Modify MoveOfflineLogs/InstallXLogFileSegment to avoid O(N^2) behavior

when recycling a large number of xlog segments during checkpoint.
The former behavior searched from the same start point each time,
requiring O(checkpoint_segments^2) stat() calls to relocate all the
segments.  Instead keep track of where we stopped last time through.

diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index 078e1956964beb3665551908d5b525b0f9ff91ae..37f6b59acc281ead0acbbb41b5e3bf8670590310 100644 (file)
--- a/src/backend/access/transam/xlog.c
+++ b/src/backend/access/transam/xlog.c
@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1996-2005, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/backend/access/transam/xlog.c,v 1.184 2005/04/13 18:54:56 tgl Exp $
+ * $PostgreSQL: pgsql/src/backend/access/transam/xlog.c,v 1.185 2005/04/15 18:48:10 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -444,8 +444,8 @@ static bool AdvanceXLInsertBuffer(void);
 static void XLogWrite(XLogwrtRqst WriteRqst);
 static int XLogFileInit(uint32 log, uint32 seg,
             bool *use_existent, bool use_lock);
-static bool InstallXLogFileSegment(uint32 log, uint32 seg, char *tmppath,
-                      bool find_free, int max_advance,
+static bool InstallXLogFileSegment(uint32 *log, uint32 *seg, char *tmppath,
+                      bool find_free, int *max_advance,
                       bool use_lock);
 static int XLogFileOpen(uint32 log, uint32 seg);
 static int XLogFileRead(uint32 log, uint32 seg, int emode);
@@ -1509,6 +1509,9 @@ XLogFileInit(uint32 log, uint32 seg,
    char        path[MAXPGPATH];
    char        tmppath[MAXPGPATH];
    char        zbuffer[BLCKSZ];
+   uint32      installed_log;
+   uint32      installed_seg;
+   int         max_advance;
    int         fd;
    int         nbytes;
 
@@ -1601,8 +1604,11 @@ XLogFileInit(uint32 log, uint32 seg,
     * else has created the file while we were filling ours: if so, use
     * ours to pre-create a future log segment.
     */
-   if (!InstallXLogFileSegment(log, seg, tmppath,
-                               *use_existent, XLOGfileslop,
+   installed_log = log;
+   installed_seg = seg;
+   max_advance = XLOGfileslop;
+   if (!InstallXLogFileSegment(&installed_log, &installed_seg, tmppath,
+                               *use_existent, &max_advance,
                                use_lock))
    {
        /* No need for any more future segments... */
@@ -1722,7 +1728,7 @@ XLogFileCopy(uint32 log, uint32 seg,
    /*
     * Now move the segment into place with its final name.
     */
-   if (!InstallXLogFileSegment(log, seg, tmppath, false, 0, false))
+   if (!InstallXLogFileSegment(&log, &seg, tmppath, false, NULL, false))
        elog(PANIC, "InstallXLogFileSegment should not have failed");
 }
 
@@ -1732,7 +1738,9 @@ XLogFileCopy(uint32 log, uint32 seg,
  * This is used both to install a newly-created segment (which has a temp
  * filename while it's being created) and to recycle an old segment.
  *
- * log, seg: identify segment to install as (or first possible target).
+ * *log, *seg: identify segment to install as (or first possible target).
+ * When find_free is TRUE, these are modified on return to indicate the
+ * actual installation location or last segment searched.
  *
  * tmppath: initial name of file to install.  It will be renamed into place.
  *
@@ -1740,9 +1748,10 @@ XLogFileCopy(uint32 log, uint32 seg,
  * number at or after the passed numbers.  If FALSE, install the new segment
  * exactly where specified, deleting any existing segment file there.
  *
- * max_advance: maximum number of log/seg slots to advance past the starting
- * point.  Fail if no free slot is found in this range.  (Irrelevant if
- * find_free is FALSE.)
+ * *max_advance: maximum number of log/seg slots to advance past the starting
+ * point.  Fail if no free slot is found in this range.  On return, reduced
+ * by the number of slots skipped over.  (Irrelevant, and may be NULL,
+ * when find_free is FALSE.)
  *
  * use_lock: if TRUE, acquire ControlFileLock while moving file into
  * place.  This should be TRUE except during bootstrap log creation.  The
@@ -1752,14 +1761,14 @@ XLogFileCopy(uint32 log, uint32 seg,
  * exceeding max_advance limit.  (Any other kind of failure causes ereport().)
  */
 static bool
-InstallXLogFileSegment(uint32 log, uint32 seg, char *tmppath,
-                      bool find_free, int max_advance,
+InstallXLogFileSegment(uint32 *log, uint32 *seg, char *tmppath,
+                      bool find_free, int *max_advance,
                       bool use_lock)
 {
    char        path[MAXPGPATH];
    struct stat stat_buf;
 
