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SubjectRe: [Q] cannot fork w/ 1000s of procs (but still mem avail.)

hopefully a simple question to answer: I get "cannot fork" messages on my
machine running some 20000 processes and threads (1 master proc, 3 threads)
where each (master) process opens a socket and does IP traffic over it.
Although there is plenty of memory left (4GB box, 2GB used, 0 swap), I get
"cannot fork - out of memory" when trying to increase the number of procs.
(If none of the procs does IP, I can start more [of course?!].)
Anything I can do to increase the number of active processes using IP? Any
kernel paramter, limit, sizing?

You run out of process numbers, I suppose.
pid is 15 bits only - look at getpid() in fork.c.
It is very easy to make it 31 bits instead, and one of my machines
has been running a system with 31 bit pids for a long time.

(Note, I am not talking about sizeof(pid_t) but just about code like

if((++last_pid) & 0xffff8000) {
last_pid = 300; /* Skip daemons etc. */
goto inside;
}

that must be removed.)

I sent Linus a patch once or twice, will include it below if I can
find it.

Andries


Yes, found it [hand-edited to remove unrelated stuff].
I have not checked whether it still applies and still is correct.

==================================================================
>From aeb Sun Jan 23 22:47:48 2000
To: alan@lxorguk.ukuu.org.uk, torvalds@transmeta.com
Subject: [PATCH] large pid and a few other fixes

Below a patch with 31-bit pids and various compilation fixes.
The fork patch makes a fork measurably faster on a system
with more than 5000 processes.

I started making CONFIG_15BIT_PID a config option,
but decided against it as long as no need had been established.
(Yesterday or so I did a grep on "all Linux sources" for
msqid_ds and shmid_ds, more in particular msg_lspid, msg_lrpid,
shm_cpid, shm_lpid, and these are used in only a handful of places,
but I have not yet studied these places. Maybe it will turn out
to be necessary to have a config option after all, say for ibcs users,
we'll see. In case you like the opposite default, change the
#ifdef CONFIG_15BIT_PID
into
#if 1
or so, below in threads.h. (But as long as 2.4 has not been released
some testing is good, to see the effects of a larger pid.
Maybe someone will adapt ps.)

Andries



diff -u --recursive --new-file ../linux-2.3.40/linux/fs/proc/base.c ./linux/fs/proc/base.c
--- ../linux-2.3.40/linux/fs/proc/base.c Fri Jan 7 21:59:42 2000
+++ ./linux/fs/proc/base.c Fri Jan 21 18:50:53 2000
@@ -29,9 +29,12 @@
* inumbers of the rest of procfs (currently those are in 0x0000--0xffff).
* As soon as we'll get a separate superblock we will be able to forget
* about magical ranges too.
+ *
+ * For example, the define below may be replaced by
+ * #define fake_ino(pid,ino) 0x10000
*/

-#define fake_ino(pid,ino) (((pid)<<16)|(ino))
+#define fake_ino(pid,ino) (((1)<<16)|(ino))

ssize_t proc_pid_read_maps(struct task_struct*,struct file*,char*,size_t,loff_t*);
int proc_pid_stat(struct task_struct*,char*);
@@ -610,7 +613,9 @@
return 1;
p = base_stuff + i;
while (p->name) {
- if (filldir(dirent, p->name, p->len, filp->f_pos, fake_ino(pid, p->type)) < 0)
+ if (filldir(dirent, p->name, p->len,
+ filp->f_pos,
+ fake_ino(pid, p->type)) < 0)
return 0;
filp->f_pos++;
p++;
@@ -621,7 +626,8 @@

/* building an inode */

-static struct inode *proc_pid_make_inode(struct super_block * sb, struct task_struct *task, int ino)
+static struct inode *proc_pid_make_inode(struct super_block * sb,
+ struct task_struct *task, int ino)
{
struct inode * inode;

@@ -923,9 +929,7 @@
goto out;
pid *= 10;
pid += c;
- if (!pid)
- goto out;
- if (pid & 0xffff0000)
+ if (pid <= 0 || pid >= PID_MAX)
goto out;
}

diff -u --recursive --new-file ../linux-2.3.40/linux/include/linux/threads.h ./linux/include/linux/threads.h
--- ../linux-2.3.40/linux/include/linux/threads.h Tue Jan 11 03:29:07 2000
+++ ./linux/include/linux/threads.h Fri Jan 21 19:12:26 2000
@@ -15,8 +15,18 @@
#define MIN_THREADS_LEFT_FOR_ROOT 4

/*
- * This controls the maximum pid allocated to a process
+ * One more than the maximum pid allocated to a process
+ * (should be positive when assigned to an int)
*/
+#ifdef CONFIG_15BIT_PID
#define PID_MAX 0x8000
+#else
+#define PID_MAX 0x7fffffff
+#endif
+
+/*
+ * Place where we start again after a full cycle
+ */
+#define PID_MIN 300

#endif
diff -u --recursive --new-file ../linux-2.3.40/linux/kernel/fork.c ./linux/kernel/fork.c
--- ../linux-2.3.40/linux/kernel/fork.c Tue Dec 14 00:39:24 1999
+++ ./linux/kernel/fork.c Fri Jan 21 14:30:04 2000
@@ -194,26 +194,19 @@
return current->pid;

spin_lock(&lastpid_lock);
- if((++last_pid) & 0xffff8000) {
- last_pid = 300; /* Skip daemons etc. */
- goto inside;
- }
- if(last_pid >= next_safe) {
-inside:
- next_safe = PID_MAX;
+ if(++last_pid >= next_safe) {
read_lock(&tasklist_lock);
repeat:
+ if (last_pid >= PID_MAX)
+ last_pid = PID_MIN;
+ next_safe = PID_MAX;
+
for_each_task(p) {
if(p->pid == last_pid ||
p->pgrp == last_pid ||
- p->session == last_pid) {
- if(++last_pid >= next_safe) {
- if(last_pid & 0xffff8000)
- last_pid = 300;
- next_safe = PID_MAX;
- }
- goto repeat;
- }
+ p->session == last_pid)
+ if(++last_pid >= next_safe)
+ goto repeat;
if(p->pid > last_pid && next_safe > p->pid)
next_safe = p->pid;
if(p->pgrp > last_pid && next_safe > p->pgrp)
-
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