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Reduce path length for locking leaf B-tree pages during insertion

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In our B-tree implementation appropriate leaf page for new tuple
insertion is acquired using _bt_search() function.  This function always
returns leaf page locked in shared mode.  In order to obtain exclusive
lock, caller have to relock the page.

This commit makes _bt_search() function lock leaf page immediately in
exclusive mode when needed.  That removes unnecessary relock and, in
turn reduces lock contention for B-tree leaf pages.  Our experiments
on multi-core systems showed acceleration up to 4.5 times in corner
case.

Discussion: https://postgr.es/m/CAPpHfduAMDFMNYTCN7VMBsFg_hsf0GqiqXnt%2BbSeaJworwFoig%40mail.gmail.com
Author: Alexander Korotkov
Reviewed-by: Yoshikazu Imai, Simon Riggs, Peter Geoghegan
This commit is contained in:
Alexander Korotkov 2018-07-28 00:31:40 +03:00
parent 8a9b72c3ea
commit d2086b08b0
2 changed files with 41 additions and 22 deletions
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19
src/backend/access/nbtree/nbtinsert.c
View File

@ -216,23 +216,12 @@ top:
if (!fastpath)
{
/* find the first page containing this key */
/*
* Find the first page containing this key. Buffer returned by
* _bt_search() is locked in exclusive mode.
*/
stack = _bt_search(rel, indnkeyatts, itup_scankey, false, &buf, BT_WRITE,
NULL);
/* trade in our read lock for a write lock */
LockBuffer(buf, BUFFER_LOCK_UNLOCK);
LockBuffer(buf, BT_WRITE);
/*
* If the page was split between the time that we surrendered our read
* lock and acquired our write lock, then this page may no longer be
* the right place for the key we want to insert. In this case, we
* need to move right in the tree. See Lehman and Yao for an
* excruciatingly precise description.
*/
buf = _bt_moveright(rel, buf, indnkeyatts, itup_scankey, false,
true, stack, BT_WRITE, NULL);
}
/*

44
src/backend/access/nbtree/nbtsearch.c
View File

@ -87,17 +87,18 @@ _bt_drop_lock_and_maybe_pin(IndexScanDesc scan, BTScanPos sp)
* place during the descent through the tree. This is not needed when
* positioning for an insert or delete, so NULL is used for those cases.
*
* NOTE that the returned buffer is read-locked regardless of the access
* parameter. However, access = BT_WRITE will allow an empty root page
* to be created and returned. When access = BT_READ, an empty index
* will result in *bufP being set to InvalidBuffer. Also, in BT_WRITE mode,
* any incomplete splits encountered during the search will be finished.
* The returned buffer is locked according to access parameter. Additionally,
* access = BT_WRITE will allow an empty root page to be created and returned.
* When access = BT_READ, an empty index will result in *bufP being set to
* InvalidBuffer. Also, in BT_WRITE mode, any incomplete splits encountered
* during the search will be finished.
*/
BTStack
_bt_search(Relation rel, int keysz, ScanKey scankey, bool nextkey,
Buffer *bufP, int access, Snapshot snapshot)
{
BTStack stack_in = NULL;
int page_access = BT_READ;
/* Get the root page to start with */
*bufP = _bt_getroot(rel, access);
@ -132,7 +133,7 @@ _bt_search(Relation rel, int keysz, ScanKey scankey, bool nextkey,
*/
*bufP = _bt_moveright(rel, *bufP, keysz, scankey, nextkey,
(access == BT_WRITE), stack_in,
BT_READ, snapshot);
page_access, snapshot);
/* if this is a leaf page, we're done */
page = BufferGetPage(*bufP);
@ -166,13 +167,42 @@ _bt_search(Relation rel, int keysz, ScanKey scankey, bool nextkey,
new_stack->bts_btentry = blkno;
new_stack->bts_parent = stack_in;
/*
* Page level 1 is lowest non-leaf page level prior to leaves. So,
* if we're on the level 1 and asked to lock leaf page in write mode,
* then lock next page in write mode, because it must be a leaf.
*/
if (opaque->btpo.level == 1 && access == BT_WRITE)
page_access = BT_WRITE;
/* drop the read lock on the parent page, acquire one on the child */
*bufP = _bt_relandgetbuf(rel, *bufP, blkno, BT_READ);
*bufP = _bt_relandgetbuf(rel, *bufP, blkno, page_access);
/* okay, all set to move down a level */
stack_in = new_stack;
}
/*
* If we're asked to lock leaf in write mode, but didn't manage to, then
* relock. That may happend when root page appears to be leaf.
*/
if (access == BT_WRITE && page_access == BT_READ)
{
/* trade in our read lock for a write lock */
LockBuffer(*bufP, BUFFER_LOCK_UNLOCK);
LockBuffer(*bufP, BT_WRITE);
/*
* If the page was split between the time that we surrendered our read
* lock and acquired our write lock, then this page may no longer be
* the right place for the key we want to insert. In this case, we
* need to move right in the tree. See Lehman and Yao for an
* excruciatingly precise description.
*/
*bufP = _bt_moveright(rel, *bufP, keysz, scankey, nextkey,
true, stack_in, BT_WRITE, snapshot);
}
return stack_in;
}