, BlockNumber oldPage,
Size oldSpaceAvail, Size spaceNeeded)
{
int old_cat;
int search_cat;
int old_cat = fsm_space_avail_to_cat(oldSpaceAvail);
int search_cat = fsm_space_needed_to_cat(spaceNeeded);
FSMAddress addr;
uint16 slot;
int search_slot;
BlockNumber nblocks = InvalidBlockNumber;
/* First try the local map, if it exists. */
if (FSM_LOCAL_MAP_EXISTS)
{
Assert((rel->rd_rel->relkind == RELKIND_RELATION ||
rel->rd_rel->relkind == RELKIND_TOASTVALUE) &&
fsm_local_map.map[oldPage] == FSM_LOCAL_AVAIL);
fsm_local_map.map[oldPage] = FSM_LOCAL_NOT_AVAIL;
return fsm_local_search();
}
if (!fsm_allow_writes(rel, oldPage, InvalidBlockNumber, &nblocks))
{
/*
* If we have neither a local map nor a FSM, we probably just tried
* the target block in the smgr relation entry and failed, so we'll
* need to create the local map.
*/
fsm_local_set(rel, nblocks);
return fsm_local_search();
}
/* Normal FSM logic follows */
old_cat = fsm_space_avail_to_cat(oldSpaceAvail);
search_cat = fsm_space_needed_to_cat(spaceNeeded);
/* Get the location of the FSM byte representing the heap block */
addr = fsm_get_location(oldPage, &slot);
@@ -278,44 +176,20 @@ RecordAndGetPageWithFreeSpace(Relation rel, BlockNumber oldPage,
* Note that if the new spaceAvail value is higher than the old value stored
* in the FSM, the space might not become visible to searchers until the next
* FreeSpaceMapVacuum call, which updates the upper level pages.
*
* Callers have no need for a local map.
*/
void
RecordPageWithFreeSpace(Relation rel, BlockNumber heapBlk,
Size spaceAvail, BlockNumber nblocks)
RecordPageWithFreeSpace(Relation rel, BlockNumber heapBlk, Size spaceAvail)
{
int new_cat;
int new_cat = fsm_space_avail_to_cat(spaceAvail);
FSMAddress addr;
uint16 slot;
BlockNumber dummy;
if (!fsm_allow_writes(rel, heapBlk, nblocks, &dummy))
/* No FSM to update and no local map either */
return;
/* Get the location of the FSM byte representing the heap block */
addr = fsm_get_location(heapBlk, &slot);
new_cat = fsm_space_avail_to_cat(spaceAvail);
fsm_set_and_search(rel, addr, slot, new_cat, 0);
}
/*
* Clear the local map. We must call this when we have found a block with
* enough free space, when we extend the relation, or on transaction abort.
*/
void
FSMClearLocalMap(void)
{
if (FSM_LOCAL_MAP_EXISTS)
{
fsm_local_map.nblocks = 0;
memset(&fsm_local_map.map, FSM_LOCAL_NOT_AVAIL,
sizeof(fsm_local_map.map));
}
}
/*
* XLogRecordPageWithFreeSpace - like RecordPageWithFreeSpace, for use in
* WAL replay
@@ -330,31 +204,6 @@ XLogRecordPageWithFreeSpace(RelFileNode rnode, BlockNumber heapBlk,
BlockNumber blkno;
Buffer buf;
Page page;
bool write_to_fsm;
/* This is meant to mirror the logic in fsm_allow_writes() */
if (heapBlk >= HEAP_FSM_CREATION_THRESHOLD)
write_to_fsm = true;
else
{
/* Open the relation at smgr level */
SMgrRelation smgr = smgropen(rnode, InvalidBackendId);
if (smgrexists(smgr, FSM_FORKNUM))
write_to_fsm = true;
else
{
BlockNumber heap_nblocks = smgrnblocks(smgr, MAIN_FORKNUM);
if (heap_nblocks > HEAP_FSM_CREATION_THRESHOLD)
write_to_fsm = true;
else
write_to_fsm = false;
}
}
if (!write_to_fsm)
return;
/* Get the location of the FSM byte representing the heap block */
addr = fsm_get_location(heapBlk, &slot);
@@ -1055,141 +904,3 @@ fsm_vacuum_page(Relation rel, FSMAddress addr,
return max_avail;
}
/*
* For heaps, we prevent creation of the FSM unless the number of pages
* exceeds HEAP_FSM_CREATION_THRESHOLD. For tables that don't already have
* a FSM, this will save an inode and a few kB of space.
*
* XXX The API is a little awkward -- if the caller passes a valid nblocks
* value, it can avoid invoking a system call. If the caller passes
* InvalidBlockNumber and receives a false return value, it can get an
* up-to-date relation size from get_nblocks. This saves a few cycles in
* the caller, which would otherwise need to get the relation size by itself.
