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# sudo mount -t cgroup
cgroup on /sys/fs/cgroup/systemd type cgroup (rw,nosuid,nodev,noexec,relatime,xattr,release_agent=/usr/lib/systemd/systemd-cgroups-agent,name=systemd)
cgroup on /sys/fs/cgroup/devices type cgroup (rw,nosuid,nodev,noexec,relatime,devices)
cgroup on /sys/fs/cgroup/cpu,cpuacct type cgroup (rw,nosuid,nodev,noexec,relatime,cpu,cpuacct)
cgroup on /sys/fs/cgroup/net_cls,net_prio type cgroup (rw,nosuid,nodev,noexec,relatime,net_cls,net_prio)
cgroup on /sys/fs/cgroup/pids type cgroup (rw,nosuid,nodev,noexec,relatime,pids)
cgroup on /sys/fs/cgroup/blkio type cgroup (rw,nosuid,nodev,noexec,relatime,blkio)
cgroup on /sys/fs/cgroup/freezer type cgroup (rw,nosuid,nodev,noexec,relatime,freezer)
cgroup on /sys/fs/cgroup/perf_event type cgroup (rw,nosuid,nodev,noexec,relatime,perf_event)
cgroup on /sys/fs/cgroup/hugetlb type cgroup (rw,nosuid,nodev,noexec,relatime,hugetlb)
cgroup on /sys/fs/cgroup/cpuset type cgroup (rw,nosuid,nodev,noexec,relatime,cpuset)
cgroup on /sys/fs/cgroup/rdma type cgroup (rw,nosuid,nodev,noexec,relatime,rdma)
cgroup on /sys/fs/cgroup/memory type cgroup (rw,nosuid,nodev,noexec,relatime,memory)

# mkdir /sys/fs/cgroup/cpu/test

-rw-r--r-- 1 root root 0 Aug  9 00:51 cgroup.clone_children
-rw-r--r-- 1 root root 0 Aug  9 00:51 cgroup.procs
-r--r--r-- 1 root root 0 Aug  9 00:51 cpuacct.stat
-rw-r--r-- 1 root root 0 Aug  9 00:51 cpuacct.usage
-r--r--r-- 1 root root 0 Aug  9 00:51 cpuacct.usage_all
-r--r--r-- 1 root root 0 Aug  9 00:51 cpuacct.usage_percpu
-r--r--r-- 1 root root 0 Aug  9 00:51 cpuacct.usage_percpu_sys
-r--r--r-- 1 root root 0 Aug  9 00:51 cpuacct.usage_percpu_user
-r--r--r-- 1 root root 0 Aug  9 00:51 cpuacct.usage_sys
-r--r--r-- 1 root root 0 Aug  9 00:51 cpuacct.usage_user
-rw-r--r-- 1 root root 0 Aug  9 00:51 cpu.cfs_period_us
-rw-r--r-- 1 root root 0 Aug  9 00:51 cpu.cfs_quota_us
-rw-r--r-- 1 root root 0 Aug  9 00:51 cpu.rt_period_us
-rw-r--r-- 1 root root 0 Aug  9 00:51 cpu.rt_runtime_us
-rw-r--r-- 1 root root 0 Aug  9 00:51 cpu.shares
-r--r--r-- 1 root root 0 Aug  9 00:51 cpu.stat
-rw-r--r-- 1 root root 0 Aug  9 00:51 notify_on_release
-rw-r--r-- 1 root root 0 Aug  9 00:51 tasks

# mkdir /sys/fs/cgroup/memory/test

-rw-r--r-- 1 root root 0 Aug  9 00:55 cgroup.clone_children
--w--w--w- 1 root root 0 Aug  9 00:55 cgroup.event_control
-rw-r--r-- 1 root root 0 Aug  9 00:55 cgroup.procs
-rw-r--r-- 1 root root 0 Aug  9 00:55 memory.failcnt
--w------- 1 root root 0 Aug  9 00:55 memory.force_empty
-rw-r--r-- 1 root root 0 Aug  9 00:55 memory.kmem.failcnt
-rw-r--r-- 1 root root 0 Aug  9 00:55 memory.kmem.limit_in_bytes
-rw-r--r-- 1 root root 0 Aug  9 00:55 memory.kmem.max_usage_in_bytes
-r--r--r-- 1 root root 0 Aug  9 00:55 memory.kmem.slabinfo
-rw-r--r-- 1 root root 0 Aug  9 00:55 memory.kmem.tcp.failcnt
-rw-r--r-- 1 root root 0 Aug  9 00:55 memory.kmem.tcp.limit_in_bytes
-rw-r--r-- 1 root root 0 Aug  9 00:55 memory.kmem.tcp.max_usage_in_bytes
-r--r--r-- 1 root root 0 Aug  9 00:55 memory.kmem.tcp.usage_in_bytes
-r--r--r-- 1 root root 0 Aug  9 00:55 memory.kmem.usage_in_bytes
-rw-r--r-- 1 root root 0 Aug  9 00:55 memory.limit_in_bytes
-rw-r--r-- 1 root root 0 Aug  9 00:55 memory.max_usage_in_bytes
-rw-r--r-- 1 root root 0 Aug  9 00:55 memory.memsw.failcnt
-rw-r--r-- 1 root root 0 Aug  9 00:55 memory.memsw.limit_in_bytes
-rw-r--r-- 1 root root 0 Aug  9 00:55 memory.memsw.max_usage_in_bytes
-r--r--r-- 1 root root 0 Aug  9 00:55 memory.memsw.usage_in_bytes
-rw-r--r-- 1 root root 0 Aug  9 00:55 memory.move_charge_at_immigrate
-r--r--r-- 1 root root 0 Aug  9 00:55 memory.numa_stat
-rw-r--r-- 1 root root 0 Aug  9 00:55 memory.oom_control
---------- 1 root root 0 Aug  9 00:55 memory.pressure_level
-rw-r--r-- 1 root root 0 Aug  9 00:55 memory.soft_limit_in_bytes
-r--r--r-- 1 root root 0 Aug  9 00:55 memory.stat
-rw-r--r-- 1 root root 0 Aug  9 00:55 memory.swappiness
-r--r--r-- 1 root root 0 Aug  9 00:55 memory.usage_in_bytes
-rw-r--r-- 1 root root 0 Aug  9 00:55 memory.use_hierarchy
-rw-r--r-- 1 root root 0 Aug  9 00:55 notify_on_release
-rw-r--r-- 1 root root 0 Aug  9 00:55 tasks

