Linker performance related to swap space?

Question

Sometimes it\'s handy to mock up something with a little C program that uses a big chunk of static memory. I noticed after changing to Fedora 15 the program took a long

Answer 1

I don't observe this behavior (with Debian/Sid/AMD64 on a 8Gb desktop, gcc 4.6.2, binutils gold ld (GNU Binutils for Debian 2.22) 1.11). Here is the changed program (displaying its memory map with pmap).

#include <string.h>
#include <stdlib.h>
#include <stdio.h>
#define M 1000000
#define GIANT_SIZE (2000*M)
size_t g_arr[GIANT_SIZE];
int main( int argc, char **argv){   
  int i;
  char cmd[80];
  for(i = 0; i<10; i++){
      printf("This should be zero: %d\n",g_arr[i*1000]);
  }
  sprintf (cmd, "pmap %d", (int)getpid());
  system(cmd);
  exit(0);
}

Here is its compilation:

% time gcc -v -O big.c -o big
Using built-in specs.
COLLECT_GCC=/usr/bin/gcc-4.6.real
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/4.6/lto-wrapper
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Debian 4.6.2-4' --with-bugurl=file:///usr/share/doc/gcc-4.6/README.Bugs --enable-languages=c,c++,fortran,objc,obj-c++,go --prefix=/usr --program-suffix=-4.6 --enable-shared --enable-linker-build-id --with-system-zlib --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --with-gxx-include-dir=/usr/include/c++/4.6 --libdir=/usr/lib --enable-nls --enable-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx-time=yes --enable-plugin --enable-objc-gc --with-arch-32=i586 --with-tune=generic --enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu
Thread model: posix
gcc version 4.6.2 (Debian 4.6.2-4) 
COLLECT_GCC_OPTIONS='-v' '-O' '-o' 'big' '-mtune=generic' '-march=x86-64'
 /usr/lib/gcc/x86_64-linux-gnu/4.6/cc1 -quiet -v -imultilib . -imultiarch x86_64-linux-gnu big.c -quiet -dumpbase big.c -mtune=generic -march=x86-64 -auxbase big -O -version -o /tmp/ccWThBP5.s
GNU C (Debian 4.6.2-4) version 4.6.2 (x86_64-linux-gnu)
    compiled by GNU C version 4.6.2, GMP version 5.0.2, MPFR version 3.1.0, MPC version 0.9
warning: MPFR header version 3.1.0 differs from library version 3.1.0-p3.
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
ignoring nonexistent directory "/usr/local/include/x86_64-linux-gnu"
ignoring nonexistent directory "/usr/lib/gcc/x86_64-linux-gnu/4.6/../../../../x86_64-linux-gnu/include"
#include "..." search starts here:
#include <...> search starts here:
 /usr/lib/gcc/x86_64-linux-gnu/4.6/include
 /usr/local/include
 /usr/lib/gcc/x86_64-linux-gnu/4.6/include-fixed
 /usr/include/x86_64-linux-gnu
 /usr/include
End of search list.
GNU C (Debian 4.6.2-4) version 4.6.2 (x86_64-linux-gnu)
    compiled by GNU C version 4.6.2, GMP version 5.0.2, MPFR version 3.1.0, MPC version 0.9
warning: MPFR header version 3.1.0 differs from library version 3.1.0-p3.
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
Compiler executable checksum: 4b128876859f8f310615c7040fa3cb67
COLLECT_GCC_OPTIONS='-v' '-O' '-o' 'big' '-mtune=generic' '-march=x86-64'
 as --64 -o /tmp/ccm7905b.o /tmp/ccWThBP5.s
COMPILER_PATH=/usr/lib/gcc/x86_64-linux-gnu/4.6/:/usr/lib/gcc/x86_64-linux-gnu/4.6/:/usr/lib/gcc/x86_64-linux-gnu/:/usr/lib/gcc/x86_64-linux-gnu/4.6/:/usr/lib/gcc/x86_64-linux-gnu/
LIBRARY_PATH=/usr/lib/gcc/x86_64-linux-gnu/4.6/:/usr/lib/gcc/x86_64-linux-gnu/4.6/../../../x86_64-linux-gnu/:/usr/lib/gcc/x86_64-linux-gnu/4.6/../../../../lib/:/lib/x86_64-linux-gnu/:/lib/../lib/:/usr/lib/x86_64-linux-gnu/:/usr/lib/../lib/:/usr/lib/gcc/x86_64-linux-gnu/4.6/../../../:/lib/:/usr/lib/
COLLECT_GCC_OPTIONS='-v' '-O' '-o' 'big' '-mtune=generic' '-march=x86-64'
 /usr/lib/gcc/x86_64-linux-gnu/4.6/collect2 --build-id --no-add-needed --eh-frame-hdr -m elf_x86_64 --hash-style=both -dynamic-linker /lib64/ld-linux-x86-64.so.2 -o big /usr/lib/gcc/x86_64-linux-gnu/4.6/../../../x86_64-linux-gnu/crt1.o /usr/lib/gcc/x86_64-linux-gnu/4.6/../../../x86_64-linux-gnu/crti.o /usr/lib/gcc/x86_64-linux-gnu/4.6/crtbegin.o -L/usr/lib/gcc/x86_64-linux-gnu/4.6 -L/usr/lib/gcc/x86_64-linux-gnu/4.6/../../../x86_64-linux-gnu -L/usr/lib/gcc/x86_64-linux-gnu/4.6/../../../../lib -L/lib/x86_64-linux-gnu -L/lib/../lib -L/usr/lib/x86_64-linux-gnu -L/usr/lib/../lib -L/usr/lib/gcc/x86_64-linux-gnu/4.6/../../.. /tmp/ccm7905b.o -lgcc --as-needed -lgcc_s --no-as-needed -lc -lgcc --as-needed -lgcc_s --no-as-needed /usr/lib/gcc/x86_64-linux-gnu/4.6/crtend.o /usr/lib/gcc/x86_64-linux-gnu/4.6/../../../x86_64-linux-gnu/crtn.o
gcc -v -O big.c -o big  0.07s user 0.01s system 90% cpu 0.089 total

