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Improve kconfig symbol hashing

PATCH Applied Andi Kleen 2 participants 4 messages Jan 13, 2010 – Jan 18, 2010

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Andi Kleen

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#0 lore raw

PATCH 1/1 Improve kconfig symbol hashing

Improve kconfig symbol hashing

While looking for something else I noticed that the symbol hash function used by kconfig is quite poor. It doesn't use any of the standard hash techniques but simply adds up the string and then uses power of two masking, which is both known to perform poorly.

The current x86 kconfig has over 7000 symbols.

When I instrumented it showed that the minimum hash chain length was 16 and a significant number of them was over 30.

It didn't help that the hash table size was only 256 buckets.

This patch increases the hash table size to a larger prime and switches to a FNV32 hash. I played around with a couple of hash functions, but that one seemed to perform best with reasonable hash table sizes.

Increasing the hash table size even further didn't seem like a good idea, because there are a couple of global walks which walk the complete hash table.

I also moved the unnamed bucket to 0. It's still the longest of all the buckets (44 entries), but hopefully it's not often hit except for the global walk which doesn't care.

The result is a much nicer distribution:
(first column bucket length, second number of buckets with that length)

1: 3505
2: 1236
3: 294
4: 52
5: 3
47: 1 <--- this is the unnamed symbols bucket

There are still some 5+ buckets, but increasing the hash table even more would be likely not worth it.

This also cleans up the code slightly by removing hard coded magic numbers.

I didn't notice a big performance difference either way on my Nehalem system, but I presume it'll help somewhat on slower systems.

---
 scripts/kconfig/expr.h              |    5 ++---
 scripts/kconfig/symbol.c            |   29 +++++++++++++++++------------
 scripts/kconfig/zconf.tab.c_shipped |    2 +-
 scripts/kconfig/zconf.y             |    2 +-
 4 files changed, 21 insertions(+), 17 deletions(-)
scripts/kconfig/symbol.c +17 -12
Index: linux-2.6.33-rc3-ak/scripts/kconfig/symbol.c
===================================================================
@@ -651,12 +651,20 @@ bool sym_is_changable(struct symbol *sym
 	return sym->visible > sym->rev_dep.tri;
 }
 
+static unsigned strhash(const char *s)
+{
+	/* fnv32 hash */
+	unsigned hash = 2166136261U;
+	for (; *s; s++)
+		hash = (hash ^ *s) * 0x01000193;
+	return hash;
+}
+
 struct symbol *sym_lookup(const char *name, int flags)
 {
 	struct symbol *symbol;
-	const char *ptr;
 	char *new_name;
-	int hash = 0;
+	int hash;
 
 	if (name) {
 		if (name[0] && !name[1]) {

@@ -666,12 +674,11 @@ struct symbol *sym_lookup(const char *na
 			case 'n': return &symbol_no;
 			}
 		}
-		for (ptr = name; *ptr; ptr++)
-			hash += *ptr;
-		hash &= 0xff;
+		hash = strhash(name) % SYMBOL_HASHSIZE;
 
 		for (symbol = symbol_hash[hash]; symbol; symbol = symbol->next) {
-			if (!strcmp(symbol->name, name) &&
+			if (symbol->name &&
+			    !strcmp(symbol->name, name) &&
 			    (flags ? symbol->flags & flags
 				   : !(symbol->flags & (SYMBOL_CONST|SYMBOL_CHOICE))))
 				return symbol;

@@ -679,7 +686,7 @@ struct symbol *sym_lookup(const char *na
 		new_name = strdup(name);
 	} else {
 		new_name = NULL;
-		hash = 256;
+		hash = 0;
 	}
 
 	symbol = malloc(sizeof(*symbol));

@@ -703,7 +710,6 @@ struct symbol *sym_lookup(const char *na
 struct symbol *sym_find(const char *name)
 {
 	struct symbol *symbol = NULL;
-	const char *ptr;
 	int hash = 0;
 
 	if (!name)