-rw-r--r--  1 root root 0 Aug  9 00:23 cgroup.clone_children
--w--w--w-  1 root root 0 Aug  9 00:23 cgroup.event_control
-rw-r--r--  1 root root 0 Aug  9 00:23 cgroup.procs
-r--r--r--  1 root root 0 Aug  9 00:23 cgroup.sane_behavior
drwxr-xr-x  3 root root 0 Aug  9 00:23 docker
-rw-r--r--  1 root root 0 Aug  9 00:23 memory.failcnt
--w-------  1 root root 0 Aug  9 00:23 memory.force_empty

docker run -it -m=1g xxx

-rw-r--r--  1 root root 0 Aug  9 00:24 cpu.shares
-r--r--r--  1 root root 0 Aug  9 00:24 cpu.stat
drwxr-xr-x  3 root root 0 Aug  9 00:24 docker
-rw-r--r--  1 root root 0 Aug  9 00:24 notify_on_release
-rw-r--r--  1 root root 0 Aug  9 00:24 release_agent

-rw-r--r-- 1 root root 0 Aug  9 01:07 cgroup.clone_children
--w--w--w- 1 root root 0 Aug  9 00:22 cgroup.event_control
-rw-r--r-- 1 root root 0 Aug  9 00:22 cgroup.procs
-rw-r--r-- 1 root root 0 Aug  9 00:22 memory.failcnt
--w------- 1 root root 0 Aug  9 01:07 memory.force_empty
-rw-r--r-- 1 root root 0 Aug  9 01:07 memory.kmem.failcnt
-rw-r--r-- 1 root root 0 Aug  9 01:07 memory.kmem.limit_in_bytes
-rw-r--r-- 1 root root 0 Aug  9 01:07 memory.kmem.max_usage_in_bytes
-r--r--r-- 1 root root 0 Aug  9 01:07 memory.kmem.slabinfo
-rw-r--r-- 1 root root 0 Aug  9 01:07 memory.kmem.tcp.failcnt
-rw-r--r-- 1 root root 0 Aug  9 01:07 memory.kmem.tcp.limit_in_bytes
-rw-r--r-- 1 root root 0 Aug  9 01:07 memory.kmem.tcp.max_usage_in_bytes
-r--r--r-- 1 root root 0 Aug  9 01:07 memory.kmem.tcp.usage_in_bytes
-r--r--r-- 1 root root 0 Aug  9 01:07 memory.kmem.usage_in_bytes
-rw-r--r-- 1 root root 0 Aug  9 00:22 memory.limit_in_bytes
-rw-r--r-- 1 root root 0 Aug  9 00:22 memory.max_usage_in_bytes
-rw-r--r-- 1 root root 0 Aug  9 01:07 memory.memsw.failcnt
-rw-r--r-- 1 root root 0 Aug  9 00:22 memory.memsw.limit_in_bytes
-rw-r--r-- 1 root root 0 Aug  9 01:07 memory.memsw.max_usage_in_bytes
-r--r--r-- 1 root root 0 Aug  9 01:07 memory.memsw.usage_in_bytes
-rw-r--r-- 1 root root 0 Aug  9 01:07 memory.move_charge_at_immigrate
-r--r--r-- 1 root root 0 Aug  9 01:07 memory.numa_stat
-rw-r--r-- 1 root root 0 Aug  9 00:22 memory.oom_control
---------- 1 root root 0 Aug  9 01:07 memory.pressure_level
-rw-r--r-- 1 root root 0 Aug  9 01:07 memory.soft_limit_in_bytes
-r--r--r-- 1 root root 0 Aug  9 01:07 memory.stat
-rw-r--r-- 1 root root 0 Aug  9 01:07 memory.swappiness
-r--r--r-- 1 root root 0 Aug  9 00:22 memory.usage_in_bytes
-rw-r--r-- 1 root root 0 Aug  9 01:07 memory.use_hierarchy
-rw-r--r-- 1 root root 0 Aug  9 01:07 notify_on_release
-rw-r--r-- 1 root root 0 Aug  9 01:07 tasks

drwxr-xr-x 2 root root 0 Aug  9 00:22 3da8ee4dd5c8af2d33b9301c948bb57e9ed56534a6d980084a25d9306572608b
-rw-r--r-- 1 root root 0 Aug  9 01:05 cgroup.clone_children
--w--w--w- 1 root root 0 Aug  9 01:05 cgroup.event_control
-rw-r--r-- 1 root root 0 Aug  9 01:05 cgroup.procs

# cat memory.limit_in_bytes 
1073741824

unsigned int Abstract_VM_Version::nof_parallel_worker_threads(
                                                      unsigned int num,
                                                      unsigned int den,
                                                      unsigned int switch_pt) {
  if (FLAG_IS_DEFAULT(ParallelGCThreads)) {
    assert(ParallelGCThreads == 0, "Default ParallelGCThreads is not 0");
    // For very large machines, there are diminishing returns
    // for large numbers of worker threads.  Instead of
    // hogging the whole system, use a fraction of the workers for every
    // processor after the first 8.  For example, on a 72 cpu machine
    // and a chosen fraction of 5/8
    // use 8 + (72 - 8) * (5/8) == 48 worker threads.
    unsigned int ncpus = (unsigned int) os::active_processor_count();
    return (ncpus <= switch_pt) ?
           ncpus :
          (switch_pt + ((ncpus - switch_pt) * num) / den);
  } else {
    return ParallelGCThreads;
  }
}

int os::active_processor_count() {
  // Linux doesn't yet have a (official) notion of processor sets,
  // so just return the number of online processors.
  int online_cpus = ::sysconf(_SC_NPROCESSORS_ONLN);
  assert(online_cpus > 0 && online_cpus <= processor_count(), "sanity check");
  return online_cpus;
}

