Dept. of second thoughts: fix loop in BgBufferSync so that the exit when

bgwriter_lru_maxpages is exceeded leaves the loop variables in the
expected state.  In the original coding, we'd fail to advance
next_to_clean, causing that buffer to be probably-uselessly rechecked next
time, and also have an off-by-one idea of the number of buffers scanned.

diff --git a/src/backend/storage/buffer/bufmgr.c b/src/backend/storage/buffer/bufmgr.c
index 6ba935a09af243a18f0e67c2760d6ab61044d1b0..ce7c54deeec68b37621f43056a511b350e337e0b 100644 (file)
--- a/src/backend/storage/buffer/bufmgr.c
+++ b/src/backend/storage/buffer/bufmgr.c
@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *   $PostgreSQL: pgsql/src/backend/storage/buffer/bufmgr.c,v 1.225 2007/09/25 20:03:37 tgl Exp $
+ *   $PostgreSQL: pgsql/src/backend/storage/buffer/bufmgr.c,v 1.226 2007/09/25 22:11:48 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -1324,10 +1324,17 @@ BgBufferSync(void)
    reusable_buffers = reusable_buffers_est;
 
    /* Execute the LRU scan */
-   while (num_to_scan-- > 0 && reusable_buffers < upcoming_alloc_est)
+   while (num_to_scan > 0 && reusable_buffers < upcoming_alloc_est)
    {
        int     buffer_state = SyncOneBuffer(next_to_clean, true);
 
+       if (++next_to_clean >= NBuffers)
+       {
+           next_to_clean = 0;
+           next_passes++;
+       }
+       num_to_scan--;
+
        if (buffer_state & BUF_WRITTEN)
        {
            reusable_buffers++;
@@ -1339,12 +1346,6 @@ BgBufferSync(void)
        }
        else if (buffer_state & BUF_REUSABLE)
            reusable_buffers++;
-
-       if (++next_to_clean >= NBuffers)
-       {
-           next_to_clean = 0;
-           next_passes++;
-       }
    }
 
    BgWriterStats.m_buf_written_clean += num_written;
@@ -1353,7 +1354,7 @@ BgBufferSync(void)
    elog(DEBUG1, "bgwriter: recent_alloc=%u smoothed=%.2f delta=%ld ahead=%d density=%.2f reusable_est=%d upcoming_est=%d scanned=%d wrote=%d reusable=%d",
         recent_alloc, smoothed_alloc, strategy_delta, bufs_ahead,
         smoothed_density, reusable_buffers_est, upcoming_alloc_est,
-        bufs_to_lap - num_to_scan - 1,
+        bufs_to_lap - num_to_scan,
         num_written,
         reusable_buffers - reusable_buffers_est);
 #endif
@@ -1366,7 +1367,7 @@ BgBufferSync(void)
     * scanning, which is helpful because a long memory isn't as desirable
     * on the density estimates.
     */
-   strategy_delta = bufs_to_lap - num_to_scan - 1;
+   strategy_delta = bufs_to_lap - num_to_scan;
    recent_alloc = reusable_buffers - reusable_buffers_est;
    if (strategy_delta > 0 && recent_alloc > 0)
    {