*/
static bool
fsm_allow_writes(Relation rel, BlockNumber heapblk,
BlockNumber nblocks, BlockNumber *get_nblocks)
{
bool skip_get_nblocks;
if (heapblk >= HEAP_FSM_CREATION_THRESHOLD)
return true;
/* Non-heap rels can always create a FSM. */
if (rel->rd_rel->relkind != RELKIND_RELATION &&
rel->rd_rel->relkind != RELKIND_TOASTVALUE)
return true;
/*
* If the caller knows nblocks, we can avoid a system call later. If it
* doesn't, maybe we have relpages from a previous VACUUM. Since the table
* may have extended since then, we still have to count the pages later if
* we can't return now.
*/
if (nblocks != InvalidBlockNumber)
{
if (nblocks > HEAP_FSM_CREATION_THRESHOLD)
return true;
else
skip_get_nblocks = true;
}
else
{
if (rel->rd_rel->relpages != InvalidBlockNumber &&
rel->rd_rel->relpages > HEAP_FSM_CREATION_THRESHOLD)
return true;
else
skip_get_nblocks = false;
}
RelationOpenSmgr(rel);
if (smgrexists(rel->rd_smgr, FSM_FORKNUM))
return true;
if (skip_get_nblocks)
return false;
/* last resort */
*get_nblocks = RelationGetNumberOfBlocks(rel);
if (*get_nblocks > HEAP_FSM_CREATION_THRESHOLD)
return true;
else
return false;
}
/*
* Initialize the local map of blocks to try, for when there is no FSM.
*
* When we initialize the map, the whole heap is potentially available to
* try. Testing revealed that trying every block can cause a small
* performance dip compared to when we use a FSM, so we try every other
* block instead.
*/
static void
fsm_local_set(Relation rel, BlockNumber cur_nblocks)
{
BlockNumber blkno,
cached_target_block;
/* The local map must not be set already. */
Assert(!FSM_LOCAL_MAP_EXISTS);
/*
* Starting at the current last block in the relation and working
* backwards, mark alternating blocks as available.
*/
blkno = cur_nblocks - 1;
while (true)
{
fsm_local_map.map[blkno] = FSM_LOCAL_AVAIL;
if (blkno >= 2)
blkno -= 2;
else
break;
}
/* Cache the number of blocks. */
fsm_local_map.nblocks = cur_nblocks;
/* Set the status of the cached target block to 'unavailable'. */
cached_target_block = RelationGetTargetBlock(rel);
if (cached_target_block != InvalidBlockNumber &&
cached_target_block < cur_nblocks)
fsm_local_map.map[cached_target_block] = FSM_LOCAL_NOT_AVAIL;
}
/*
* Search the local map for an available block to try, in descending order.
* As such, there is no heuristic available to decide which order will be
* better to try, but the probability of having space in the last block in the
* map is higher because that is the most recent block added to the heap.
*
* This function is used when there is no FSM.
*/
static BlockNumber
fsm_local_search(void)
{
BlockNumber target_block;
/* Local map must be set by now. */
Assert(FSM_LOCAL_MAP_EXISTS);
target_block = fsm_local_map.nblocks;
do
{
target_block--;
if (fsm_local_map.map[target_block] == FSM_LOCAL_AVAIL)
return target_block;
} while (target_block > 0);
/*
* If we didn't find any available block to try in the local map, then
* clear it. This prevents us from using the map again without setting it
* first, which would otherwise lead to the same conclusion again and
* again.
*/
FSMClearLocalMap();
return InvalidBlockNumber;
}

6
src/backend/storage/freespace/indexfsm.c
View File

@@ -37,7 +37,7 @@
BlockNumber
GetFreeIndexPage(Relation rel)
{
BlockNumber blkno = GetPageWithFreeSpace(rel, BLCKSZ / 2, true);
BlockNumber blkno = GetPageWithFreeSpace(rel, BLCKSZ / 2);
if (blkno != InvalidBlockNumber)
RecordUsedIndexPage(rel, blkno);
@@ -51,7 +51,7 @@ GetFreeIndexPage(Relation rel)
void
RecordFreeIndexPage(Relation rel, BlockNumber freeBlock)
{
RecordPageWithFreeSpace(rel, freeBlock, BLCKSZ - 1, InvalidBlockNumber);
RecordPageWithFreeSpace(rel, freeBlock, BLCKSZ - 1);
}
@@ -61,7 +61,7 @@ RecordFreeIndexPage(Relation rel, BlockNumber freeBlock)
void
RecordUsedIndexPage(Relation rel, BlockNumber usedBlock)
{
RecordPageWithFreeSpace(rel, usedBlock, 0, InvalidBlockNumber);
RecordPageWithFreeSpace(rel, usedBlock, 0);
}
/*