Avoid XLogFlush for clean buffers in BufferSync.

diff --git a/src/backend/storage/buffer/bufmgr.c b/src/backend/storage/buffer/bufmgr.c
index afc8c8cc053d55cc5d446823c9ce3cd0c2d92197..9395f068d6493284269490e19066aa036fbb100a 100644 (file)
--- a/src/backend/storage/buffer/bufmgr.c
+++ b/src/backend/storage/buffer/bufmgr.c
@@ -723,6 +723,7 @@ BufferSync()
        RelFileNode     rnode;
        XLogRecPtr      recptr;
        Relation        reln = NULL;
+       bool            dirty = false;
 
        for (i = 0, bufHdr = BufferDescriptors; i < NBuffers; i++, bufHdr++)
        {
@@ -745,6 +746,7 @@ BufferSync()
 
                buffer = BufferDescriptorGetBuffer(bufHdr);
                rnode = bufHdr->tag.rnode;
+               dirty = bufHdr->flags & BM_DIRTY;
 
                SpinRelease(BufMgrLock);
 
@@ -758,6 +760,17 @@ BufferSync()
                 */
                LockBuffer(buffer, BUFFER_LOCK_SHARE);
 
+               if (!dirty && !(bufHdr->cntxDirty))
+               {
+                       LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
+                       SpinAcquire(BufMgrLock);
+                       UnpinBuffer(bufHdr);
+                       SpinRelease(BufMgrLock);
+                       if (reln != (Relation) NULL)
+                               RelationDecrementReferenceCount(reln);
+                       continue;
+               }
+
                /*
                 * Force XLOG flush for buffer' LSN
                 */
@@ -766,9 +779,8 @@ BufferSync()
 
                /*
                 * Now it's safe to write buffer to disk
-                * (if needed at all -:))
+                * (if no one else already)
                 */
-
                SpinAcquire(BufMgrLock);
                if (bufHdr->flags & BM_IO_IN_PROGRESS)
                        WaitIO(bufHdr, BufMgrLock);
@@ -824,20 +836,19 @@ BufferSync()
                         */
                        if (!(bufHdr->flags & BM_JUST_DIRTIED))
                                bufHdr->flags &= ~BM_DIRTY;
+                       UnpinBuffer(bufHdr);
+                       SpinRelease(BufMgrLock);
                }
                else
+               {
+                       UnpinBuffer(bufHdr);
+                       SpinRelease(BufMgrLock);
                        LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
-
-               UnpinBuffer(bufHdr);
-
-               SpinRelease(BufMgrLock);
+               }
 
                /* drop refcnt obtained by RelationNodeCacheGetRelation */
                if (reln != (Relation) NULL)
-               {
                        RelationDecrementReferenceCount(reln);
-                       reln = NULL;
-               }
        }
 
 }