On Thu, 2015-10-29 at 04:16 +0000, Al Viro wrote:
> Have you tried to experiment with that in userland? I mean, emulate that
> thing in normal userland code, count the cacheline accesses and drive it
> with the use patterns collected from actual applications.
Sure.
>
> I can sit down and play with math expectations, but I suspect that it's
> easier to experiment. It's nothing but an intuition (I hadn't seriously
> done probability theory in quite a while, and my mathematical tastes run
> more to geometry and topology anyway), but... I would expect it to degrade
> badly when the bitmap is reasonably dense.
>
> Note, BTW, that vmalloc'ed memory gets populated as you read it, and it's
> not cheap - it's done via #PF triggered in kernel mode, with handler
> noticing that the faulting address is in vmalloc range and doing the
> right thing. IOW, if your bitmap is very sparse, the price of page faults
> needs to be taken into account.
This vmalloc PF is pure noise.
This only matters for the very first allocations.
We target programs opening zillions of fd in their lifetime ;)
Not having to expand a 4,000,000 slots fd array while fully loaded also
removes a latency spike that is very often not desirable.
>
> AFAICS, the only benefit of that thing is keeping dirtied cachelines far
> from each other. Which might be a win overall, but I'm not convinced that
> the rest won't offset the effect of that...
Well, I already tested the O_FD_FASTALLOC thing, and I can tell you
find_next_zero_bit() is nowhere to be found in kernel profiles anymore.
It also lowers time we hold the fd array spinlock while doing fd alloc.
User land test program I wrote few months back
Current kernel :
64.98% [kernel] [k] queued_spin_lock_slowpath
14.88% opensock [.] memset // this part simulates user land actual work ;)
11.15% [kernel] [k] _find_next_bit.part.0
0.69% [kernel] [k] _raw_spin_lock
0.46% [kernel] [k] memset_erms
0.38% [kernel] [k] sk_alloc
0.37% [kernel] [k] kmem_cache_alloc
0.33% [kernel] [k] get_empty_filp
0.31% [kernel] [k] kmem_cache_free
0.26% [kernel] [k] __alloc_fd
0.26% opensock [.] child_function
0.18% [kernel] [k] inode_init_always
0.17% opensock [.] __random_r
/*
* test for b/9072743 : fd scaling on gigantic process (with ~ 10,000,000 TCP sockets)
* - Size fd arrays in kernel to avoid resizings that kill latencies.
* - Then launch xx threads doing
* populate the fd array of the process, opening 'max' files.
*
* - Loop : close(randomfd()), socket(AF_INET, SOCK_STREAM, 0);
*
* Usage : opensock [ -n fds_count ] [ -t threads_count] [-f]
*/
#include <pthread.h>
#include <stdio.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <stdlib.h>
#include <errno.h>
#include <fcntl.h>
#include <unistd.h>
#include <stdlib.h>
#include <string.h>
unsigned int count;
int skflags;
#define NBTHREADS_MAX 4096
pthread_t tid[NBTHREADS_MAX];
int nbthreads;
int nbthreads_req = 24;
int stop_all;
#ifndef O_FD_FASTALLOC
#define O_FD_FASTALLOC 0x40000000
#endif
#ifndef SOCK_FD_FASTALLOC
#define SOCK_FD_FASTALLOC O_FD_FASTALLOC
#endif
/* expand kernel fd array for optimal perf.
* This could be done by doing a loop on dup(),
* or can be done using dup2()
*/
int expand_fd_array(int max)
{
int target, res;
int fd = socket(AF_INET, SOCK_STREAM, 0);
if (fd == -1) {
perror("socket()");
return -1;
}
for (;;) {
count = max;
target = count;
if (skflags & SOCK_FD_FASTALLOC)
target += count/10;
res = dup2(fd, target);
if (res != -1) {
close(res);
break;
}
max -= max/10;
}
printf("count=%u (check/increase ulimit -n)\n", count);
return 0;
}
static char state[32] = {
0, 1, 2, 3, 4, 5, 6, 7,
8, 9, 10, 11, 12, 13, 14, 15,
16, 17, 18, 19, 20, 21, 22, 23,
24, 25, 26, 27, 28, 29, 30, 31
};
/* each thread is using ~400 KB of data per unit of work */
#define WORKING_SET_SIZE 400000
static void *child_function(void *arg)
{
unsigned int max = count / nbthreads_req;
struct random_data buf;
unsigned int idx;
int *tab;
unsigned long iter = 0;
unsigned long *work_set = malloc(WORKING_SET_SIZE);
int i;
if (!work_set)
return NULL;
tab = malloc(max * sizeof(int));
if (!tab) {
free(work_set);
return NULL;
}
memset(tab, 255, max * sizeof(int));
initstate_r(getpid(), state, sizeof(state), &buf);
tab[0] = socket(AF_INET, SOCK_STREAM | skflags, 0);
for (i = 1; i < max; i++)
tab[i] = dup(tab[0]);
while (!stop_all) {
random_r(&buf, &idx);
idx = idx % max;
close(tab[idx]);
/* user space needs typically to use a bit of the memory. */
memset(work_set, idx, WORKING_SET_SIZE);
tab[idx] = socket(AF_INET, SOCK_STREAM | skflags, 0);
if (tab[idx] == -1) {
perror("socket");
break;
}
iter++;
}
for (i = 0; i < max; i++)
close(tab[idx]);
free(tab);
free(work_set);
printf("%lu\n", iter);
return NULL;
}
static int launch_threads(void)
{
int i, err;
for (i = 0; i < nbthreads_req; i++) {
err = pthread_create(&tid[i], NULL, child_function, NULL);
if (err)
return err;
nbthreads++;
}
return 0;
}
static void wait_end(void)
{
int i;
for (i = 0; i < nbthreads; i++)
pthread_join(tid[i], NULL);
}
static void usage(int code)
{
fprintf(stderr, "Usage : opensock [ -n fds_count ] [ -t threads_count] [-f]\n");
exit(code);
}
int main(int argc, char *argv[])
{
int c;
int max = 1000000;
int duration = 10;
while ((c = getopt(argc, argv, "fn:t:l:")) != -1) {
switch (c) {
case 'f':
skflags = SOCK_FD_FASTALLOC;
break;
case 'n':
max = atoi(optarg);
break;
case 't':
nbthreads_req = atoi(optarg);
if (nbthreads_req > NBTHREADS_MAX)
usage(1);
break;
case 'l':
duration = atoi(optarg);
break;
default:
usage(1);
}
}
system("sysctl -w fs.file-max=8000000");
expand_fd_array(max);
launch_threads();
sleep(duration);
stop_all = 1;
wait_end();
}
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