Gprof results: what is “alloc_mmap”?

Question

My results for a short run of my program are as follows:

 67.93      3.24     3.24                             grid::rKfour(int, int)
  9.43      3.69     0.45                             alloc_mmap
  5.03      3.93     0.24    30001     0.01     0.01  grid::timeStep()
  3.04      4.08     0.15 42007105     0.00     0.00  linkers::linkers(linkers const&)
  2.94      4.22     0.14  6360900     0.00     0.00  particle::fulldistance(particle&)
  2.73      4.35     0.13                             blas_thread_server
...

The output from ldd is

linux-vdso.so.1 =>  (0x00007fffe2bea000)
    libpthread.so.0 => /lib/x86_64-linux-gnu/libpthread.so.0 (0x00007eff34595000)
    libboost_filesystem.so.1.46.1 => /usr/lib/libboost_filesystem.so.1.46.1 (0x00007eff34377000)
    libboost_system.so.1.46.1 => /usr/lib/libboost_system.so.1.46.1 (0x00007eff34172000)
    libGL.so.1 => /usr/lib/x86_64-linux-gnu/mesa/libGL.so.1 (0x00007eff33f16000)
    libglut.so.3 => /usr/lib/libglut.so.3 (0x00007eff33cd0000)
    libGLU.so.1 => /usr/lib/x86_64-linux-gnu/libGLU.so.1 (0x00007eff33a62000)
    libGLEW.so.1.5 => /usr/lib/libGLEW.so.1.5 (0x00007eff3380c000)
    libboost_thread.so.1.46.1 => /usr/lib/libboost_thread.so.1.46.1 (0x00007eff335f3000)
    libstdc++.so.6 => /usr/lib/x86_64-linux-gnu/libstdc++.so.6 (0x00007eff332eb000)
    libm.so.6 => /lib/x86_64-linux-gnu/libm.so.6 (0x00007eff33067000)
    libgcc_s.so.1 => /lib/x86_64-linux-gnu/libgcc_s.so.1 (0x00007eff32e51000)
    libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007eff32ab1000)
    /lib64/ld-linux-x86-64.so.2 (0x00007eff347c4000)
    libglapi.so.0 => /usr/lib/x86_64-linux-gnu/libglapi.so.0 (0x00007eff3288d000)
    libX11.so.6 => /usr/lib/x86_64-linux-gnu/libX11.so.6 (0x00007eff32555000)
    libXext.so.6 => /usr/lib/x86_64-linux-gnu/libXext.so.6 (0x00007eff32341000)
    libXdamage.so.1 => /usr/lib/x86_64-linux-gnu/libXdamage.so.1 (0x00007eff3213e000)
    libXfixes.so.3 => /usr/lib/x86_64-linux-gnu/libXfixes.so.3 (0x00007eff31f38000)
    libXxf86vm.so.1 => /usr/lib/x86_64-linux-gnu/libXxf86vm.so.1 (0x00007eff31d31000)
    libdrm.so.2 => /usr/lib/x86_64-linux-gnu/libdrm.so.2 (0x00007eff31b26000)
    libdl.so.2 => /lib/x86_64-linux-gnu/libdl.so.2 (0x00007eff31922000)
    libxcb.so.1 => /usr/lib/x86_64-linux-gnu/libxcb.so.1 (0x00007eff31705000)
    librt.so.1 => /lib/x86_64-linux-gnu/librt.so.1 (0x00007eff314fd000)
    libXau.so.6 => /usr/lib/x86_64-linux-gnu/libXau.so.6 (0x00007eff312f9000)
    libXdmcp.so.6 => /usr/lib/x86_64-linux-gnu/libXdmcp.so.6 (0x00007eff310f3000)

Can anybody identify "alloc_mmap"?

Answer 1

I assume you're asking because you want to see what you could do to improve the program's speed.
If not, forget this.

In gprof output, the number that matters is the second column, cumulative seconds, because if that routine could be made to take no time, that is the amount by which your total time would shrink.

One of the problems with gprof is it ignores blocked time like I/O. Since your program is using alloc_mmap (directly or indirectly) it is mapping a file to memory, so it is doing I/O, which is often not a small cost. gprof doesn't see it.

There are more problems with gprof. If you are on linux, you could try a profiler like Zoom. It samples on wall-clock time, so it is not blind to I/O. It also gives you percent time usage by line/instruction, not just by function, so it will pinpoint the lines in your code that, if you could improve/remove them, would give you the most speedup. (Usually these are function calls. "Self time" is rarely relevant except in heavy math or tight CPU loops, and it doesn't matter anyway. Zoom will spot it.)

The method I rely on is this.

Answer 2

Maybe your memory allocator is using mmap for large allocations. You should first confirm this (gdb breakpoint in alloc_mmap should work) and perhaps increase the threshold with mallopt.