[PATCH] kallsyms data size reduction / lookup speedup

This patch is an improvement over my first kallsyms speedup patch posted about
2 weeks ago.

It changes scripts/kallsyms as to produce a different format for
kallsyms_names and extra data to speedup lookups.  The compression algorithm
is quite simple: it uses all the char codes not actually used in symbols to
build a lookup table that translates these codes into small strings.  For
instance, in my test runs the code 0xFE was being translated into "acpi_"
giving a 4 byte save on every translation.

The advantage of this algorithm is that to translate a symbol we only require
information that is stored on that symbol position, and never need to go back
on the compressed stream to get information from other symbols.

To give an idea about the benefits of this algorithm here are some benchmark
results on a P4 2.8GHz with a symbol table with 10000 entries:

kallsyms_lookup average time:
  vanilla           1346.0 us
  speedup             14.4 us
  with this patch      0.5 us

total data produced by scripts/kallsyms:
  uncompressed         169 Kb
  vanilla              134 Kb
  with this patch       91 Kb

(speedup was my latest patch, that only changed the way kallsyms_lookup worked
and not the data format)

I removed a cond_resched() from the proc/kallsyms handling code path, because
using stem compression, if the current position went backwards, the hole
stream would be uncompressed up to the current position.  It seemed that by
removing this loop it would be safe to remove the conditional reschedule
altogether.

There is just one catch with this patch: the time it takes to compile the
kernel goes up just a bit (about 0.8s on a P4 2.8GHz with defconfig).  If this
delay is not acceptable I can change the compression algorithm so that it can
use the previous table (calculating a new table is what consumes most of the
time, and not doing the actual compression) and check to see if it obtains a
similar compression ratio.  If it does, then this is a sign that the symbol
patterns haven't changed that much and this table is still good to use.  This
would not only cut the time down to half on any compilation (because of the 2
pass symbol build method), but in frequent cases where a developer is
compiling a single file and linking everything over and over again, the table
optimization process would never run.

I'm CC'ing Brent Casavant on this email, because last june he sent a patch
trying a different approach that used a 32 entry symbol cache, because there
was a problem with the time "top" took to read "proc/<pid>/wchan".  I was
hopping he would be willing to test this patch and comment on the results.

Signed-off-by: Paulo Marques <pmarques@grupopie.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 515 insertions, 38 deletions

diff --git a/scripts/kallsyms.c b/scripts/kallsyms.c
index e21a5d1a255aea..2e0e07afb84c28 100644
--- a/scripts/kallsyms.c
+++ b/scripts/kallsyms.c
@@ -6,6 +6,22 @@
  * of the GNU General Public License, incorporated herein by reference.
  *
  * Usage: nm -n vmlinux | scripts/kallsyms [--all-symbols] > symbols.S
+ *
+ * ChangeLog:
+ *
+ * (25/Aug/2004) Paulo Marques <pmarques@grupopie.com>
+ *      Changed the compression method from stem compression to "table lookup"
+ *      compression
+ *
+ *      Table compression uses all the unused char codes on the symbols and
+ *  maps these to the most used substrings (tokens). For instance, it might
+ *  map char code 0xF7 to represent "write_" and then in every symbol where
+ *  "write_" appears it can be replaced by 0xF7, saving 5 bytes.
+ *      The used codes themselves are also placed in the table so that the
+ *  decompresion can work without "special cases".
+ *      Applied to kernel symbols, this usually produces a compression ratio
+ *  of about 50%.
+ *
  */
 
