Messages in this thread | | | Date | Fri, 13 Jul 2007 16:21:30 +0300 | From | "Pekka Enberg" <> | Subject | Re: [PATCH] replace kmem_cache_alloc with kmem_cache_zalloc to remove some following zero initializations. |
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Hi Kirill,
On 7/13/07, Kirill Korotaev <dev@sw.ru> wrote: > Look, until you have any numbers in hands it's impossible to say > which one is faster.
Well, it's not too hard to guess: struct dentry is 124 bytes on i386 so kmem_cache_zalloc over it is bound to be slower tha kmem_cache_alloc as most members are initialized to non-NULL. And if you're into micro-benchmarks, here's one:
d_alloc takes 182.75ns d_alloc_zalloc takes 315.89ns
So definitely NAK for Denis' patch.
Pekka
#include <stddef.h> #include <stdio.h> #include <stdlib.h> #include <sys/time.h> #include <string.h> #include <time.h>
struct dentry { void *a; void *b; void *c; void *d; void *e; void *f; void *g; void *h; unsigned char d_iname[36]; };
#define NR_ITERS 1000000
#define MAX_DENTRIES (NR_ITERS*2)
static unsigned long offset; static struct dentry dentries[MAX_DENTRIES];
static inline void *__kmalloc(void) { void *p = &dentries[offset++]; if (offset == MAX_DENTRIES) offset = 0; return p; }
void *kmalloc(size_t sz) { return __kmalloc(); }
struct dentry *d_alloc(const char *name) { struct dentry *ret = kmalloc(sizeof *ret); if (ret) { ret->a = NULL; ret->b = NULL; ret->c = NULL; ret->d = NULL; ret->e = (void *) 0xdeadbeef; ret->f = (void *) 0xdeadbeef; ret->g = (void *) 0xdeadbeef; ret->h = (void *) 0xdeadbeef; strcpy(ret->d_iname, name); } return ret; }
static inline void * __memset_generic(void * s, char c,size_t count) { int d0, d1; __asm__ __volatile__( "rep\n\t" "stosb" : "=&c" (d0), "=&D" (d1) :"a" (c),"1" (s),"0" (count) :"memory"); return s; }
void *kzalloc(size_t sz) { void *p = __kmalloc(); if (p) __memset_generic(p, 0, sz); return p; }
struct dentry *d_alloc_zalloc(const char *name) { struct dentry *ret = kzalloc(sizeof *ret); if (ret) { ret->e = (void *) 0xdeadbeef; ret->f = (void *) 0xdeadbeef; ret->g = (void *) 0xdeadbeef; ret->h = (void *) 0xdeadbeef; strcpy(ret->d_iname, name); } return ret; }
int main(int argc, char *argv[]) { struct timeval start, end; unsigned long long usec; int i;
gettimeofday(&start, NULL); for (i = 0; i < NR_ITERS; i++) { struct dentry *dentry = d_alloc("root");
if (!dentry || dentry->a != NULL) abort(); } gettimeofday(&end, NULL); usec = end.tv_usec + 1000000*(end.tv_sec - start.tv_sec) - start.tv_usec; printf("d_alloc takes %0.2lfns\n", (double)usec * 1000 / NR_ITERS);
gettimeofday(&start, NULL); for (i = 0; i < NR_ITERS; i++) { struct dentry *dentry = d_alloc_zalloc("root");
if (!dentry || dentry->a != NULL) abort(); } gettimeofday(&end, NULL); usec = end.tv_usec + 1000000*(end.tv_sec - start.tv_sec) - start.tv_usec; printf("d_alloc_zalloc takes %0.2lfns\n", (double)usec * 1000 / NR_ITERS);
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