From 84495354e9d02c3c1d87229516f97b393f15d590 Mon Sep 17 00:00:00 2001
From: Wayne Davison <wayned@samba.org>
Date: Tue, 9 Oct 2007 06:17:07 +0000
Subject: [PATCH] Improved to avoid a hashtable slowdown for files that don't
 need the extra-large hashtable.

---
 dynamic_hash.diff | 69 +++++++++++++++++++++++++++++++++--------------
 1 file changed, 49 insertions(+), 20 deletions(-)

diff --git a/dynamic_hash.diff b/dynamic_hash.diff
index f9a41d5..3388bd9 100644
--- a/dynamic_hash.diff
+++ b/dynamic_hash.diff
@@ -1,9 +1,11 @@
 This patch makes the processing of large really large files more efficient
 by making sure that the sender's hash table is large enough to hold all the
-checksum entries without being overloaded.  Unfortunately, the code adds a
-modulus calculation for (up to) every byte of the source file, which slows
-down the code for normal file sizes (e.g. 4 CPU seconds slower on a Pentium
-III when copying a 65 MB file without very much matching data).
+checksum entries without being overloaded.
+
+Updated to use the current hashtable method when possible, and the new
+hashtable method (which requires a modulus calculation for up to every byte
+of the source file) only on large files that need a larger hashtable size.
+This avoids slowing down files that don't need the extra-large hashtable.
 
 This was updated for the latest codebase from a patch written by Shachar
 Shemesh.
@@ -16,23 +18,25 @@ To use this patch, run these commands for a successful build:
 
 --- old/match.c
 +++ new/match.c
-@@ -40,24 +40,31 @@ static int total_matches;
+@@ -39,29 +39,50 @@ static int total_matches;
  
  extern struct stats stats;
  
 -#define TABLESIZE (1<<16)
--
++#define TRADITIONAL_TABLESIZE (1<<16)
+ 
 +static uint32 tablesize;
  static int32 *hash_table;
  
--#define SUM2HASH2(s1,s2) (((s1) + (s2)) & 0xFFFF)
--#define SUM2HASH(sum) SUM2HASH2((sum)&0xFFFF,(sum)>>16)
-+#define SUM2HASH(sum) ((sum)%tablesize)
+ #define SUM2HASH2(s1,s2) (((s1) + (s2)) & 0xFFFF)
+ #define SUM2HASH(sum) SUM2HASH2((sum)&0xFFFF,(sum)>>16)
  
++#define BIG_SUM2HASH(sum) ((sum)%tablesize)
++
  static void build_hash_table(struct sum_struct *s)
  {
++	static uint32 alloc_size;
  	int32 i;
-+	uint32 prior_size = tablesize;
  
 -	if (!hash_table) {
 -		hash_table = new_array(int32, TABLESIZE);
@@ -40,30 +44,55 @@ To use this patch, run these commands for a successful build:
 +	 * for big files is about 80%.  This number must be odd or s2 will
 +	 * not be able to span the entire set. */
 +	tablesize = (uint32)(s->count/8) * 10 + 11;
-+	if (tablesize < 65537)
-+		tablesize = 65537; /* a prime number */
-+	if (tablesize != prior_size) {
++	if (tablesize < TRADITIONAL_TABLESIZE)
++		tablesize = TRADITIONAL_TABLESIZE;
++	if (tablesize > alloc_size || tablesize < alloc_size - 16*1024) {
 +		if (hash_table)
 +			free(hash_table);
 +		hash_table = new_array(int32, tablesize);
  		if (!hash_table)
  			out_of_memory("build_hash_table");
++		alloc_size = tablesize;
  	}
  
 -	memset(hash_table, 0xFF, TABLESIZE * sizeof hash_table[0]);
 +	memset(hash_table, 0xFF, tablesize * sizeof hash_table[0]);
  
- 	for (i = 0; i < s->count; i++) {
- 		uint32 t = SUM2HASH(s->sums[i].sum1);
-@@ -165,11 +172,11 @@ static void hash_search(int f,struct sum
+-	for (i = 0; i < s->count; i++) {
+-		uint32 t = SUM2HASH(s->sums[i].sum1);
+-		s->sums[i].chain = hash_table[t];
+-		hash_table[t] = i;
++	if (tablesize == TRADITIONAL_TABLESIZE) {
++		for (i = 0; i < s->count; i++) {
++			uint32 t = SUM2HASH(s->sums[i].sum1);
++			s->sums[i].chain = hash_table[t];
++			hash_table[t] = i;
++		}
++	} else {
++		for (i = 0; i < s->count; i++) {
++			uint32 t = BIG_SUM2HASH(s->sums[i].sum1);
++			s->sums[i].chain = hash_table[t];
++			hash_table[t] = i;
++		}
+ 	}
+ }
+ 
+@@ -164,11 +185,16 @@ static void hash_search(int f,struct sum
  				(double)offset, s2 & 0xFFFF, s1 & 0xFFFF);
  		}
  
 -		i = hash_table[SUM2HASH2(s1,s2)];
-+		sum = (s1 & 0xffff) | (s2 << 16);
-+		i = hash_table[SUM2HASH(sum)];
- 		if (i < 0)
- 			goto null_hash;
+-		if (i < 0)
+-			goto null_hash;
++		if (tablesize == TRADITIONAL_TABLESIZE) {
++			if ((i = hash_table[SUM2HASH2(s1,s2)]) < 0)
++				goto null_hash;
++			sum = (s1 & 0xffff) | (s2 << 16);
++		} else {
++			sum = (s1 & 0xffff) | (s2 << 16);
++			if ((i = hash_table[BIG_SUM2HASH(sum)]) < 0)
++				goto null_hash;
++		}
  
 -		sum = (s1 & 0xffff) | (s2 << 16);
  		hash_hits++;
-- 
2.34.1