and its execution:

  % time ./big
 This should be zero: 0
 This should be zero: 0
 This should be zero: 0
 This should be zero: 0
 This should be zero: 0
 This should be zero: 0
 This should be zero: 0
 This should be zero: 0
 This should be zero: 0
 This should be zero: 0
 8835:   ./big
 0000000000400000      4K r-x--  /home/basile/tmp/big
 0000000000401000      4K rw---  /home/basile/tmp/big
 0000000000402000 15625000K rw---    [ anon ]
 00007f2d15a44000   1512K r-x--  /lib/x86_64-linux-gnu/libc-2.13.so
 00007f2d15bbe000   2048K -----  /lib/x86_64-linux-gnu/libc-2.13.so
 00007f2d15dbe000     16K r----  /lib/x86_64-linux-gnu/libc-2.13.so
 00007f2d15dc2000      4K rw---  /lib/x86_64-linux-gnu/libc-2.13.so
 00007f2d15dc3000     20K rw---    [ anon ]
 00007f2d15dc8000    124K r-x--  /lib/x86_64-linux-gnu/ld-2.13.so
 00007f2d15fb4000     12K rw---    [ anon ]
 00007f2d15fe4000     12K rw---    [ anon ]
 00007f2d15fe7000      4K r----  /lib/x86_64-linux-gnu/ld-2.13.so
 00007f2d15fe8000      4K rw---  /lib/x86_64-linux-gnu/ld-2.13.so
 00007f2d15fe9000      4K rw---    [ anon ]
 00007ffff5b5b000    132K rw---    [ stack ]
 00007ffff5bff000      4K r-x--    [ anon ]
 ffffffffff600000      4K r-x--    [ anon ]
  total         15628908K
 ./big  0.00s user 0.00s system 0% cpu 0.004 total

I believe that installing a recent GCC (e.g. a GCC 4.6) with a binutils Gold linker is significant for such programs.

I don't hear any swapping involved.

