Re: "Read my lips: no more merges" - aka Linux 2.6.14-rc1

[Eric Dumazet]

Benjamin LaHaise a écrit :
> On Tue, Sep 13, 2005 at 09:04:32AM +0200, Eric Dumazet wrote:
>
> > I wish a process param could allow open() to take any free fd available, 
> > not the lowest one. One can always use fcntl(fd, F_DUPFD, slot) to move a 
> > fd on a specific high slot and always keep the 64 first fd slots free to 
> > speedup the kernel part at open()/dup()/socket() time.
>
>
> The overhead is easy to avoid by making use of dup2() and close() to keep 
> the lowest file descriptors in the table free, allowing open() and socket() 
> to always return 3 or 4.

Yes, this is what I described :) Maybe this was not clear.

> Alternatively, the kernel could track available file descriptors using a 
> tree to efficiently insert freed slots into an ordered list of free 
> regions (something similar to the avl tree used in vmas).  Is it worth 
> doing?

Well no, since a user app can manage itself this part if it happens to be 
performance critical.


Sample of a user land lib : Each time a new fd is returned by 
open()/socket()/pipe()/accept()... the thread should call

fd = fdcache_dupfd(fd);

And close the file using  fdcache_closefd(fd) instead of close(fd);

Eric
/*
 * Unix kernel has an expensive get_unused_fd() function :
 * This is because semantics of Unix mandates that a open()/pipe()/socket()/ call always returns the lowest fd, not a random one.
 * Linux use a linear scan of a table of bits.
 * A program handling 1.000.000 files scans about 128 KB of ram, with a spinlock held : No other thread can get a fd.
 *
 * The trick is to use this library to make sure 64 low fds are available, so that the standard unix functions 
 * dont have to scan a lot of fd before finding a free one.
 * And remap them using fcntl(F_DUPFD) at precise slots we manage ourselfs.
 */
#include <pthread.h>
#include <fcntl.h>
#include <stdlib.h>
#include <unistd.h>

# define MAXFDS 1500000

struct {
	pthread_mutex_t lock;
	unsigned int cache_fd;
	unsigned int next_alloc;
	unsigned int *cache_tab;
	} fdd;


void fdcache_init()
{
	pthread_mutex_init(&fdd.lock, NULL);
	fdd.cache_tab = calloc(MAXFDS, sizeof(unsigned int));
	fdd.next_alloc = 64;
}

int fdcache_dupfd(int fd)
{
	int ret;
	pthread_mutex_lock(&fdd.lock);
	if (fdd.cache_fd == 0)
		fdd.cache_fd = fdd.next_alloc++;
	ret = fcntl(fd, F_DUPFD, fdd.cache_fd);
	if (ret != -1) {
		fdd.cache_fd = fdd.cache_tab[ret];
		pthread_mutex_unlock(&fdd.lock);
		close(fd);
		return ret;
	}
	else {
		pthread_mutex_unlock(&fdd.lock);
		return fd;
	}
}

void fdcache_closefd(int fd)
{
	if (fd == -1)
		return;

	close(fd);

	pthread_mutex_lock(&fdd.lock);
	fdd.cache_tab[fd] = fdd.cache_fd;
	fdd.cache_fd = fd;
	pthread_mutex_unlock(&fdd.lock);
}