-   XLogFilePath(path, ThisTimeLineID, log, seg);
+   XLogFilePath(path, ThisTimeLineID, *log, *seg);
 
    /*
     * We want to be sure that only one process does this at a time.
@@ -1777,15 +1786,16 @@ InstallXLogFileSegment(uint32 log, uint32 seg, char *tmppath,
        /* Find a free slot to put it in */
        while (stat(path, &stat_buf) == 0)
        {
-           if (--max_advance < 0)
+           if (*max_advance <= 0)
            {
                /* Failed to find a free slot within specified range */
                if (use_lock)
                    LWLockRelease(ControlFileLock);
                return false;
            }
-           NextLogSeg(log, seg);
-           XLogFilePath(path, ThisTimeLineID, log, seg);
+           NextLogSeg(*log, *seg);
+           (*max_advance)--;
+           XLogFilePath(path, ThisTimeLineID, *log, *seg);
        }
    }
 
@@ -1799,14 +1809,14 @@ InstallXLogFileSegment(uint32 log, uint32 seg, char *tmppath,
        ereport(PANIC,
                (errcode_for_file_access(),
                 errmsg("could not link file \"%s\" to \"%s\" (initialization of log file %u, segment %u): %m",
-                       tmppath, path, log, seg)));
+                       tmppath, path, *log, *seg)));
    unlink(tmppath);
 #else
    if (rename(tmppath, path) < 0)
        ereport(PANIC,
                (errcode_for_file_access(),
                 errmsg("could not rename file \"%s\" to \"%s\" (initialization of log file %u, segment %u): %m",
-                       tmppath, path, log, seg)));
+                       tmppath, path, *log, *seg)));
 #endif
 
    if (use_lock)
@@ -2129,6 +2139,7 @@ MoveOfflineLogs(uint32 log, uint32 seg, XLogRecPtr endptr,
 {
    uint32      endlogId;
    uint32      endlogSeg;
+   int         max_advance;
    DIR        *xldir;
    struct dirent *xlde;
    char        lastoff[MAXFNAMELEN];
@@ -2137,7 +2148,12 @@ MoveOfflineLogs(uint32 log, uint32 seg, XLogRecPtr endptr,
    *nsegsremoved = 0;
    *nsegsrecycled = 0;
 
+   /*
+    * Initialize info about where to try to recycle to.  We allow recycling
+    * segments up to XLOGfileslop segments beyond the current XLOG location.
+    */
    XLByteToPrevSeg(endptr, endlogId, endlogSeg);
+   max_advance = XLOGfileslop;
 
    xldir = AllocateDir(XLogDir);
    if (xldir == NULL)
@@ -2179,12 +2195,10 @@ MoveOfflineLogs(uint32 log, uint32 seg, XLogRecPtr endptr,
 
                /*
                 * Before deleting the file, see if it can be recycled as
-                * a future log segment.  We allow recycling segments up
-                * to XLOGfileslop segments beyond the current XLOG
-                * location.
+                * a future log segment.
                 */
-               if (InstallXLogFileSegment(endlogId, endlogSeg, path,
-                                          true, XLOGfileslop,
+               if (InstallXLogFileSegment(&endlogId, &endlogSeg, path,
+                                          true, &max_advance,
                                           true))
                {
                    ereport(DEBUG2,