Answer 2

I am able to reproduce this on an Ubuntu 10.10 system (GNU ld (GNU Binutils for Ubuntu) 2.20.51-system.20100908), and I think I have your answer. First, some methodology.

After confirming this happens to me in a small VM (512MB ram, 2GB swap), from here I decided the easiest thing to do would be to strace gcc and see what exactly was going on when everything went to hell:

~# strace -f gcc swap.c

It illuminated the following:

vfork()                                 = 3589
[pid  3589] execve("/usr/lib/gcc/x86_64-linux-gnu/4.4.5/collect2", ["/usr/lib/gcc/x86_64-linux-gnu/4."..., "--build-id", "--eh-frame-hdr", "-m", "elf_x86_64", "--hash-style=gnu", "-dynamic-linker", "/lib64/ld-linux-x86-64.so.2", "-o", "swap", "-z", "relro", "/usr/lib/gcc/x86_64-linux-gnu/4."..., "/usr/lib/gcc/x86_64-linux-gnu/4."..., "/usr/lib/gcc/x86_64-linux-gnu/4."..., "-L/usr/lib/gcc/x86_64-linux-gnu/"..., ...], [/* 26 vars */]) = 0

...

[pid  3589] vfork()                     = 3590

...

[pid  3590] execve("/usr/bin/ld", ["/usr/bin/ld", "--build-id", "--eh-frame-hdr", "-m", "elf_x86_64", "--hash-style=gnu", "-dynamic-linker", "/lib64/ld-linux-x86-64.so.2", "-o", "swap", "-z", "relro", "/usr/lib/gcc/x86_64-linux-gnu/4."..., "/usr/lib/gcc/x86_64-linux-gnu/4."..., "/usr/lib/gcc/x86_64-linux-gnu/4."..., "-L/usr/lib/gcc/x86_64-linux-gnu/"..., ...], [/* 27 vars */]) = 0     

...

[pid  3590] lseek(13, 4096, SEEK_SET)   = 4096
[pid  3590] read(13, ".\4@\0\0\0\0\0>\4@\0\0\0\0\0N\4@\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 4096) = 4096
[pid  3590] mmap(NULL, 1600004096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f1771931000
<system comes to screeching halt>

It would appear that, as we might have suspected, it looks like ld is actually trying to anonymously mmap the entire static memory space of this array (or possibly the entire program, it's hard to tell since the rest of the program is so small, it might all fit in that extra 4096).

So that's all well and good, but why does it work when we exceed the available swap on the system? Let's turn swapoff and run strace -f again...

[pid  3618] lseek(13, 4096, SEEK_SET)   = 4096
[pid  3618] read(13, ".\4@\0\0\0\0\0>\4@\0\0\0\0\0N\4@\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 4096) = 4096
[pid  3618] mmap(NULL, 1600004096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = -1 ENOMEM (Cannot allocate memory)
[pid  3618] brk(0x60638000)             = 0x1046000
[pid  3618] mmap(NULL, 1600135168, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = -1 ENOMEM (Cannot allocate memory)
[pid  3618] mmap(NULL, 134217728, PROT_NONE, MAP_PRIVATE|MAP_ANONYMOUS|MAP_NORESERVE, -1, 0) = 0x7fd011864000

...

Unsurprisingly, ld seems to do the same thing it tried last time, to mmap the entire space. but the system is no longer able to do that, it fails! ld tries again, and it fails again, then ld does something unexpected... it moves on with less memory.

Weird, I guess we'd better have a look at the ld code then. Drat, it doesn't do an explicit mmap. This must be coming from inside of a plain old malloc. We'll have to build ld with some debug symbols to track this down. Unfortunately, when I built bin-utils 2.21.1 the problem went away. Perhap it's been fixed in newer versions of bin-utils?

Answer 3

I tortured tested my OpenSuse 11.4 (going for 12.1 in a week)

I have 4GiB ram + 2GiB swap and did not notice serious slow down, the system might be trashing at times, but still the compile time was short.

The longest was 6 seconds while heavy swapping.