@@ -716,12 +722,11 @@ struct symbol *sym_find(const char *name
 		case 'n': return &symbol_no;
 		}
 	}
-	for (ptr = name; *ptr; ptr++)
-		hash += *ptr;
-	hash &= 0xff;
+	hash = strhash(name) % SYMBOL_HASHSIZE;
 
 	for (symbol = symbol_hash[hash]; symbol; symbol = symbol->next) {
-		if (!strcmp(symbol->name, name) &&
+		if (symbol->name &&
+		    !strcmp(symbol->name, name) &&
 		    !(symbol->flags & SYMBOL_CONST))
 				break;
 	}
scripts/kconfig/zconf.tab.c_shipped +1 -1
Index: linux-2.6.33-rc3-ak/scripts/kconfig/zconf.tab.c_shipped
===================================================================
@@ -104,7 +104,7 @@ static void zconf_error(const char *err,
 static void zconferror(const char *err);
 static bool zconf_endtoken(struct kconf_id *id, int starttoken, int endtoken);
 
-struct symbol *symbol_hash[257];
+struct symbol *symbol_hash[SYMBOL_HASHSIZE];
 
 static struct menu *current_menu, *current_entry;
scripts/kconfig/zconf.y +1 -1
Index: linux-2.6.33-rc3-ak/scripts/kconfig/zconf.y
===================================================================
@@ -27,7 +27,7 @@ static void zconf_error(const char *err,
 static void zconferror(const char *err);
 static bool zconf_endtoken(struct kconf_id *id, int starttoken, int endtoken);
 
-struct symbol *symbol_hash[257];
+struct symbol *symbol_hash[SYMBOL_HASHSIZE];
 
 static struct menu *current_menu, *current_entry;
scripts/kconfig/expr.h +2 -3
Index: linux-2.6.33-rc3-ak/scripts/kconfig/expr.h
===================================================================
@@ -86,7 +86,7 @@ struct symbol {
 	struct expr_value rev_dep;
 };
 
-#define for_all_symbols(i, sym) for (i = 0; i < 257; i++) for (sym = symbol_hash[i]; sym; sym = sym->next) if (sym->type != S_OTHER)
+#define for_all_symbols(i, sym) for (i = 0; i < SYMBOL_HASHSIZE; i++) for (sym = symbol_hash[i]; sym; sym = sym->next) if (sym->type != S_OTHER)
 
 #define SYMBOL_CONST      0x0001  /* symbol is const */
 #define SYMBOL_CHECK      0x0008  /* used during dependency checking */

@@ -108,8 +108,7 @@ struct symbol {
 #define SYMBOL_DEF4       0x80000  /* symbol.def[S_DEF_4] is valid */
 
 #define SYMBOL_MAXLENGTH	256
-#define SYMBOL_HASHSIZE		257
-#define SYMBOL_HASHMASK		0xff
+#define SYMBOL_HASHSIZE		9973
 
 /* A property represent the config options that can be associated
  * with a config "symbol".
#1 lore raw

On 13.1.2010 17:02, Andi Kleen wrote:
[...]

Quoted 33 lines

This patch increases the hash table size to a larger prime and switches to a FNV32 hash. I played around with a couple of hash functions, but that one seemed to perform best with reasonable hash table sizes.

Increasing the hash table size even further didn't seem like a good idea, because there are a couple of global walks which walk the complete hash table.

I also moved the unnamed bucket to 0. It's still the longest of all the buckets (44 entries), but hopefully it's not often hit except for the global walk which doesn't care.

The result is a much nicer distribution:
(first column bucket length, second number of buckets with that length)

1: 3505
2: 1236
3: 294
4: 52
5: 3
47: 1 <--- this is the unnamed symbols bucket

There are still some 5+ buckets, but increasing the hash table even more would be likely not worth it.

This also cleans up the code slightly by removing hard coded magic numbers.

I didn't notice a big performance difference either way on my Nehalem system, but I presume it'll help somewhat on slower systems.

Quoted 6 lines
---
 scripts/kconfig/expr.h              |    5 ++---
 scripts/kconfig/symbol.c            |   29 +++++++++++++++++------------
 scripts/kconfig/zconf.tab.c_shipped |    2 +-
 scripts/kconfig/zconf.y             |    2 +-
 4 files changed, 21 insertions(+), 17 deletions(-)

Hi Andi,

The patch looks ok, but you forgot your Signed-off-by line?

Michal