最初の C++ (m) 割り当てが常に 72 KB になるのはなぜですか?

LD_PRELOAD=/home/joel/mymalloc/libmymalloc.so ls

$ LOG_ALLOC=log.txt LD_PRELOAD=/home/joel/mymalloc/libmymalloc.so ls

$ head -n 1 log.txt
73728

$ gdb --args ls
...
(gdb) set environment LD_PRELOAD=/home/joel/mymalloc/libmymalloc.so
(gdb) b MallocWrapper.cpp:malloc
...
(gdb) r
Starting program: /usr/bin/ls
Breakpoint 1, malloc (size=73728) at src/MallocWrapper.cpp:44
...
(gdb) bt
#0  malloc (size=73728) at src/MallocWrapper.cpp:44
#1  0x00007ffff78bd17f in ?? () from /lib/x86_64-linux-gnu/libstdc++.so.6
#2  0x00007ffff7fca71f in call_init (l=, argc=argc@entry=1, argv=argv@entry=0x7fffffffdce8, env=env@entry=0x7fffffffdcf8) at ./elf/dl-init.c:74
#3  0x00007ffff7fca824 in call_init (env=, argv=, argc=, l=) at ./elf/dl-init.c:120
#4  _dl_init (main_map=0x7ffff7ffe2e0, argc=1, argv=0x7fffffffdce8, env=0x7fffffffdcf8) at ./elf/dl-init.c:121
#5  0x00007ffff7fe45a0 in _dl_start_user () from /lib64/ld-linux-x86-64.so.2
# ...

(gdb) info sharedlibrary
From                To                  Syms Read   Shared Object Library
...
0x00007ffff78aa600  0x00007ffff79eef42  Yes (*)     /lib/x86_64-linux-gnu/libstdc++.so.6
...
(*): Shared library is missing debugging information.
(gdb) add-symbol-file /lib/x86_64-linux-gnu/debug/libstdc++.so.6 0x00007ffff78aa600
...
Reading symbols from /lib/x86_64-linux-gnu/debug/libstdc++.so.6...
(gdb) bt
#0  malloc (size=73728) at src/MallocWrapper.cpp:44
#1  0x00007ffff78bd17f in __gnu_cxx::__pool_alloc_base::_M_allocate_chunk (this=0x7ffff79ef08d <:filesystem::filesystem_error::_impl::>, __n=8, __nobjs=)
#   at ../../../../../../src/libstdc++-v3/src/c++98/pool_allocator.cc:114
# ...

char* __pool_alloc_base::_M_allocate_chunk(size_t __n, int& __nobjs) {
  // ...
  __try {
    _S_start_free = static_castchar*>(::operator new(__bytes_to_get));
  } __catch(const std::bad_alloc&) { /* ... */ }
  // ...
}

pool::pool() noexcept {
  // ...
  arena_size = buffer_size_in_bytes(obj_count, obj_size);
  if (arena_size == 0)
    return;
  arena = (char *)malloc (arena_size);
  if (!arena) {
    // If the allocation failed go without an emergency pool.
    arena_size = 0;
    return;
  }

  // Populate the free-list with a single entry covering the whole arena
  first_free_entry = reinterpret_cast free_entry *> (arena);
  new (first_free_entry) free_entry;
  first_free_entry->size = arena_size;
  first_free_entry->next = NULL;
}

extern "C" void *
__cxxabiv1::__cxa_allocate_exception(std::size_t thrown_size) noexcept {
  // ..
  void *ret = malloc (thrown_size);

#if USE_POOL
  if (!ret)
    ret = emergency_pool.allocate (thrown_size);
#endif
  // ...
}

// The size of the buffer is N * (S * P + R + D), where:
// N == The number of objects to reserve space for.
//      Defaults to EMERGENCY_OBJ_COUNT, defined below.
// S == Estimated size of exception objects to account for.
//      This size is in units of sizeof(void*) not bytes.
//      Defaults to EMERGENCY_OBJ_SIZE, defined below.
// P == sizeof(void*).
// R == sizeof(__cxa_refcounted_exception).
// D == sizeof(__cxa_dependent_exception).

// ...
#define EMERGENCY_OBJ_SIZE 6
#define EMERGENCY_OBJ_COUNT (4 * __SIZEOF_POINTER__ * __SIZEOF_POINTER__)

$ GLIBCXX_TUNABLES=glibcxx.eh_pool.obj_count=10 \
  LOG_ALLOC=log.txt \
  LD_PRELOAD=/home/joel/mymalloc/libmymalloc.so \
  ls
$ head -n 1 log.txt
2880

==1174489== HEAP SUMMARY:
==1174489==     in use at exit: 0 bytes in 0 blocks
==1174489==   total heap usage: 1 allocs, 1 frees, 73,728 bytes allocated

==8511== HEAP SUMMARY:
==8511==     in use at exit: 72,704 bytes in 1 blocks
==8511==   total heap usage: 1 allocs, 0 frees, 72,704 bytes allocated
==8511==
==8511== LEAK SUMMARY:
==8511==    definitely lost: 0 bytes in 0 blocks
==8511==    indirectly lost: 0 bytes in 0 blocks
==8511==      possibly lost: 0 bytes in 0 blocks
==8511==    still reachable: 72,704 bytes in 1 blocks
==8511==         suppressed: 0 bytes in 0 blocks

/* g++ mangled __gnu_cxx::__freeres yields -> _ZN9__gnu_cxx9__freeresEv */
extern void _ZN9__gnu_cxx9__freeresEv(void) __attribute__((weak));
if (((to_run & VG_RUN__GNU_CXX__FREERES) != 0) &&
  (_ZN9__gnu_cxx9__freeresEv != NULL)) {
  _ZN9__gnu_cxx9__freeresEv();
}

namespace __gnu_cxx {
  __attribute__((cold)) void __freeres() noexcept {
#ifndef _GLIBCXX_EH_POOL_STATIC
    if (emergency_pool.arena) {
      ::free(emergency_pool.arena);
      emergency_pool.arena = 0;
    }
#endif
  }
}