[tester@ulises ~]$ free -m
             total       used       free     shared    buffers     cached
Mem:          3456       3426         30          0          4        249
-/+ buffers/cache:       3172        284
Swap:         2055       1382        672
[tester@ulises ~]$ time cc -Wall -O test2.c
test2.c: In function ‘main’:
test2.c:13:2: warning: format ‘%d’ expects type ‘int’, but argument 2 has type ‘size_t’

real    0m6.501s
user    0m0.101s
sys     0m0.078s
[tester@ulises ~]$ free -m
             total       used       free     shared    buffers     cached
Mem:          3456       3389         67          0          5        289
-/+ buffers/cache:       3094        362
Swap:         2055       1455        599
[tester@ulises ~]$ free -m
             total       used       free     shared    buffers     cached
Mem:          3456       3373         82          0          4        264
-/+ buffers/cache:       3104        352
Swap:         2055       1442        612
[tester@ulises ~]$ time cc -Wall -O test2.c
test2.c: In function ‘main’:
test2.c:13:2: warning: format ‘%d’ expects type ‘int’, but argument 2 has type ‘size_t’

real    0m1.122s
user    0m0.086s
sys     0m0.045s
[tester@ulises ~]$ time cc -Wall -O test2.c
test2.c: In function ‘main’:
test2.c:13:2: warning: format ‘%d’ expects type ‘int’, but argument 2 has type ‘size_t’

real    0m0.095s
user    0m0.047s
sys     0m0.032s
[tester@ulises ~]$ free -m
             total       used       free     shared    buffers     cached
Mem:          3456       3376         79          0          4        252
-/+ buffers/cache:       3119        336
Swap:         2055       1436        618
[tester@ulises ~]$ time cc -Wall -O test2.c
test2.c: In function ‘main’:
test2.c:13:2: warning: format ‘%d’ expects type ‘int’, but argument 2 has type ‘size_t’

real    0m0.641s
user    0m0.054s
sys     0m0.040s

Between running I have loaded and unloaded Virtualbox Box VM's, Eclipse, large pdf files, mi firefox alone using 800+ MiB. I didi not go the limit, otherwise many Apps would be killed by the OS. It has a preference for killing Firefox.. :-)

I also went to the extreme defining:

#define M 1048576
#define GIANT_SIZE (20000*M)

and even then nothing change significantly.

[tester@ulises ~]$ time cc -Wall -O test2.c
test2.c:7:14: warning: integer overflow in expression
test2.c:7:8: error: size of array ‘g_arr’ is negative
test2.c:7:1: warning: variably modified ‘g_arr’ at file scope
test2.c: In function ‘main’:
test2.c:13:2: warning: format ‘%d’ expects type ‘int’, but argument 2 has type ‘size_t’

real    0m0.661s
user    0m0.043s
sys     0m0.031s

Edit: I re-tested using Fedora16 on a VM with 512MiB RAM and 1.5GiB swap, and things were similar except for an error message on my "maximum stress version" where 20000 megabytes were assigned to the array. The error say the array size was negative.

[ricardo@localhost ~]$ time gcc -Wall test2.c 
test2.c:7:14: warning: integer overflow in expression [-Woverflow]
test2.c:7:8: error: size of array ‘g_arr’ is negative
test2.c:7:1: warning: variably modified ‘g_arr’ at file scope [enabled by default]
test2.c: In function ‘main’:
test2.c:13:2: warning: format ‘%d’ expects argument of type ‘int’, but argument 2 has type ‘size_t’ [-Wformat]

real    0m1.053s
user    0m0.050s
sys     0m0.137s

The same response happens in opensuse 12.1 VM. The Fedora 16 install seamed verry slow and memory hungry(during install I had to use 800MiB versus OpenSuse 512 MiB), I could not use swapoff on Fedora because it was using a lot of swap space. I had not sluggishness nor memory problems on OpenSuse 12.1 and . Both have essentially the same versions of kernel, gcc, etc. Both using stock installs with KDE as the Desktop environment

I could not reproduce you issues, Maybe is a gcc related issue. Try downloading an older version like 4.5 and see what happens