 #include <stdio.h>
@@ -13,10 +29,39 @@
 #include <string.h>
 #include <ctype.h>
 
+/* maximum token length used. It doesn't pay to increase it a lot, because
+ * very long substrings probably don't repeat themselves too often. */
+#define MAX_TOK_SIZE		11
+#define KSYM_NAME_LEN		127
+
+/* we use only a subset of the complete symbol table to gather the token count,
+ * to speed up compression, at the expense of a little compression ratio */
+#define WORKING_SET		1024
+
+/* first find the best token only on the list of tokens that would profit more
+ * than GOOD_BAD_THRESHOLD. Only if this list is empty go to the "bad" list.
+ * Increasing this value will put less tokens on the "good" list, so the search
+ * is faster. However, if the good list runs out of tokens, we must painfully
+ * search the bad list. */
+#define GOOD_BAD_THRESHOLD	10
+
+/* token hash parameters */
+#define HASH_BITS		18
+#define HASH_TABLE_SIZE		(1 << HASH_BITS)
+#define HASH_MASK		(HASH_TABLE_SIZE - 1)
+#define HASH_BASE_OFFSET	2166136261U
+#define HASH_FOLD(a)		((a)&(HASH_MASK))
+
+/* flags to mark symbols */
+#define SYM_FLAG_VALID		1
+#define SYM_FLAG_SAMPLED	2
+
 struct sym_entry {
 	unsigned long long addr;
 	char type;
-	char *sym;
+	unsigned char flags;
+	unsigned char len;
+	unsigned char *sym;
 };
 
 
@@ -25,6 +70,26 @@ static int size, cnt;
 static unsigned long long _stext, _etext, _sinittext, _einittext;
 static int all_symbols = 0;
 
+struct token {
+	unsigned char data[MAX_TOK_SIZE];
+	unsigned char len;
+	/* profit: the number of bytes that could be saved by inserting this
+	 * token into the table */
+	int profit;
+	struct token *next;	/* next token on the hash list */
+	struct token *right;	/* next token on the good/bad list */
+	struct token *left;    /* previous token on the good/bad list */
+	struct token *smaller; /* token that is less one letter than this one */
+	};
+
+struct token bad_head, good_head;
+struct token *hash_table[HASH_TABLE_SIZE];
+
+/* the table that holds the result of the compression */
+unsigned char best_table[256][MAX_TOK_SIZE+1];
+unsigned char best_table_len[256];
+
+
 static void
 usage(void)
 {
@@ -59,34 +124,53 @@ read_symbol(FILE *in, struct sym_entry *s)
 	else if (toupper(s->type) == 'A' || toupper(s->type) == 'U')
 		return -1;
 
-	s->sym = strdup(str);
+	/* include the type field in the symbol name, so that it gets
+	 * compressed together */
+	s->len = strlen(str) + 1;
+	s->sym = (char *) malloc(s->len + 1);
+	strcpy(s->sym + 1, str);
+	s->sym[0] = s->type;
+
 	return 0;
 }
 
 static int
 symbol_valid(struct sym_entry *s)
 {
+	/* Symbols which vary between passes.  Passes 1 and 2 must have
+	 * identical symbol lists.  The kallsyms_* symbols below are only added
+	 * after pass 1, they would be included in pass 2 when --all-symbols is
+	 * specified so exclude them to get a stable symbol list.
+	 */
+	static char *special_symbols[] = {
+		"kallsyms_addresses",
+		"kallsyms_num_syms",
+		"kallsyms_names",
+		"kallsyms_markers",
+		"kallsyms_token_table",
+		"kallsyms_token_index",
+
+	/* Exclude linker generated symbols which vary between passes */
+		"_SDA_BASE_",		/* ppc */
+		"_SDA2_BASE_",		/* ppc */
+		NULL };
+	int i;
+
+	/* if --all-symbols is not specified, then symbols outside the text
+	 * and inittext sections are discarded */
 	if (!all_symbols) {
 		if ((s->addr < _stext || s->addr > _etext)
 		    && (s->addr < _sinittext || s->addr > _einittext))
 			return 0;
 	}
 
