Ingo Molnar wrote:

>On Sun, 16 Mar 2003, Manfred Spraul wrote:
>
>  
>
>>Below is a proposal to get rid of the quadratic behaviour of
>>proc_pid_readir(): Instead of storing the task number in f_pos and
>>walking tasks by tasklist order, the pid is stored in f_pos and the
>>tasks are walked by (hash-mangled) pid order.
>>    
>>
>
>have you seen my "procfs/procps threading performance speedup" patch? It
>does something like this.
>  
>
The implementation is flawed:

>+static int get_pid_list(int index, int *pids, struct file *filp)
> {
>-	struct task_struct *p;
>-	int nr_pids = 0;
>+	int nr_pids = 0, pid = 0, pid_cursor = (int)filp->private_data;
>+	struct task_struct *g = NULL, *p = NULL;
> 
>-	index--;
> 	read_lock(&tasklist_lock);
>-	for_each_process(p) {
>-		int pid = p->pid;
>-		if (!pid)
>-			continue;
>+	if (pid_cursor) {
>+		p = find_task_by_pid(pid_cursor);
>+		g = p->group_leader;
>+	}
>+	if (!p) {
>+		g = p = &init_task;
>+		index--;
>+	} else
>+		index = 0;
>+
>+	goto inside;
>+
>+	__do_each_thread(g, p) {
> 		if (--index >= 0)
> 			continue;
>-		pids[nr_pids] = pid;
>+		pid = p->pid;
>+		if (!pid)
>+			BUG();
>+		if (p->tgid != p->pid)
>+			pids[nr_pids] = -pid;
>+		else
>+			pids[nr_pids] = pid;
> 		nr_pids++;
> 		if (nr_pids >= PROC_MAXPIDS)
>-			break;
>-	}
>+			goto out;
>+inside:
>+		;
>+	} while_each_thread(g, p);
>+out:
>+	filp->private_data = (void *)pid;
> 	read_unlock(&tasklist_lock);
>+
> 	return nr_pids;
> }
> 
>
get_pid_list stores up to 20 pids in an array, and the last pid in the 
array is stored in filp->private_data, for the next call.

> int proc_pid_readdir(struct file * filp, void * dirent, filldir_t filldir)
> {
>-	unsigned int pid_array[PROC_MAXPIDS];
>+	int pid_array[PROC_MAXPIDS];
> 	char buf[PROC_NUMBUF];
> 	unsigned int nr = filp->f_pos - FIRST_PROCESS_ENTRY;
> 	unsigned int nr_pids, i;
>@@ -1192,14 +1217,16 @@ int proc_pid_readdir(struct file * filp,
> 		nr++;
> 	}
> 
>-	nr_pids = get_pid_list(nr, pid_array);
>+	nr_pids = get_pid_list(nr, pid_array, filp);
> 
> 	for (i = 0; i < nr_pids; i++) {
>-		int pid = pid_array[i];
>+		int pid = abs(pid_array[i]);
> 		ino_t ino = fake_ino(pid,PROC_PID_INO);
> 		unsigned long j = PROC_NUMBUF;
> 
> 		do buf[--j] = '0' + (pid % 10); while (pid/=10);
>+		if (pid_array[i] < 0)
>+			buf[--j] = '.';
> 
> 		if (filldir(dirent, buf+j, PROC_NUMBUF-j, filp->f_pos, ino, DT_DIR) < 0)
> 			break;
>  
>
But:  proc_pid_readdir() doesn't use all 20 pids: It may return early if 
the user space buffer is full. Then your patch skips a few threads. 
Attached is my test app:

./getdents /proc 0 24 returns just 3 threads,
./getdents /proc 0 4000 returns (probably) all: 66 threads or processes.

--
    Manfred