// Get the current number of available processors for this process.
// This value can change at any time during a process's lifetime.
// sched_getaffinity gives an accurate answer as it accounts for cpusets.
// If anything goes wrong we fallback to returning the number of online
// processors - which can be greater than the number available to the process.
int os::active_processor_count() {
  cpu_set_t cpus;  // can represent at most 1024 (CPU_SETSIZE) processors
  int cpus_size = sizeof(cpu_set_t);
  int cpu_count = 0;

  // pid 0 means the current thread - which we have to assume represents the process
  if (sched_getaffinity(0, cpus_size, &cpus) == 0) {
    cpu_count = os_cpu_count(&cpus);
    if (PrintActiveCpus) {
      tty->print_cr("active_processor_count: sched_getaffinity processor count: %d", cpu_count);
    }
  }
  else {
    cpu_count = ::sysconf(_SC_NPROCESSORS_ONLN);
    warning("sched_getaffinity failed (%s)- using online processor count (%d) "
            "which may exceed available processors", strerror(errno), cpu_count);
  }

  assert(cpu_count > 0 && cpu_count <= processor_count(), "sanity check");
  return cpu_count;
}

// Determine the active processor count from one of
// three different sources:
//
// 1. User option -XX:ActiveProcessorCount
// 2. kernel os calls (sched_getaffinity or sysconf(_SC_NPROCESSORS_ONLN)
// 3. extracted from cgroup cpu subsystem (shares and quotas)
//
// Option 1, if specified, will always override.
// If the cgroup subsystem is active and configured, we
// will return the min of the cgroup and option 2 results.
// This is required since tools, such as numactl, that
// alter cpu affinity do not update cgroup subsystem
// cpuset configuration files.
int os::active_processor_count() {
  // User has overridden the number of active processors
  if (ActiveProcessorCount > 0) {
    if (PrintActiveCpus) {
      tty->print_cr("active_processor_count: "
                    "active processor count set by user : %d",
                    ActiveProcessorCount);
    }
    return ActiveProcessorCount;
  }

  int active_cpus;
  if (OSContainer::is_containerized()) {
    active_cpus = OSContainer::active_processor_count();
    if (PrintActiveCpus) {
      tty->print_cr("active_processor_count: determined by OSContainer: %d",
                     active_cpus);
    }
  } else {
    active_cpus = os::Linux::active_processor_count();
  }

  return active_cpus;
}

inline bool OSContainer::is_containerized() {
  assert(_is_initialized, "OSContainer not initialized");
  return _is_containerized;
}

/* init
 *
 * Initialize the container support and determine if
 * we are running under cgroup control.
 */
void OSContainer::init() {
  int mountid;
  int parentid;
  int major;
  int minor;
  FILE *mntinfo = NULL;
  FILE *cgroup = NULL;
  char buf[MAXPATHLEN+1];
  char tmproot[MAXPATHLEN+1];
  char tmpmount[MAXPATHLEN+1];
  char tmpbase[MAXPATHLEN+1];
  char *p;
  jlong mem_limit;

  assert(!_is_initialized, "Initializing OSContainer more than once");

  _is_initialized = true;
  _is_containerized = false;

  _unlimited_memory = (LONG_MAX / os::vm_page_size()) * os::vm_page_size();

  if (PrintContainerInfo) {
    tty->print_cr("OSContainer::init: Initializing Container Support");
  }
  if (!UseContainerSupport) {
    if (PrintContainerInfo) {
      tty->print_cr("Container Support not enabled");
    }
    return;
  }

  /*
   * Find the cgroup mount point for memory and cpuset
   * by reading /proc/self/mountinfo
   *
   * Example for docker:
   * 219 214 0:29 /docker/7208cebd00fa5f2e342b1094f7bed87fa25661471a4637118e65f1c995be8a34 /sys/fs/cgroup/memory ro,nosuid,nodev,noexec,relatime - cgroup cgroup rw,memory
   *
   * Example for host:
   * 34 28 0:29 / /sys/fs/cgroup/memory rw,nosuid,nodev,noexec,relatime shared:16 - cgroup cgroup rw,memory
   */
  mntinfo = fopen("/proc/self/mountinfo", "r");
  if (mntinfo == NULL) {
      if (PrintContainerInfo) {
        tty->print_cr("Can't open /proc/self/mountinfo, %s",
                       strerror(errno));
      }
      return;
  }

  while ( (p = fgets(buf, MAXPATHLEN, mntinfo)) != NULL) {
    // Look for the filesystem type and see if it's cgroup
    char fstype[MAXPATHLEN+1];
    fstype[0] = '\0';
    char *s =  strstr(p, " - ");
    if (s != NULL &&
        sscanf(s, " - %s", fstype) == 1 &&
        strcmp(fstype, "cgroup") == 0) {