-	/* Exclude symbols which vary between passes.  Passes 1 and 2 must have
-	 * identical symbol lists.  The kallsyms_* symbols below are only added
-	 * after pass 1, they would be included in pass 2 when --all-symbols is
-	 * specified so exclude them to get a stable symbol list.
-	 */
-	if (strstr(s->sym, "_compiled.") ||
-	    strcmp(s->sym, "kallsyms_addresses") == 0 ||
-	    strcmp(s->sym, "kallsyms_num_syms") == 0 ||
-	    strcmp(s->sym, "kallsyms_names") == 0)
+	/* Exclude symbols which vary between passes. */
+	if (strstr(s->sym + 1, "_compiled."))
 		return 0;
 
-	/* Exclude linker generated symbols which vary between passes */
-	if (strcmp(s->sym, "_SDA_BASE_") == 0 ||	/* ppc */
-	    strcmp(s->sym, "_SDA2_BASE_") == 0)		/* ppc */
-		return 0;
+	for (i = 0; special_symbols[i]; i++)
+		if( strcmp(s->sym + 1, special_symbols[i]) == 0 )
+			return 0;
 
 	return 1;
 }
@@ -108,11 +192,47 @@ read_map(FILE *in)
 	}
 }
 
+static void output_label(char *label)
+{
+	printf(".globl %s\n",label);
+	printf("\tALGN\n");
+	printf("%s:\n",label);
+}
+
+/* uncompress a compressed symbol. When this function is called, the best table
+ * might still be compressed itself, so the function needs to be recursive */
+static int expand_symbol(unsigned char *data, int len, char *result)
+{
+	int c, rlen, total=0;
+
+	while (len) {
+		c = *data;
+		/* if the table holds a single char that is the same as the one
+		 * we are looking for, then end the search */
+		if (best_table[c][0]==c && best_table_len[c]==1) {
+			*result++ = c;
+			total++;
+		} else {
+			/* if not, recurse and expand */
+			rlen = expand_symbol(best_table[c], best_table_len[c], result);
+			total += rlen;
+			result += rlen;
+		}
+		data++;
+		len--;
+	}
+	*result=0;
+
+	return total;
+}
+
 static void
 write_src(void)
 {
-	int i, valid = 0;
-	char *prev;
+	int i, k, off, valid;
+	unsigned int best_idx[256];
+	unsigned int *markers;
+	char buf[KSYM_NAME_LEN+1];
 
 	printf("#include <asm/types.h>\n");
 	printf("#if BITS_PER_LONG == 64\n");
@@ -125,43 +245,399 @@ write_src(void)
 
 	printf(".data\n");
 
-	printf(".globl kallsyms_addresses\n");
-	printf("\tALGN\n");
-	printf("kallsyms_addresses:\n");
+	output_label("kallsyms_addresses");
+	valid = 0;
 	for (i = 0; i < cnt; i++) {
-		if (!symbol_valid(&table[i]))
-			continue;
-
-		printf("\tPTR\t%#llx\n", table[i].addr);
-		valid++;
+		if (table[i].flags & SYM_FLAG_VALID) {
+			printf("\tPTR\t%#llx\n", table[i].addr);
+			valid++;
+		}
 	}
 	printf("\n");
 
-	printf(".globl kallsyms_num_syms\n");
-	printf("\tALGN\n");
-	printf("kallsyms_num_syms:\n");
+	output_label("kallsyms_num_syms");
 	printf("\tPTR\t%d\n", valid);
 	printf("\n");
 
-	printf(".globl kallsyms_names\n");
-	printf("\tALGN\n");
-	printf("kallsyms_names:\n");
-	prev = ""; 
+	/* table of offset markers, that give the offset in the compressed stream
+	 * every 256 symbols */
+	markers = (unsigned int *) malloc(sizeof(unsigned int)*((valid + 255) / 256));
+
+	output_label("kallsyms_names");
+	valid = 0;
+	off = 0;
 	for (i = 0; i < cnt; i++) {
-		int k;
 
-		if (!symbol_valid(&table[i]))
+		if (!table[i].flags & SYM_FLAG_VALID)
 			continue;
 
