[Cluster-devel] [GFS2] Faster gfs2_bitfit algorithm

[Bob Peterson]

Hi,

This version of the gfs2_bitfit algorithm includes the latest
suggestions from Steve Whitehouse.  It is typically eight to
ten times faster than the version we're using today.  If there
is a lot of metadata mixed in (lots of small files) the
algorithm is often 15 times faster, and given the right
conditions, I've seen peaks of 20 times faster.

Regards,

Bob Peterson

Signed-off-by: Bob Peterson <rpeterso at redhat.com>
--
 fs/gfs2/rgrp.c |   93 ++++++++++++++++++++++++++++++++++++-------------------
 1 files changed, 61 insertions(+), 32 deletions(-)

diff --git a/fs/gfs2/rgrp.c b/fs/gfs2/rgrp.c
index 4291375..7e8f0b1 100644
--- a/fs/gfs2/rgrp.c
+++ b/fs/gfs2/rgrp.c
@@ -14,6 +14,7 @@
 #include <linux/fs.h>
 #include <linux/gfs2_ondisk.h>
 #include <linux/lm_interface.h>
+#include <linux/prefetch.h>
 
 #include "gfs2.h"
 #include "incore.h"
@@ -33,6 +34,16 @@
 #define BFITNOENT ((u32)~0)
 #define NO_BLOCK ((u64)~0)
 
+#if BITS_PER_LONG == 32
+#define LBITMASK   (0x55555555UL)
+#define LBITSKIP55 (0x55555555UL)
+#define LBITSKIP00 (0x00000000UL)
+#else
+#define LBITMASK   (0x5555555555555555UL)
+#define LBITSKIP55 (0x5555555555555555UL)
+#define LBITSKIP00 (0x0000000000000000UL)
+#endif
+
 /*
  * These routines are used by the resource group routines (rgrp.c)
  * to keep track of block allocation.  Each block is represented by two
@@ -138,45 +149,63 @@ static inline unsigned char gfs2_testbit(struct gfs2_rgrpd *rgd,
 static u32 gfs2_bitfit(const u8 *buffer, unsigned int buflen, u32 goal,
 		       u8 old_state)
 {
-	const u8 *byte;
-	u32 blk = goal;
-	unsigned int bit, bitlong;
-	const unsigned long *plong;
-#if BITS_PER_LONG == 32
-	const unsigned long plong55 = 0x55555555;
-#else
-	const unsigned long plong55 = 0x5555555555555555;
-#endif
-
-	byte = buffer + (goal / GFS2_NBBY);
-	plong = (const unsigned long *)(buffer + (goal / GFS2_NBBY));
-	bit = (goal % GFS2_NBBY) * GFS2_BIT_SIZE;
-	bitlong = bit;
-
-	while (byte < buffer + buflen) {
-
-		if (bitlong == 0 && old_state == 0 && *plong == plong55) {
-			plong++;
-			byte += sizeof(unsigned long);
-			blk += sizeof(unsigned long) * GFS2_NBBY;
-			continue;
+	const u8 *byte, *start, *end;
+	int bit, startbit;
+	u32 g1, g2, misaligned;
+	unsigned long *plong;
+	unsigned long lskipval;
+
+	lskipval = (old_state & GFS2_BLKST_USED) ? LBITSKIP00 : LBITSKIP55;
+	g1 = (goal / GFS2_NBBY);
+	start = buffer + g1;
+	byte = start;
+        end = buffer + buflen;
+	g2 = ALIGN(g1, sizeof(unsigned long));
+	plong = (unsigned long *)(buffer + g2);
+	startbit = bit = (goal % GFS2_NBBY) * GFS2_BIT_SIZE;
+	misaligned = g2 - g1;
+	if (!misaligned)
+		goto ulong_aligned;
+/* parse the bitmap a byte at a time */
+misaligned:
+	while (byte < end) {
+		if (((*byte >> bit) & GFS2_BIT_MASK) == old_state) {
+			return goal +
+				(((byte - start) * GFS2_NBBY) +
+				 ((bit - startbit) >> 1));
 		}
-		if (((*byte >> bit) & GFS2_BIT_MASK) == old_state)
-			return blk;
 		bit += GFS2_BIT_SIZE;
-		if (bit >= 8) {
+		if (bit >= GFS2_NBBY * GFS2_BIT_SIZE) {
 			bit = 0;
 			byte++;
+			misaligned--;
+			if (!misaligned) {
+				plong = (unsigned long *)byte;
+				goto ulong_aligned;
+			}
 		}
-		bitlong += GFS2_BIT_SIZE;
-		if (bitlong >= sizeof(unsigned long) * 8) {
-			bitlong = 0;
-			plong++;
-		}
-
-		blk++;
 	}
+	return BFITNOENT;
 
+/* parse the bitmap a unsigned long at a time */
+ulong_aligned:
+	/* Stop at "end - 1" or else prefetch can go past the end and segfault.
+	   We could "if" it but we'd lose some of the performance gained.
+	   This way will only slow down searching the very last 4/8 bytes
+	   depending on architecture.  I've experimented with several ways
+	   of writing this section such as using an else before the goto
+	   but this one seems to be the fastest. */
+	while ((unsigned char *)plong < end - 1) {
+		prefetch(plong + 1);
+		if (((*plong) & LBITMASK) != lskipval)
+			break;
+		plong++;
+	}
+	if ((unsigned char *)plong < end) {
+		byte = (const u8 *)plong;
+		misaligned += sizeof(unsigned long) - 1;
+		goto misaligned;
+	}
 	return BFITNOENT;
 }