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Commitd2086b0

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Reduce path length for locking leaf B-tree pages during insertion
In our B-tree implementation appropriate leaf page for new tupleinsertion is acquired using _bt_search() function. This function alwaysreturns leaf page locked in shared mode. In order to obtain exclusivelock, caller have to relock the page.This commit makes _bt_search() function lock leaf page immediately inexclusive mode when needed. That removes unnecessary relock and, inturn reduces lock contention for B-tree leaf pages. Our experimentson multi-core systems showed acceleration up to 4.5 times in cornercase.Discussion:https://postgr.es/m/CAPpHfduAMDFMNYTCN7VMBsFg_hsf0GqiqXnt%2BbSeaJworwFoig%40mail.gmail.comAuthor: Alexander KorotkovReviewed-by: Yoshikazu Imai, Simon Riggs, Peter Geoghegan
1 parent8a9b72c commitd2086b0

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2 files changed

+41
-22
lines changed

2 files changed

+41
-22
lines changed

‎src/backend/access/nbtree/nbtinsert.c

Lines changed: 4 additions & 15 deletions
Original file line numberDiff line numberDiff line change
@@ -216,23 +216,12 @@ _bt_doinsert(Relation rel, IndexTuple itup,
216216

217217
if (!fastpath)
218218
{
219-
/* find the first page containing this key */
220-
stack=_bt_search(rel,indnkeyatts,itup_scankey, false,&buf,BT_WRITE,
221-
NULL);
222-
223-
/* trade in our read lock for a write lock */
224-
LockBuffer(buf,BUFFER_LOCK_UNLOCK);
225-
LockBuffer(buf,BT_WRITE);
226-
227219
/*
228-
* If the page was split between the time that we surrendered our read
229-
* lock and acquired our write lock, then this page may no longer be
230-
* the right place for the key we want to insert. In this case, we
231-
* need to move right in the tree. See Lehman and Yao for an
232-
* excruciatingly precise description.
220+
* Find the first page containing this key. Buffer returned by
221+
* _bt_search() is locked in exclusive mode.
233222
*/
234-
buf=_bt_moveright(rel,buf,indnkeyatts,itup_scankey, false,
235-
true,stack,BT_WRITE,NULL);
223+
stack=_bt_search(rel,indnkeyatts,itup_scankey, false,&buf,BT_WRITE,
224+
NULL);
236225
}
237226

238227
/*

‎src/backend/access/nbtree/nbtsearch.c

Lines changed: 37 additions & 7 deletions
Original file line numberDiff line numberDiff line change
@@ -87,17 +87,18 @@ _bt_drop_lock_and_maybe_pin(IndexScanDesc scan, BTScanPos sp)
8787
* place during the descent through the tree. This is not needed when
8888
* positioning for an insert or delete, so NULL is used for those cases.
8989
*
90-
*NOTE that thereturned buffer isread-lockedregardless of theaccess
91-
*parameter. However,access = BT_WRITE will allow an empty root page
92-
*to be created and returned.When access = BT_READ, an empty index
93-
*will result in *bufP being set toInvalidBuffer. Also, in BT_WRITE mode,
94-
*any incomplete splits encounteredduring the search will be finished.
90+
*Thereturned buffer is lockedaccording toaccess parameter. Additionally,
91+
* access = BT_WRITE will allow an empty root page to be created and returned.
92+
* When access = BT_READ, an empty index will result in *bufP being set to
93+
* InvalidBuffer. Also, in BT_WRITE mode, any incomplete splits encountered
94+
* during the search will be finished.
9595
*/
9696
BTStack
9797
_bt_search(Relationrel,intkeysz,ScanKeyscankey,boolnextkey,
9898
Buffer*bufP,intaccess,Snapshotsnapshot)
9999
{
100100
BTStackstack_in=NULL;
101+
intpage_access=BT_READ;
101102

102103
/* Get the root page to start with */
103104
*bufP=_bt_getroot(rel,access);
@@ -132,7 +133,7 @@ _bt_search(Relation rel, int keysz, ScanKey scankey, bool nextkey,
132133
*/
133134
*bufP=_bt_moveright(rel,*bufP,keysz,scankey,nextkey,
134135
(access==BT_WRITE),stack_in,
135-
BT_READ,snapshot);
136+
page_access,snapshot);
136137

137138
/* if this is a leaf page, we're done */
138139
page=BufferGetPage(*bufP);
@@ -166,13 +167,42 @@ _bt_search(Relation rel, int keysz, ScanKey scankey, bool nextkey,
166167
new_stack->bts_btentry=blkno;
167168
new_stack->bts_parent=stack_in;
168169

170+
/*
171+
* Page level 1 is lowest non-leaf page level prior to leaves. So,
172+
* if we're on the level 1 and asked to lock leaf page in write mode,
173+
* then lock next page in write mode, because it must be a leaf.
174+
*/
175+
if (opaque->btpo.level==1&&access==BT_WRITE)
176+
page_access=BT_WRITE;
177+
169178
/* drop the read lock on the parent page, acquire one on the child */
170-
*bufP=_bt_relandgetbuf(rel,*bufP,blkno,BT_READ);
179+
*bufP=_bt_relandgetbuf(rel,*bufP,blkno,page_access);
171180

172181
/* okay, all set to move down a level */
173182
stack_in=new_stack;
174183
}
175184

185+
/*
186+
* If we're asked to lock leaf in write mode, but didn't manage to, then
187+
* relock. That may happend when root page appears to be leaf.
188+
*/
189+
if (access==BT_WRITE&&page_access==BT_READ)
190+
{
191+
/* trade in our read lock for a write lock */
192+
LockBuffer(*bufP,BUFFER_LOCK_UNLOCK);
193+
LockBuffer(*bufP,BT_WRITE);
194+
195+
/*
196+
* If the page was split between the time that we surrendered our read
197+
* lock and acquired our write lock, then this page may no longer be
198+
* the right place for the key we want to insert. In this case, we
199+
* need to move right in the tree. See Lehman and Yao for an
200+
* excruciatingly precise description.
201+
*/
202+
*bufP=_bt_moveright(rel,*bufP,keysz,scankey,nextkey,
203+
true,stack_in,BT_WRITE,snapshot);
204+
}
205+
176206
returnstack_in;
177207
}
178208

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