-		for (k = 0; table[i].sym[k] && table[i].sym[k] == prev[k]; ++k)
-			; 
+		if ((valid & 0xFF) == 0)
+			markers[valid >> 8] = off;
 
-		printf("\t.byte 0x%02x\n\t.asciz\t\"%s\"\n", k, table[i].sym + k);
-		prev = table[i].sym;
+		printf("\t.byte 0x%02x", table[i].len);
+		for (k = 0; k < table[i].len; k++)
+			printf(", 0x%02x", table[i].sym[k]);
+		printf("\n");
+
+		off += table[i].len + 1;
+		valid++;
+	}
+	printf("\n");
+
+	output_label("kallsyms_markers");
+	for (i = 0; i < ((valid + 255) >> 8); i++)
+		printf("\tPTR\t%d\n", markers[i]);
+	printf("\n");
+
+	free(markers);
+
+	output_label("kallsyms_token_table");
+	off = 0;
+	for (i = 0; i < 256; i++) {
+		best_idx[i] = off;
+		expand_symbol(best_table[i],best_table_len[i],buf);
+		printf("\t.asciz\t\"%s\"\n", buf);
+		off += strlen(buf) + 1;
 	}
 	printf("\n");
+
+	output_label("kallsyms_token_index");
+	for (i = 0; i < 256; i++)
+		printf("\t.short\t%d\n", best_idx[i]);
+	printf("\n");
+}
+
+
+/* table lookup compression functions */
+
+static inline unsigned int rehash_token(unsigned int hash, unsigned char data)
+{
+	return ((hash * 16777619) ^ data);
+}
+
+static unsigned int hash_token(unsigned char *data, int len)
+{
+	unsigned int hash=HASH_BASE_OFFSET;
+	int i;
+
+	for (i = 0; i < len; i++)
+		hash = rehash_token(hash, data[i]);
+
+	return HASH_FOLD(hash);
+}
+
+/* find a token given its data and hash value */
+static struct token *find_token_hash(unsigned char *data, int len, unsigned int hash)
+{
+	struct token *ptr;
+
+	ptr = hash_table[hash];
+
+	while (ptr) {
+		if ((ptr->len == len) && (memcmp(ptr->data, data, len) == 0))
+			return ptr;
+		ptr=ptr->next;
+	}
+
+	return NULL;
+}
+
+static inline void insert_token_in_group(struct token *head, struct token *ptr)
+{
+	ptr->right = head->right;
+	ptr->right->left = ptr;
+	head->right = ptr;
+	ptr->left = head;
+}
+
+static inline void remove_token_from_group(struct token *ptr)
+{
+	ptr->left->right = ptr->right;
+	ptr->right->left = ptr->left;
+}
+
+
+/* build the counts for all the tokens that start with "data", and have lenghts
+ * from 2 to "len" */
+static void learn_token(unsigned char *data, int len)
+{
+	struct token *ptr,*last_ptr;
+	int i, newprofit;
+	unsigned int hash = HASH_BASE_OFFSET;
+	unsigned int hashes[MAX_TOK_SIZE + 1];