      if (strstr(p, "memory") != NULL) {
        int matched = sscanf(p, "%d %d %d:%d %s %s",
                             &mountid,
                             &parentid,
                             &major,
                             &minor,
                             tmproot,
                             tmpmount);
        if (matched == 6) {
          memory = new CgroupSubsystem(tmproot, tmpmount);
        }
        else
          if (PrintContainerInfo) {
            tty->print_cr("Incompatible str containing cgroup and memory: %s", p);
          }
      } else if (strstr(p, "cpuset") != NULL) {
        int matched = sscanf(p, "%d %d %d:%d %s %s",
                             &mountid,
                             &parentid,
                             &major,
                             &minor,
                             tmproot,
                             tmpmount);
        if (matched == 6) {
          cpuset = new CgroupSubsystem(tmproot, tmpmount);
        }
        else {
          if (PrintContainerInfo) {
            tty->print_cr("Incompatible str containing cgroup and cpuset: %s", p);
          }
        }
      } else if (strstr(p, "cpu,cpuacct") != NULL || strstr(p, "cpuacct,cpu") != NULL) {
        int matched = sscanf(p, "%d %d %d:%d %s %s",
                             &mountid,
                             &parentid,
                             &major,
                             &minor,
                             tmproot,
                             tmpmount);
        if (matched == 6) {
          cpu = new CgroupSubsystem(tmproot, tmpmount);
          cpuacct = new CgroupSubsystem(tmproot, tmpmount);
        }
        else {
          if (PrintContainerInfo) {
            tty->print_cr("Incompatible str containing cgroup and cpu,cpuacct: %s", p);
          }
        }
      } else if (strstr(p, "cpuacct") != NULL) {
        int matched = sscanf(p, "%d %d %d:%d %s %s",
                             &mountid,
                             &parentid,
                             &major,
                             &minor,
                             tmproot,
                             tmpmount);
        if (matched == 6) {
          cpuacct = new CgroupSubsystem(tmproot, tmpmount);
        }
        else {
          if (PrintContainerInfo) {
            tty->print_cr("Incompatible str containing cgroup and cpuacct: %s", p);
          }
        }
      } else if (strstr(p, "cpu") != NULL) {
        int matched = sscanf(p, "%d %d %d:%d %s %s",
                             &mountid,
                             &parentid,
                             &major,
                             &minor,
                             tmproot,
                             tmpmount);
        if (matched == 6) {
          cpu = new CgroupSubsystem(tmproot, tmpmount);
        }
        else {
          if (PrintContainerInfo) {
            tty->print_cr("Incompatible str containing cgroup and cpu: %s", p);
          }
        }
      }
    }
  }

  fclose(mntinfo);

  if (memory == NULL) {
    if (PrintContainerInfo) {
      tty->print_cr("Required cgroup memory subsystem not found");
    }
    return;
  }
  if (cpuset == NULL) {
    if (PrintContainerInfo) {
      tty->print_cr("Required cgroup cpuset subsystem not found");
    }
    return;
  }
  if (cpu == NULL) {
    if (PrintContainerInfo) {
      tty->print_cr("Required cgroup cpu subsystem not found");
    }
    return;
  }
  if (cpuacct == NULL) {
    if (PrintContainerInfo) {
      tty->print_cr("Required cgroup cpuacct subsystem not found");
    }
    return;
  }

  /*
   * Read /proc/self/cgroup and map host mount point to
   * local one via /proc/self/mountinfo content above
   *
   * Docker example:
   * 5:memory:/docker/6558aed8fc662b194323ceab5b964f69cf36b3e8af877a14b80256e93aecb044
   *
   * Host example:
   * 5:memory:/user.slice
   *
   * Construct a path to the process specific memory and cpuset
   * cgroup directory.
   *
   * For a container running under Docker from memory example above
   * the paths would be:
   *
   * /sys/fs/cgroup/memory
   *
   * For a Host from memory example above the path would be:
   *
   * /sys/fs/cgroup/memory/user.slice
   *
   */
  cgroup = fopen("/proc/self/cgroup", "r");
  if (cgroup == NULL) {
    if (PrintContainerInfo) {
      tty->print_cr("Can't open /proc/self/cgroup, %s",
                     strerror(errno));
      }
    return;
  }

  while ( (p = fgets(buf, MAXPATHLEN, cgroup)) != NULL) {
    int cgno;
    int matched;
    char *controller;
    char *base;

    /* Skip cgroup number */
    strsep(&p, ":");
    /* Get controller and base */
    controller = strsep(&p, ":");
    base = strsep(&p, "\n");

    if (controller != NULL) {
      if (strstr(controller, "memory") != NULL) {
        memory->set_subsystem_path(base);
      } else if (strstr(controller, "cpuset") != NULL) {
        cpuset->set_subsystem_path(base);
      } else if (strstr(controller, "cpu,cpuacct") != NULL || strstr(controller, "cpuacct,cpu") != NULL) {
        cpu->set_subsystem_path(base);
        cpuacct->set_subsystem_path(base);
      } else if (strstr(controller, "cpuacct") != NULL) {
        cpuacct->set_subsystem_path(base);
      } else if (strstr(controller, "cpu") != NULL) {
        cpu->set_subsystem_path(base);
      }
    }
  }

  fclose(cgroup);

  // We need to update the amount of physical memory now that
  // command line arguments have been processed.
  if ((mem_limit = memory_limit_in_bytes()) > 0) {
    os::Linux::set_physical_memory(mem_limit);
  }

  _is_containerized = true;

}

/* active_processor_count
 *
 * Calculate an appropriate number of active processors for the
 * VM to use based on these three inputs.
 *
 * cpu affinity
 * cgroup cpu quota & cpu period
 * cgroup cpu shares
 *
 * Algorithm:
 *
 * Determine the number of available CPUs from sched_getaffinity
 *
 * If user specified a quota (quota != -1), calculate the number of
 * required CPUs by dividing quota by period.
 *
 * If shares are in effect (shares != -1), calculate the number
 * of CPUs required for the shares by dividing the share value
 * by PER_CPU_SHARES.
 *
 * All results of division are rounded up to the next whole number.
 *
 * If neither shares or quotas have been specified, return the
 * number of active processors in the system.
 *
 * If both shares and quotas have been specified, the results are
 * based on the flag PreferContainerQuotaForCPUCount.  If true,
 * return the quota value.  If false return the smallest value
 * between shares or quotas.
 *
 * If shares and/or quotas have been specified, the resulting number
 * returned will never exceed the number of active processors.
 *
 * return:
 *    number of CPUs
 */
int OSContainer::active_processor_count() {
  int quota_count = 0, share_count = 0;
  int cpu_count, limit_count;
  int result;

  cpu_count = limit_count = os::Linux::active_processor_count();
  int quota  = cpu_quota();
  int period = cpu_period();
  int share  = cpu_shares();

  if (quota > -1 && period > 0) {
    quota_count = ceilf((float)quota / (float)period);
    if (PrintContainerInfo) {
      tty->print_cr("CPU Quota count based on quota/period: %d", quota_count);
    }
  }
  if (share > -1) {
    share_count = ceilf((float)share / (float)PER_CPU_SHARES);
    if (PrintContainerInfo) {
      tty->print_cr("CPU Share count based on shares: %d", share_count);
    }
  }

