One way to figure that out is to attach gdb to a hanging fcgi process and get a 
backtrace (bt) from gdb which should show where it is stuck.

From you description I traced back to the line 1475 in ./etc/session/session.c,
it is the only line which will be affect by the php.ini variable session.gc_divisor you describe.

The line is show below:
nrand = (int) ((float) PS(gc_divisor) * php_combined_lcg(TSRMLS_C));

So, I think it may be a compiling error.

May be i expressed myself bad about the option session.gc_divisor. Setting the option to 100000 is nearly like disabling garbage collection. The chance to reproduce the problem is very, very low but possible.

How can i findout that it's a compile error?

Nevertheless i will make the backtrace.

Maybe you can post your backtrace result here first, i just guess it is a compiler error, cause php_combined_lcg function should have no error.

I'm sorry, i didn't get gdb running on the Asus Router (Flash too small).

But i've a strace for you, may be it helps:

accept(0, {sa_family=AF_FILE, NULL}, [2]) = 3
poll([{fd=3, events=POLLIN}], 1, 5000)  = 1 ([{fd=3, revents=POLLIN}])
read(3, "\1\1\0\1\0\10\0\0", 8)         = 8
read(3, "\0\1\0\0\0\0\0\0", 8)          = 8
read(3, "\1\4\0\1\3H\0\0", 8)           = 8
read(3, "\17\17SERVER_SOFTWARElighttpd/1.4.25"..., 840) = 840
read(3, "\1\4\0\1\0\0\0\0", 8)          = 8
time(NULL)                              = 946686768
lstat("/www/pc/power.php", {st_mode=S_IFREG|0644, st_size=8269, ...}) = 0
lstat("/www/pc", {st_mode=0, st_size=97, ...}) = 0
lstat("/www", {st_mode=S_IFDIR|0755, st_size=0, ...}) = 0
old_mmap(NULL, 266240, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, 0, 0) = 0x2afaa000
time(NULL)                              = 946686768
stat("/www/.user.ini", 0x7fff46e8)      = -1 ENOENT (No such file or directory)
stat("/www/pc/.user.ini", 0x7fff46e8)   = -1 ENOENT (No such file or directory)
setitimer(ITIMER_PROF, {it_interval={0, 0}, it_value={60, 0}}, NULL) = 0
rt_sigaction(SIGPROF, {0x10000000, [RT_68 RT_69 RT_76 RT_77 RT_79 RT_81 RT_82 RT_83 RT_84 RT_85 RT_86 RT_87 RT_88 RT_89 RT_90 RT_91 RT_92 RT_93 RT_94], SA_SIGINFO|0x66b990}, {SIG_DFL, [RT_65 RT_66], 0}, 16) = 0
rt_sigprocmask(SIG_UNBLOCK, [PROF], NULL, 16) = 0
time(NULL)                              = 946686768
open("/www/pc/power.php", O_RDONLY)     = 4
ioctl(4, TIOCNXCL, 0x7fff5888)          = -1 ENOTTY (Inappropriate ioctl for device)
brk(0x956000)                           = 0x956000
fstat(4, {st_mode=S_IFREG|0644, st_size=2606, ...}) = 0
read(4, "<?php\n\nrequire_once(\"/www/power_"..., 4096) = 2606
_llseek(4, 0, [0], SEEK_SET)            = 0
getcwd("", 4095)                        = 9
chdir("\35\2")                          = 0
setitimer(ITIMER_PROF, {it_interval={0, 0}, it_value={30, 0}}, NULL) = 0
ioctl(4, TIOCNXCL, 0x7fff3678)          = -1 ENOTTY (Inappropriate ioctl for device)
fstat(4, {st_mode=S_IFREG|0644, st_size=2606, ...}) = 0
old_mmap(NULL, 2638, PROT_READ, MAP_PRIVATE, 718909632, 0) = 0x2aaae000
_llseek(4, 0, [0], SEEK_SET)            = 0
munmap(0x2aaae000, 2606)                = 0
close(4)                                = 0
time(NULL)                              = 946686768
lstat("/www/power_logic.inc.php", {st_mode=S_IFREG|0644, st_size=2385, ...}) = 0
time(NULL)                              = 946686768