+
+	if (len > MAX_TOK_SIZE)
+		len = MAX_TOK_SIZE;
+
+	/* calculate and store the hash values for all the sub-tokens */
+	hash = rehash_token(hash, data[0]);
+	for (i = 2; i <= len; i++) {
+		hash = rehash_token(hash, data[i-1]);
+		hashes[i] = HASH_FOLD(hash);
+	}
+
+	last_ptr = NULL;
+	ptr = NULL;
+
+	for (i = len; i >= 2; i--) {
+		hash = hashes[i];
+
+		if (!ptr) ptr = find_token_hash(data, i, hash);
+
+		if (!ptr) {
+			/* create a new token entry */
+			ptr = (struct token *) malloc(sizeof(*ptr));
+
+			memcpy(ptr->data, data, i);
+			ptr->len = i;
+
+			/* when we create an entry, it's profit is 0 because
+			 * we also take into account the size of the token on
+			 * the compressed table. We then subtract GOOD_BAD_THRESHOLD
+			 * so that the test to see if this token belongs to
+			 * the good or bad list, is a comparison to zero */
+			ptr->profit = -GOOD_BAD_THRESHOLD;
+
+			ptr->next = hash_table[hash];
+			hash_table[hash] = ptr;
+
+			insert_token_in_group(&bad_head, ptr);
+
+			ptr->smaller = NULL;
+		} else {
+			newprofit = ptr->profit + (ptr->len - 1);
+			/* check to see if this token needs to be moved to a
+			 * different list */
+			if((ptr->profit < 0) && (newprofit >= 0)) {
+				remove_token_from_group(ptr);
+				insert_token_in_group(&good_head,ptr);
+			}
+			ptr->profit = newprofit;
+		}
+
+		if (last_ptr) last_ptr->smaller = ptr;
+		last_ptr = ptr;
+
+		ptr = ptr->smaller;
+	}
+}
+
+/* decrease the counts for all the tokens that start with "data", and have lenghts
+ * from 2 to "len". This function is much simpler than learn_token because we have
+ * more guarantees (tho tokens exist, the ->smaller pointer is set, etc.)
+ * The two separate functions exist only because of compression performance */
+static void forget_token(unsigned char *data, int len)
+{
+	struct token *ptr;
+	int i, newprofit;
+	unsigned int hash=0;
+
+	if (len > MAX_TOK_SIZE) len = MAX_TOK_SIZE;
+
+	hash = hash_token(data, len);
+	ptr = find_token_hash(data, len, hash);
+
+	for (i = len; i >= 2; i--) {
+
+		newprofit = ptr->profit - (ptr->len - 1);
+		if ((ptr->profit >= 0) && (newprofit < 0)) {
+			remove_token_from_group(ptr);
+			insert_token_in_group(&bad_head, ptr);
+		}
+		ptr->profit=newprofit;
+
+		ptr=ptr->smaller;
+	}
 }
 