  // If both shares and quotas are setup results depend
  // on flag PreferContainerQuotaForCPUCount.
  // If true, limit CPU count to quota
  // If false, use minimum of shares and quotas
  if (quota_count !=0 && share_count != 0) {
    if (PreferContainerQuotaForCPUCount) {
      limit_count = quota_count;
    } else {
      limit_count = MIN2(quota_count, share_count);
    }
  } else if (quota_count != 0) {
    limit_count = quota_count;
  } else if (share_count != 0) {
    limit_count = share_count;
  }

  result = MIN2(cpu_count, limit_count);
  if (PrintContainerInfo) {
    tty->print_cr("OSContainer::active_processor_count: %d", result);
  }
  return result;
}

void Arguments::set_heap_size() {
  if (!FLAG_IS_DEFAULT(DefaultMaxRAMFraction)) {
    // Deprecated flag
    FLAG_SET_CMDLINE(uintx, MaxRAMFraction, DefaultMaxRAMFraction);
  }

  const julong phys_mem =
    FLAG_IS_DEFAULT(MaxRAM) ? MIN2(os::physical_memory(), (julong)MaxRAM)
                            : (julong)MaxRAM;

  // If the maximum heap size has not been set with -Xmx,
  // then set it as fraction of the size of physical memory,
  // respecting the maximum and minimum sizes of the heap.
  if (FLAG_IS_DEFAULT(MaxHeapSize)) {
    julong reasonable_max = phys_mem / MaxRAMFraction;

    if (phys_mem <= MaxHeapSize * MinRAMFraction) {
      // Small physical memory, so use a minimum fraction of it for the heap
      reasonable_max = phys_mem / MinRAMFraction;
    } else {
      // Not-small physical memory, so require a heap at least
      // as large as MaxHeapSize
      reasonable_max = MAX2(reasonable_max, (julong)MaxHeapSize);
    }
    if (!FLAG_IS_DEFAULT(ErgoHeapSizeLimit) && ErgoHeapSizeLimit != 0) {
      // Limit the heap size to ErgoHeapSizeLimit
      reasonable_max = MIN2(reasonable_max, (julong)ErgoHeapSizeLimit);
    }
    if (UseCompressedOops) {
      // Limit the heap size to the maximum possible when using compressed oops
      julong max_coop_heap = (julong)max_heap_for_compressed_oops();
      if (HeapBaseMinAddress + MaxHeapSize < max_coop_heap) {
        // Heap should be above HeapBaseMinAddress to get zero based compressed oops
        // but it should be not less than default MaxHeapSize.
        max_coop_heap -= HeapBaseMinAddress;
      }
      reasonable_max = MIN2(reasonable_max, max_coop_heap);
    }
    reasonable_max = limit_by_allocatable_memory(reasonable_max);

    if (!FLAG_IS_DEFAULT(InitialHeapSize)) {
      // An initial heap size was specified on the command line,
      // so be sure that the maximum size is consistent.  Done
      // after call to limit_by_allocatable_memory because that
      // method might reduce the allocation size.
      reasonable_max = MAX2(reasonable_max, (julong)InitialHeapSize);
    }

    if (PrintGCDetails && Verbose) {
      // Cannot use gclog_or_tty yet.
      tty->print_cr("  Maximum heap size " SIZE_FORMAT, reasonable_max);
    }
    FLAG_SET_ERGO(uintx, MaxHeapSize, (uintx)reasonable_max);
  }

  // If the minimum or initial heap_size have not been set or requested to be set
  // ergonomically, set them accordingly.
  if (InitialHeapSize == 0 || min_heap_size() == 0) {
    julong reasonable_minimum = (julong)(OldSize + NewSize);

    reasonable_minimum = MIN2(reasonable_minimum, (julong)MaxHeapSize);

    reasonable_minimum = limit_by_allocatable_memory(reasonable_minimum);

    if (InitialHeapSize == 0) {
      julong reasonable_initial = phys_mem / InitialRAMFraction;

      reasonable_initial = MAX3(reasonable_initial, reasonable_minimum, (julong)min_heap_size());
      reasonable_initial = MIN2(reasonable_initial, (julong)MaxHeapSize);

      reasonable_initial = limit_by_allocatable_memory(reasonable_initial);

      if (PrintGCDetails && Verbose) {
        // Cannot use gclog_or_tty yet.
        tty->print_cr("  Initial heap size " SIZE_FORMAT, (uintx)reasonable_initial);
      }
      FLAG_SET_ERGO(uintx, InitialHeapSize, (uintx)reasonable_initial);
    }
    // If the minimum heap size has not been set (via -Xms),
    // synchronize with InitialHeapSize to avoid errors with the default value.
    if (min_heap_size() == 0) {
      set_min_heap_size(MIN2((uintx)reasonable_minimum, InitialHeapSize));
      if (PrintGCDetails && Verbose) {
        // Cannot use gclog_or_tty yet.
        tty->print_cr("  Minimum heap size " SIZE_FORMAT, min_heap_size());
      }
    }
  }
}

julong os::physical_memory() {
  return Linux::physical_memory();
}

$ java -XX:+PrintFlagsFinal -version|grep -i heapsize|egrep 'Initial|Max'
java version "1.8.0_74"
Java(TM) SE Runtime Environment (build 1.8.0_74-b02)
Java HotSpot(TM) 64-Bit Server VM (build 25.74-b02, mixed mode)
    uintx InitialHeapSize                          := 188743680                           {product}
    uintx MaxHeapSize                              := 2988441600                          {product}

$ docker run --rm java java  -XX:+PrintFlagsFinal -version |grep -i heapsize | egrep 'Initial|Max'
openjdk version "1.8.0_72-internal"
OpenJDK Runtime Environment (build 1.8.0_72-internal-b15)
OpenJDK 64-Bit Server VM (build 25.72-b15, mixed mode)
    uintx InitialHeapSize                          := 188743680                           {product}
    uintx MaxHeapSize                              := 2988441600                          {product}

$ docker run -m 256m --rm java java  -XX:+PrintFlagsFinal -version |grep -i heapsize | egrep 'Initial|Max'

