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Replace CAS loop with single TAS in ProcArrayGroupClearXid() 

Single pg_atomic_exchange_u32() is expected to be faster than loop of
pg_atomic_compare_exchange_u32().  Also, it would be consistent with
clog group update code.

Discussion: https://postgr.es/m/CAPpHfdtxLsC-bqfxFcHswZ91OxXcZVNDBBVfg9tAWU0jvn1tQA%40mail.gmail.com
Reviewed-by: Amit Kapila
This commit is contained in:
Alexander Korotkov 2018-09-22 16:22:30 +03:00
parent db361db2fc
commit 2f39106a20
1 changed files with 2 additions and 8 deletions
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10
src/backend/storage/ipc/procarray.c
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@ -542,14 +542,8 @@ ProcArrayGroupClearXid(PGPROC *proc, TransactionId latestXid)
* group XID clearing, saving a pointer to the head of the list. Trying
* to pop elements one at a time could lead to an ABA problem.
*/
while (true)
{
nextidx = pg_atomic_read_u32(&procglobal->procArrayGroupFirst);
if (pg_atomic_compare_exchange_u32(&procglobal->procArrayGroupFirst,
&nextidx,
INVALID_PGPROCNO))
break;
}
nextidx = pg_atomic_exchange_u32(&procglobal->procArrayGroupFirst,
INVALID_PGPROCNO);
/* Remember head of list so we can perform wakeups after dropping lock. */
wakeidx = nextidx;