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replace modulo by bitwise AND for hash table lookups

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This operation is a lot faster, but depends on the hash table size
be a power of 2 (so HTABSIZE - 1 is a row of 1's). Also a define for the
magic value cleans things up a bit.
This commit is contained in:
Phil Sutter 2009-12-06 21:34:36 +01:00
parent 12d7781c7b
commit 4baba32459
1 changed files with 9 additions and 6 deletions
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15
src/top.c
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@ -32,6 +32,9 @@
#include "top.h" #include "top.h"
#include "logging.h" #include "logging.h"
/* hash table size - always a power of 2 */
#define HTABSIZE 256
static unsigned long g_time = 0; static unsigned long g_time = 0;
static unsigned long long previous_total = 0; static unsigned long long previous_total = 0;
static struct process *first_process = 0; static struct process *first_process = 0;
@ -41,7 +44,7 @@ struct proc_hash_entry {
struct proc_hash_entry *next; struct proc_hash_entry *next;
struct process *proc; struct process *proc;
}; };
static struct proc_hash_entry proc_hash_table[256]; static struct proc_hash_entry proc_hash_table[HTABSIZE];
static void hash_process(struct process *p) static void hash_process(struct process *p)
{ {
@ -51,12 +54,12 @@ static void hash_process(struct process *p)
/* better make sure all next pointers are zero upon first access */ /* better make sure all next pointers are zero upon first access */
if (first_run) { if (first_run) {
memset(proc_hash_table, 0, sizeof(struct proc_hash_entry) * 256); memset(proc_hash_table, 0, sizeof(struct proc_hash_entry) * HTABSIZE);
first_run = 0; first_run = 0;
} }
/* get the bucket index */ /* get the bucket index */
bucket = p->pid % 256; bucket = p->pid & (HTABSIZE - 1);
/* insert a new element on bucket's top */ /* insert a new element on bucket's top */
phe = malloc(sizeof(struct proc_hash_entry)); phe = malloc(sizeof(struct proc_hash_entry));
@ -70,7 +73,7 @@ static void unhash_process(struct process *p)
struct proc_hash_entry *phe, *tmp; struct proc_hash_entry *phe, *tmp;
/* get the bucket head */ /* get the bucket head */
phe = &proc_hash_table[p->pid % 256]; phe = &proc_hash_table[p->pid & (HTABSIZE - 1)];
/* find the entry pointing to p and drop it */ /* find the entry pointing to p and drop it */
while (phe->next) { while (phe->next) {
@ -95,7 +98,7 @@ static void unhash_all_processes(void)
{ {
int i; int i;
for (i = 0; i < 256; i++) { for (i = 0; i < HTABSIZE; i++) {
__unhash_all_processes(&proc_hash_table[i]); __unhash_all_processes(&proc_hash_table[i]);
proc_hash_table[i].next = NULL; proc_hash_table[i].next = NULL;
} }
@ -140,7 +143,7 @@ static struct process *find_process(pid_t pid)
{ {
struct proc_hash_entry *phe; struct proc_hash_entry *phe;
phe = &proc_hash_table[pid % 256]; phe = &proc_hash_table[pid & (HTABSIZE - 1)];
while (phe->next) { while (phe->next) {
if (phe->next->proc->pid == pid) if (phe->next->proc->pid == pid)
return phe->next->proc; return phe->next->proc;