I've already suggested it twice in the other thread, but I'm going to suggest it one last time: use strace to find out what's going on under the hood.

Had you done so in the first place (i.e. compared the strace output of a working environment against the non-working case), you'd have figured out within minutes, what took you almost a week doing it your way...

An example to illustrate (I'm using DBD::Pg here, as I don't have DBD::mysql installed — but the principle is the same):

$ strace -otrace_ok  perl -MDBD::Pg -e1 
$ ulimit -Sv 16000    # this simulates your non-working environment
$ strace -otrace_err perl -MDBD::Pg -e1 
Can't load '/usr/local/perl/5.8.8/lib/site_perl/5.8.8/x86_64-linux/aut
+o/DBD/Pg/Pg.so' for module DBD::Pg: libpq.so.3: failed to map segment
+ from shared object: Cannot allocate memory at /usr/local/perl/5.8.8/
+lib/5.8.8/x86_64-linux/DynaLoader.pm line 230.
 at -e line 0
Compilation failed in require.
BEGIN failed--compilation aborted.

[download]

The relevant parts of the traces clearly show that it's the mmap which would allocate the memory for the shared lib libpq.so.3 that is failing —> "ENOMEM (Cannot allocate memory)".

Depending on the exact memory limit you set, you can make it fail sooner or later, i.e. for libpq.so.3, libnsl.so.1, libz.so.1, or whatever other shared lib dependency is involved... 

----- working case -----
(...)
open("/usr/local/perl/5.8.8/lib/site_perl/5.8.8/x86_64-linux/auto/DBD/
+Pg/Pg.so", O_RDONLY) = 4
read(4, "\177ELF\2\1\1\0\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0\300_\0\0"..., 
+832) = 832
fstat(4, {st_mode=S_IFREG|0755, st_size=93669, ...}) = 0
mmap(NULL, 1127840, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 4,
+ 0) = 0x2b3b10e2a000
madvise(0x2b3b10e2a000, 1127840, MADV_SEQUENTIAL|0x1) = 0
mprotect(0x2b3b10e3b000, 1048576, PROT_NONE) = 0
mmap(0x2b3b10f3b000, 12288, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXE
+D|MAP_DENYWRITE, 4, 0x11000) = 0x2b3b10f3b000
close(4)                                = 0
open("/usr/local/postgresql-7.4.5/lib64/tls/x86_64/libpq.so.3", O_RDON
+LY) = -1 ENOENT (No such file or directory)
open("/usr/local/postgresql-7.4.5/lib64/tls/libpq.so.3", O_RDONLY) = -
+1 ENOENT (No such file or directory)
open("/usr/local/postgresql-7.4.5/lib64/x86_64/libpq.so.3", O_RDONLY) 
+= -1 ENOENT (No such file or directory)
open("/usr/local/postgresql-7.4.5/lib64/libpq.so.3", O_RDONLY) = 4
read(4, "\177ELF\2\1\1\0\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0\0o\0\0\0"..., 
+832) = 832
fstat(4, {st_mode=S_IFREG|0755, st_size=131049, ...}) = 0
mmap(NULL, 1158680, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 4,
+ 0) = 0x2b3b10f3e000
madvise(0x2b3b10f3e000, 1158680, MADV_SEQUENTIAL|0x1) = 0
mprotect(0x2b3b10f56000, 1044480, PROT_NONE) = 0
mmap(0x2b3b11055000, 16384, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXE
+D|MAP_DENYWRITE, 4, 0x17000) = 0x2b3b11055000
close(4)                                = 0
(...)

----- out of memory case -----
(...)
open("/usr/local/perl/5.8.8/lib/site_perl/5.8.8/x86_64-linux/auto/DBD/
+Pg/Pg.so", O_RDONLY) = 4
read(4, "\177ELF\2\1\1\0\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0\300_\0\0"..., 
+832) = 832
fstat(4, {st_mode=S_IFREG|0755, st_size=93669, ...}) = 0
mmap(NULL, 1127840, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 4,
+ 0) = 0x2b7780b99000
madvise(0x2b7780b99000, 1127840, MADV_SEQUENTIAL|0x1) = 0
mprotect(0x2b7780baa000, 1048576, PROT_NONE) = 0
mmap(0x2b7780caa000, 12288, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXE
+D|MAP_DENYWRITE, 4, 0x11000) = 0x2b7780caa000
close(4)                                = 0
open("/usr/local/postgresql-7.4.5/lib64/tls/x86_64/libpq.so.3", O_RDON
+LY) = -1 ENOENT (No such file or directory)
open("/usr/local/postgresql-7.4.5/lib64/tls/libpq.so.3", O_RDONLY) = -
+1 ENOENT (No such file or directory)
open("/usr/local/postgresql-7.4.5/lib64/x86_64/libpq.so.3", O_RDONLY) 
+= -1 ENOENT (No such file or directory)
open("/usr/local/postgresql-7.4.5/lib64/libpq.so.3", O_RDONLY) = 4
read(4, "\177ELF\2\1\1\0\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0\0o\0\0\0"..., 
+832) = 832
fstat(4, {st_mode=S_IFREG|0755, st_size=131049, ...}) = 0
mmap(NULL, 1158680, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 4,
+ 0) = -1 ENOMEM (Cannot allocate memory)
close(4)                                = 0
munmap(0x2b7780b99000, 1127840)         = 0
(...)

[download]

The second parameter of mmap also shows that the allocation of 1158680 bytes had been attempted.

If the system call says ENOMEM, then you can be rather sure there is not enough memory.  But you could still look at the individual mmapped sizes to see if there's something unexpectedly large among them...

In reply to Re^3: How to determine what the memory-limits of a CGI app are? by almut
in thread How to determine what the memory-limits of a CGI app are? [SOLVED] by locked_user sundialsvc4

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