WARNING: Your kernel does not support swap limit capabilities, memory limited without swap.
openjdk version "1.8.0_72-internal"
OpenJDK Runtime Environment (build 1.8.0_72-internal-b15)
OpenJDK 64-Bit Server VM (build 25.72-b15, mixed mode)
    uintx InitialHeapSize                          := 188743680                           {product}
    uintx MaxHeapSize                              := 2988441600                          {product}

....
"State": {
    "Running": false,
    "Paused": false,
    "Restarting": false,
    "OOMKilled": true,
    "Dead": false,
    "Pid": 0,
    "ExitCode": 137,
    "Error": "",
    "StartedAt": "2016-03-15T21:21:48.845032635Z",
    "FinishedAt": "2016-03-15T21:21:49.140794192Z"
},
....

void Arguments::set_heap_size() {
  if (!FLAG_IS_DEFAULT(DefaultMaxRAMFraction)) {
    // Deprecated flag
    FLAG_SET_CMDLINE(uintx, MaxRAMFraction, DefaultMaxRAMFraction);
  }

  julong phys_mem =
    FLAG_IS_DEFAULT(MaxRAM) ? MIN2(os::physical_memory(), (julong)MaxRAM)
                            : (julong)MaxRAM;

  // Experimental support for CGroup memory limits
  if (UseCGroupMemoryLimitForHeap) {
    // This is a rough indicator that a CGroup limit may be in force
    // for this process
    const char* lim_file = "/sys/fs/cgroup/memory/memory.limit_in_bytes";
    FILE *fp = fopen(lim_file, "r");
    if (fp != NULL) {
      julong cgroup_max = 0;
      int ret = fscanf(fp, JULONG_FORMAT, &cgroup_max);
      if (ret == 1 && cgroup_max > 0) {
        // If unlimited, cgroup_max will be a very large, but unspecified
        // value, so use initial phys_mem as a limit
        if (PrintGCDetails && Verbose) {
          // Cannot use gclog_or_tty yet.
          tty->print_cr("Setting phys_mem to the min of cgroup limit ("
                        JULONG_FORMAT "MB) and initial phys_mem ("
                        JULONG_FORMAT "MB)", cgroup_max/M, phys_mem/M);
        }
        phys_mem = MIN2(cgroup_max, phys_mem);
      } else {
        warning("Unable to read/parse cgroup memory limit from %s: %s",
                lim_file, errno != 0 ? strerror(errno) : "unknown error");
      }
      fclose(fp);
    } else {
      warning("Unable to open cgroup memory limit file %s (%s)", lim_file, strerror(errno));
    }
  }

  // If the maximum heap size has not been set with -Xmx,
  // then set it as fraction of the size of physical memory,
  // respecting the maximum and minimum sizes of the heap.
  if (FLAG_IS_DEFAULT(MaxHeapSize)) {
    julong reasonable_max = phys_mem / MaxRAMFraction;

    if (phys_mem <= MaxHeapSize * MinRAMFraction) {
      // Small physical memory, so use a minimum fraction of it for the heap
      reasonable_max = phys_mem / MinRAMFraction;
    } else {
      // Not-small physical memory, so require a heap at least
      // as large as MaxHeapSize
      reasonable_max = MAX2(reasonable_max, (julong)MaxHeapSize);
    }
    if (!FLAG_IS_DEFAULT(ErgoHeapSizeLimit) && ErgoHeapSizeLimit != 0) {
      // Limit the heap size to ErgoHeapSizeLimit
      reasonable_max = MIN2(reasonable_max, (julong)ErgoHeapSizeLimit);
    }
    if (UseCompressedOops) {
      // Limit the heap size to the maximum possible when using compressed oops
      julong max_coop_heap = (julong)max_heap_for_compressed_oops();
      if (HeapBaseMinAddress + MaxHeapSize < max_coop_heap) {
        // Heap should be above HeapBaseMinAddress to get zero based compressed oops
        // but it should be not less than default MaxHeapSize.
        max_coop_heap -= HeapBaseMinAddress;
      }
      reasonable_max = MIN2(reasonable_max, max_coop_heap);
    }
    reasonable_max = limit_by_allocatable_memory(reasonable_max);

    if (!FLAG_IS_DEFAULT(InitialHeapSize)) {
      // An initial heap size was specified on the command line,
      // so be sure that the maximum size is consistent.  Done
      // after call to limit_by_allocatable_memory because that
      // method might reduce the allocation size.
      reasonable_max = MAX2(reasonable_max, (julong)InitialHeapSize);
    }

    if (PrintGCDetails && Verbose) {
      // Cannot use gclog_or_tty yet.
      tty->print_cr("  Maximum heap size " SIZE_FORMAT, (size_t) reasonable_max);
    }
    FLAG_SET_ERGO(uintx, MaxHeapSize, (uintx)reasonable_max);
  }

  // If the minimum or initial heap_size have not been set or requested to be set
  // ergonomically, set them accordingly.
  if (InitialHeapSize == 0 || min_heap_size() == 0) {
    julong reasonable_minimum = (julong)(OldSize + NewSize);

    reasonable_minimum = MIN2(reasonable_minimum, (julong)MaxHeapSize);

    reasonable_minimum = limit_by_allocatable_memory(reasonable_minimum);

    if (InitialHeapSize == 0) {
      julong reasonable_initial = phys_mem / InitialRAMFraction;

      reasonable_initial = MAX3(reasonable_initial, reasonable_minimum, (julong)min_heap_size());
      reasonable_initial = MIN2(reasonable_initial, (julong)MaxHeapSize);

      reasonable_initial = limit_by_allocatable_memory(reasonable_initial);

      if (PrintGCDetails && Verbose) {
        // Cannot use gclog_or_tty yet.
        tty->print_cr("  Initial heap size " SIZE_FORMAT, (uintx)reasonable_initial);
      }
      FLAG_SET_ERGO(uintx, InitialHeapSize, (uintx)reasonable_initial);
    }
    // If the minimum heap size has not been set (via -Xms),
    // synchronize with InitialHeapSize to avoid errors with the default value.
    if (min_heap_size() == 0) {
      set_min_heap_size(MIN2((uintx)reasonable_minimum, InitialHeapSize));
      if (PrintGCDetails && Verbose) {
        // Cannot use gclog_or_tty yet.
        tty->print_cr("  Minimum heap size " SIZE_FORMAT, min_heap_size());
      }
    }
  }
}

void Arguments::set_heap_size() {
  if (!FLAG_IS_DEFAULT(DefaultMaxRAMFraction)) {
    // Deprecated flag
    FLAG_SET_CMDLINE(uintx, MaxRAMFraction, DefaultMaxRAMFraction);
  }

  julong phys_mem =
    FLAG_IS_DEFAULT(MaxRAM) ? MIN2(os::physical_memory(), (julong)MaxRAM)
                            : (julong)MaxRAM;