+/* count all the possible tokens in a symbol */
+static void learn_symbol(unsigned char *symbol, int len)
+{
+	int i;
+
+	for (i = 0; i < len - 1; i++)
+		learn_token(symbol + i, len - i);
+}
+
+/* decrease the count for all the possible tokens in a symbol */
+static void forget_symbol(unsigned char *symbol, int len)
+{
+	int i;
+
+	for (i = 0; i < len - 1; i++)
+		forget_token(symbol + i, len - i);
+}
+
+/* set all the symbol flags and do the initial token count */
+static void build_initial_tok_table(void)
+{
+	int i, use_it, valid;
+
+	valid = 0;
+	for (i = 0; i < cnt; i++) {
+		table[i].flags = 0;
+		if ( symbol_valid(&table[i]) ) {
+			table[i].flags |= SYM_FLAG_VALID;
+			valid++;
+		}
+	}
+
+	use_it = 0;
+	for (i = 0; i < cnt; i++) {
+
+		/* subsample the available symbols. This method is almost like
+		 * a Bresenham's algorithm to get uniformly distributed samples
+		 * across the symbol table */
+		if (table[i].flags & SYM_FLAG_VALID) {
+
+			use_it += WORKING_SET;
+
+			if (use_it >= valid) {
+				table[i].flags |= SYM_FLAG_SAMPLED;
+				use_it -= valid;
+			}
+		}
+		if (table[i].flags & SYM_FLAG_SAMPLED)
+			learn_symbol(table[i].sym, table[i].len);
+	}
+}
+
+/* replace a given token in all the valid symbols. Use the sampled symbols
+ * to update the counts */
+static void compress_symbols(unsigned char *str, int tlen, int idx)
+{
+	int i, len, learn, size;
+	unsigned char *p;
+
+	for (i = 0; i < cnt; i++) {
+
+		if (!(table[i].flags & SYM_FLAG_VALID)) continue;
+
+		len = table[i].len;
+		learn = 0;
+		p = table[i].sym;
+
+		do {
+			/* find the token on the symbol */
+			p = (unsigned char *) strstr((char *) p, (char *) str);
+			if (!p) break;
+
+			if (!learn) {
+				/* if this symbol was used to count, decrease it */
+				if (table[i].flags & SYM_FLAG_SAMPLED)
+					forget_symbol(table[i].sym, len);
+				learn = 1;
+			}
+
+			*p = idx;
+			size = (len - (p - table[i].sym)) - tlen + 1;
+			memmove(p + 1, p + tlen, size);
+			p++;
+			len -= tlen - 1;
+
+		} while (size >= tlen);
+
+		if(learn) {
+			table[i].len = len;
+			/* if this symbol was used to count, learn it again */
+			if(table[i].flags & SYM_FLAG_SAMPLED)
+				learn_symbol(table[i].sym, len);
+		}
+	}
+}
+
+/* search the token with the maximum profit */
+static struct token *find_best_token(void)
+{
+	struct token *ptr,*best,*head;
+	int bestprofit;
+
+	bestprofit=-10000;
+
+	/* failsafe: if the "good" list is empty search from the "bad" list */
+	if(good_head.right == &good_head) head = &bad_head;
+	else head = &good_head;
+
+	ptr = head->right;
+	best = NULL;
+	while (ptr != head) {
+		if (ptr->profit > bestprofit) {
+			bestprofit = ptr->profit;
+			best = ptr;
+		}
+		ptr = ptr->right;
+	}
+
+	return best;
+}
+
+/* this is the core of the algorithm: calculate the "best" table */
+static void optimize_result(void)
+{
+	struct token *best;
+	int i;
+
+	/* using the '\0' symbol last allows compress_symbols to use standard
+	 * fast string functions */
+	for (i = 255; i >= 0; i--) {
+
+		/* if this table slot is empty (it is not used by an actual
+		 * original char code */
+		if (!best_table_len[i]) {
+
+			/* find the token with the breates profit value */
+			best = find_best_token();
+
+			/* place it in the "best" table */
+			best_table_len[i] = best->len;
+			memcpy(best_table[i], best->data, best_table_len[i]);
+			/* zero terminate the token so that we can use strstr
+			   in compress_symbols */
+			best_table[i][best_table_len[i]]='\0';
+
+			/* replace this token in all the valid symbols */
+			compress_symbols(best_table[i], best_table_len[i], i);
+		}
+	}
+}
+
+/* start by placing the symbols that are actually used on the table */
+static void insert_real_symbols_in_table(void)
+{
+	int i, j, c;
+
+	memset(best_table, 0, sizeof(best_table));
+	memset(best_table_len, 0, sizeof(best_table_len));
+
+	for (i = 0; i < cnt; i++) {
+		if (table[i].flags & SYM_FLAG_VALID) {
+			for (j = 0; j < table[i].len; j++) {
+				c = table[i].sym[j];
+				best_table[c][0]=c;
+				best_table_len[c]=1;
+			}
+		}
+	}
+}
+
+static void optimize_token_table(void)
+{
+	memset(hash_table, 0, sizeof(hash_table));
+
+	good_head.left = &good_head;
+	good_head.right = &good_head;
+
+	bad_head.left = &bad_head;
+	bad_head.right = &bad_head;
+
+	build_initial_tok_table();
+
+	insert_real_symbols_in_table();
+
+	optimize_result();
+}
+
+
 int
 main(int argc, char **argv)
 {
@@ -171,6 +647,7 @@ main(int argc, char **argv)
 		usage();
 
 	read_map(stdin);
+	optimize_token_table();
 	write_src();
 
 	return 0;