Dave Chinner <david@xxxxxxxxxxxxx> wrote:
> On Sun, Dec 16, 2012 at 12:25:49AM +0000, Eric Wong wrote:
> > Alan Cox <alan@xxxxxxxxxxxxxxxxxxx> wrote:
> > > On Sat, 15 Dec 2012 00:54:48 +0000
> > > Eric Wong <normalperson@xxxxxxxx> wrote:
> > >
> > > > Applications streaming large files may want to reduce disk spinups and
> > > > I/O latency by performing large amounts of readahead up front
> > >
> > > How does it compare benchmark wise with a user thread or using the
> > > readahead() call ?
> >
> > Very well.
> >
> > My main concern is for the speed of the initial pread()/read() call
> > after open().
> >
> > Setting EARLY_EXIT means my test program _exit()s immediately after the
> > first pread(). In my test program (below), I wait for the background
> > thread to become ready before open() so I would not take overhead from
> > pthread_create() into account.
> >
> > RA=1 uses a pthread + readahead()
> > Not setting RA uses fadvise (with my patch)
>
> And if you don't use fadvise/readahead at all?
Sorry for the confusion. I believe my other reply to you summarized
what I wanted to say in my commit message and also reply to Alan.
I want all the following things:
- I want the first read to be fast.
- I want to read the whole file eventually (probably slowly,
as processing takes a while).
- I want to let my disk spin down for as long as possible.
This could also be a use case for an audio/video player.
> You're not timing how long the first pread() takes at all. You're
> timing the entire set of operations, including cloning a thread and
> for the readahead(2) call and messages to be passed back and forth
> through the eventfd interface to read the entire file.
You're right, I screwed up the measurement. Using clock_gettime(),
there's hardly a difference between the approaches and I can't
get consistent timings between them.
So no, there's no difference that matters between the approaches.
But I think doing this in the kernel is easier for userspace users.
---------------------------------- 8<----------------------------
/* gcc -O2 -Wall -lpthread -lrt -o first_read first_read.c */
#define _GNU_SOURCE
#include <sys/stat.h>
#include <sys/types.h>
#include <fcntl.h>
#include <errno.h>
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
#include <assert.h>
#include <sched.h>
#include <sys/eventfd.h>
#include <time.h>
static int efd1;
static int efd2;
static void clock_diff(struct timespec *a, const struct timespec *b)
{
a->tv_sec -= b->tv_sec;
a->tv_nsec -= b->tv_nsec;
if (a->tv_nsec < 0) {
--a->tv_sec;
a->tv_nsec += 1000000000;
}
}
static void * start_ra(void *unused)
{
struct stat st;
eventfd_t val;
int fd;
/* tell parent to open() */
assert(eventfd_write(efd1, 1) == 0);
/* wait for parent to tell us fd is ready */
assert(eventfd_read(efd2, &val) == 0);
fd = (int)val;
assert(fstat(fd, &st) == 0);
assert(readahead(fd, 0, st.st_size) == 0);
return NULL;
}
int main(int argc, char *argv[])
{
char buf[16384];
pthread_t thr;
int fd;
struct timespec start;
struct timespec finish;
char *do_ra = getenv("RA");
if (argc != 2) {
fprintf(stderr, "Usage: strace -T %s LARGE_FILE\n", argv[0]);
return 1;
}
if (do_ra) {
eventfd_t val;
efd1 = eventfd(0, 0);
efd2 = eventfd(0, 0);
assert(efd1 >= 0 && efd2 >= 0 && "eventfd failed");
assert(pthread_create(&thr, NULL, start_ra, NULL) == 0);
/* wait for child thread to spawn */
assert(eventfd_read(efd1, &val) == 0);
}
fd = open(argv[1], O_RDONLY);
assert(fd >= 0 && "open failed");
assert(clock_gettime(CLOCK_MONOTONIC, &start) == 0);
if (do_ra) {
/* wake up the child thread, give it a chance to run */
assert(eventfd_write(efd2, fd) == 0);
sched_yield();
} else
assert(posix_fadvise(fd, 0, 0, POSIX_FADV_WILLNEED) == 0);
assert(pread(fd, buf, sizeof(buf), 0) == sizeof(buf));
assert(clock_gettime(CLOCK_MONOTONIC, &finish) == 0);
clock_diff(&finish, &start);
fprintf(stderr, "elapsed: %lu.%09lu\n", finish.tv_sec, finish.tv_nsec);
if (getenv("FULL_READ")) {
ssize_t r;
do {
r = read(fd, buf, sizeof(buf));
} while (r > 0);
assert(r == 0 && "EOF not reached");
}
if (getenv("EXIT_EARLY"))
_exit(0);
if (do_ra) {
assert(pthread_join(thr, NULL) == 0);
assert(close(efd1) == 0);
assert(close(efd2) == 0);
}
assert(close(fd) == 0);
return 0;
}
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