  // Experimental support for CGroup memory limits
  if (UseCGroupMemoryLimitForHeap) {
    // This is a rough indicator that a CGroup limit may be in force
    // for this process
    const char* lim_file = "/sys/fs/cgroup/memory/memory.limit_in_bytes";
    FILE *fp = fopen(lim_file, "r");
    if (fp != NULL) {
      julong cgroup_max = 0;
      int ret = fscanf(fp, JULONG_FORMAT, &cgroup_max);
      if (ret == 1 && cgroup_max > 0) {
        // If unlimited, cgroup_max will be a very large, but unspecified
        // value, so use initial phys_mem as a limit
        if (PrintGCDetails && Verbose) {
          // Cannot use gclog_or_tty yet.
          tty->print_cr("Setting phys_mem to the min of cgroup limit ("
                        JULONG_FORMAT "MB) and initial phys_mem ("
                        JULONG_FORMAT "MB)", cgroup_max/M, phys_mem/M);
        }
        phys_mem = MIN2(cgroup_max, phys_mem);
      } else {
        warning("Unable to read/parse cgroup memory limit from %s: %s",
                lim_file, errno != 0 ? strerror(errno) : "unknown error");
      }
      fclose(fp);
    } else {
      warning("Unable to open cgroup memory limit file %s (%s)", lim_file, strerror(errno));
    }
  }

  // Convert Fraction to Precentage values
  if (FLAG_IS_DEFAULT(MaxRAMPercentage) &&
      !FLAG_IS_DEFAULT(MaxRAMFraction))
    MaxRAMPercentage = 100.0 / MaxRAMFraction;

   if (FLAG_IS_DEFAULT(MinRAMPercentage) &&
       !FLAG_IS_DEFAULT(MinRAMFraction))
     MinRAMPercentage = 100.0 / MinRAMFraction;

   if (FLAG_IS_DEFAULT(InitialRAMPercentage) &&
       !FLAG_IS_DEFAULT(InitialRAMFraction))
     InitialRAMPercentage = 100.0 / InitialRAMFraction;

  // If the maximum heap size has not been set with -Xmx,
  // then set it as fraction of the size of physical memory,
  // respecting the maximum and minimum sizes of the heap.
  if (FLAG_IS_DEFAULT(MaxHeapSize)) {
    julong reasonable_max = (julong)((phys_mem * MaxRAMPercentage) / 100);
    const julong reasonable_min = (julong)((phys_mem * MinRAMPercentage) / 100);
    if (reasonable_min < MaxHeapSize) {
      // Small physical memory, so use a minimum fraction of it for the heap
      reasonable_max = reasonable_min;
    } else {
      // Not-small physical memory, so require a heap at least
      // as large as MaxHeapSize
      reasonable_max = MAX2(reasonable_max, (julong)MaxHeapSize);
    }

    if (!FLAG_IS_DEFAULT(ErgoHeapSizeLimit) && ErgoHeapSizeLimit != 0) {
      // Limit the heap size to ErgoHeapSizeLimit
      reasonable_max = MIN2(reasonable_max, (julong)ErgoHeapSizeLimit);
    }
    if (UseCompressedOops) {
      // Limit the heap size to the maximum possible when using compressed oops
      julong max_coop_heap = (julong)max_heap_for_compressed_oops();
      if (HeapBaseMinAddress + MaxHeapSize < max_coop_heap) {
        // Heap should be above HeapBaseMinAddress to get zero based compressed oops
        // but it should be not less than default MaxHeapSize.
        max_coop_heap -= HeapBaseMinAddress;
      }
      reasonable_max = MIN2(reasonable_max, max_coop_heap);
    }
    reasonable_max = limit_by_allocatable_memory(reasonable_max);

    if (!FLAG_IS_DEFAULT(InitialHeapSize)) {
      // An initial heap size was specified on the command line,
      // so be sure that the maximum size is consistent.  Done
      // after call to limit_by_allocatable_memory because that
      // method might reduce the allocation size.
      reasonable_max = MAX2(reasonable_max, (julong)InitialHeapSize);
    }

    if (PrintGCDetails && Verbose) {
      // Cannot use gclog_or_tty yet.
      tty->print_cr("  Maximum heap size " SIZE_FORMAT, (size_t) reasonable_max);
    }
    FLAG_SET_ERGO(uintx, MaxHeapSize, (uintx)reasonable_max);
  }

  // If the minimum or initial heap_size have not been set or requested to be set
  // ergonomically, set them accordingly.
  if (InitialHeapSize == 0 || min_heap_size() == 0) {
    julong reasonable_minimum = (julong)(OldSize + NewSize);

    reasonable_minimum = MIN2(reasonable_minimum, (julong)MaxHeapSize);

    reasonable_minimum = limit_by_allocatable_memory(reasonable_minimum);

    if (InitialHeapSize == 0) {
      julong reasonable_initial = (julong)((phys_mem * InitialRAMPercentage) / 100);

      reasonable_initial = MAX3(reasonable_initial, reasonable_minimum, (julong)min_heap_size());
      reasonable_initial = MIN2(reasonable_initial, (julong)MaxHeapSize);

      reasonable_initial = limit_by_allocatable_memory(reasonable_initial);