open("/www/power_logic.inc.php", O_RDONLY) = 4
fstat(4, {st_mode=S_IFCHR|S_ISUID|030000554, st_rdev=makedev(0, 0), ...}) = 0
fstat(4, {st_mode=05273160660, st_size=720167216, ...}) = 0
fstat(4, {st_mode=0, st_size=0, ...})   = 0
old_mmap(NULL, 2385, PROT_READ, MAP_SHARED, 4, 0) = 0x2aaae000
munmap(0x2aaae000, 2385)                = 0
close(4)                                = 0
brk(0x957000)                           = 0x957000
time(NULL)                              = 946686768
lstat("/www/defines.inc.php", {st_mode=S_IFREG|0644, st_size=5087, ...}) = 0
time(NULL)                              = 946686768
open("/www/defines.inc.php", O_RDONLY)  = 4
fstat(4, {st_mode=S_IFREG|0644, st_size=5087, ...}) = 0
fstat(4, {st_mode=0, st_size=0, ...})   = 0
fstat(4, {st_mode=S_IFREG|040000130, st_size=0, ...}) = 0
old_mmap(NULL, 5087, PROT_READ, MAP_SHARED, 4, 0) = 0x2aaae000
munmap(0x2aaae000, 5087)                = 0
close(4)                                = 0
brk(0x958000)                           = 0x958000
brk(0x959000)                           = 0x959000
time(NULL)                              = 946686769
lstat("/www/functions.inc.php", {st_mode=S_IFREG|0644, st_size=8586, ...}) = 0
time(NULL)                              = 946686769
open("/www/functions.inc.php", O_RDONLY) = 4
fstat(4, {st_mode=S_IFREG|0644, st_size=8586, ...}) = 0
fstat(4, {st_mode=S_IFREG|0644, st_size=18, ...}) = 0
fstat(4, {st_mode=S_IFREG|040000130, st_size=4, ...}) = 0
old_mmap(NULL, 8586, PROT_READ, MAP_SHARED, 4, 0) = 0x2aaae000
brk(0x95a000)                           = 0x95a000
munmap(0x2aaae000, 8586)                = 0
close(4)                                = 0
open("/tmp/sess_29fa4b85a40a392a7955de8cdc62d5f6", O_RDWR|O_CREAT, 0600) = 4
flock(4, LOCK_EX)                       = 0
fcntl(4, F_SETFD, FD_CLOEXEC)           = 0
fstat(4, {st_mode=S_IFSOCK|S_ISGID|05273600020, st_size=19, ...}) = 0
lseek(4, 0, SEEK_SET)                   = 0
read(4, "authentication|b:1;", 19)      = 19
gettimeofday({946686769, 156033}, NULL) = 0
getpid()                                = 789
gettimeofday({946686769, 156033}, NULL) = 0
open("/tmp", O_RDONLY|O_NONBLOCK|O_DIRECTORY) = 5
fstat(5, {st_mode=S_IFDIR|S_ISVTX|0777, st_size=360, ...}) = 0
fcntl(5, F_SETFD, FD_CLOEXEC)           = 0
brk(0x95b000)                           = 0x95b000
time([946686769])                       = 946686769
getdents64(5, /* 18 entries */, 4096)   = 560
rt_sigprocmask(SIG_SETMASK, NULL, [RT_0], 16) = 0
rt_sigsuspend([]

Looking at the strace, i can see that the garbage collector is invoked, after the current session is locked.

Is it possible that the garbage collector hangs because of the locked session?

Do you still have this problem or resolved by yourself?

In 2010, i could isolate the problem by writing my own session handler. The hanging appeared after the second call of the PHP function readdir(). The problem with readdir() could also reproduced outside the session handler:

Test script:

<?php

$dp = opendir("/tmp");

while (($file = readdir($dp)) !== FALSE)
{
    echo $file."< br>";
}

closedir($dp);

?>

Luckily the hanging didn't appear by using the DirectoryIterator. As a workaround, i written my own session handler with the DirectoryIterator.

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