      if (PrintGCDetails && Verbose) {
        // Cannot use gclog_or_tty yet.
        tty->print_cr("  Initial heap size " SIZE_FORMAT, (uintx)reasonable_initial);
      }
      FLAG_SET_ERGO(uintx, InitialHeapSize, (uintx)reasonable_initial);
    }
    // If the minimum heap size has not been set (via -Xms),
    // synchronize with InitialHeapSize to avoid errors with the default value.
    if (min_heap_size() == 0) {
      set_min_heap_size(MIN2((uintx)reasonable_minimum, InitialHeapSize));
      if (PrintGCDetails && Verbose) {
        // Cannot use gclog_or_tty yet.
        tty->print_cr("  Minimum heap size " SIZE_FORMAT, min_heap_size());
      }
    }
  }
}

julong os::physical_memory() {
  jlong phys_mem = 0;
  if (OSContainer::is_containerized()) {
    jlong mem_limit;
    if ((mem_limit = OSContainer::memory_limit_in_bytes()) > 0) {
      if (PrintContainerInfo) {
        tty->print_cr("total container memory: " JLONG_FORMAT, mem_limit);
      }
      return mem_limit;
    }

    if (PrintContainerInfo) {
      tty->print_cr("container memory limit %s: " JLONG_FORMAT ", using host value",
                     mem_limit == OSCONTAINER_ERROR ? "failed" : "unlimited", mem_limit);
    }
  }

  phys_mem = Linux::physical_memory();
  if (Verbose) {
    tty->print_cr("total system memory: " JLONG_FORMAT, phys_mem);
  }
  return phys_mem;
}

/* memory_limit_in_bytes
 *
 * Return the limit of available memory for this process.
 *
 * return:
 *    memory limit in bytes or
 *    -1 for unlimited
 *    OSCONTAINER_ERROR for not supported
 */
jlong OSContainer::memory_limit_in_bytes() {
  GET_CONTAINER_INFO(julong, memory, "/memory.limit_in_bytes",
                     "Memory Limit is: " JULONG_FORMAT, JULONG_FORMAT, memlimit);

  if (memlimit >= _unlimited_memory) {
    if (PrintContainerInfo) {
      tty->print_cr("Memory Limit is: Unlimited");
    }
    return (jlong)-1;
  }
  else {
    return (jlong)memlimit;
  }
}

Can't attach to the process: ptrace(PTRACE_ATTACH, ..).

// These options imply the use of a SA tool
private static String SA_TOOL_OPTIONS =
      "-heap|-heap:format=b|-clstats|-finalizerinfo";
if (option.matches(SA_TOOL_OPTIONS)) {
            useSA = true;
}

// The -F (force) option is currently not passed through to SA
private static String FORCE_SA_OPTION = "-F";

if (arg.equals(FORCE_SA_OPTION)) {
        useSA = true;
}

private void setupDebugger() {
    ....
    if (os.equals("solaris")) {
           setupDebuggerSolaris();
    } else if (os.equals("win32")) {
           setupDebuggerWin32();
    } else if (os.equals("linux")) {
           setupDebuggerLinux();
    } else if (os.equals("bsd")) {
           setupDebuggerBsd();
    } else if (os.equals("darwin")) {
           setupDebuggerDarwin();
    } else {
           // Add support for more operating systems here
           throw new DebuggerException("Operating system " + os + " not yet supported");
    }
    ....
}

private void setupDebuggerLinux() {
        setupJVMLibNamesLinux();

        if (cpu.equals("x86")) {
            machDesc = new MachineDescriptionIntelX86();
        } else if (cpu.equals("ia64")) {
            machDesc = new MachineDescriptionIA64();
        } else if (cpu.equals("amd64")) {
            machDesc = new MachineDescriptionAMD64();
        } else if (cpu.equals("sparc")) {
            if (LinuxDebuggerLocal.getAddressSize()==8) {
                    machDesc = new MachineDescriptionSPARC64Bit();
            } else {
                    machDesc = new MachineDescriptionSPARC32Bit();
            }
        } else {
          try {
            machDesc = (MachineDescription)
              Class.forName("sun.jvm.hotspot.debugger.MachineDescription" +
                            cpu.toUpperCase()).newInstance();
          } catch (Exception e) {
            throw new DebuggerException("Linux not supported on machine type " + cpu);
          }
        }

        LinuxDebuggerLocal dbg =
        new LinuxDebuggerLocal(machDesc, !isServer);
        debugger = dbg;

        attachDebugger();
    }

public synchronized void attach(int processID) throws DebuggerException {
        checkAttached();
        threadList = new ArrayList();
        loadObjectList = new ArrayList();
        class AttachTask implements WorkerThreadTask {
           int pid;
           public void doit(LinuxDebuggerLocal debugger) {
              debugger.attach0(pid);
              debugger.attached = true;
              debugger.isCore = false;
              findABIVersion();
           }
        }

        AttachTask task = new AttachTask();
        task.pid = processID;
        workerThread.execute(task);
    }

/*
 * Class:     sun_jvm_hotspot_debugger_linux_LinuxDebuggerLocal
 * Method:    attach0
 * Signature: (I)V
 */
JNIEXPORT void JNICALL Java_sun_jvm_hotspot_debugger_linux_LinuxDebuggerLocal_attach0__I
  (JNIEnv *env, jobject this_obj, jint jpid) {

  // For bitness checking, locate binary at /proc/jpid/exe
  char buf[PATH_MAX];
  snprintf((char *) &buf, PATH_MAX, "/proc/%d/exe", jpid);
  verifyBitness(env, (char *) &buf);
  CHECK_EXCEPTION;

  char err_buf[200];
  struct ps_prochandle* ph;
  if ( (ph = Pgrab(jpid, err_buf, sizeof(err_buf))) == NULL) {
    char msg[230];
    snprintf(msg, sizeof(msg), "Can't attach to the process: %s", err_buf);
    THROW_NEW_DEBUGGER_EXCEPTION(msg);
  }
  (*env)->SetLongField(env, this_obj, p_ps_prochandle_ID, (jlong)(intptr_t)ph);
  fillThreadsAndLoadObjects(env, this_obj, ph);
}

/*
 * Class:     sun_jvm_hotspot_debugger_linux_LinuxDebuggerLocal
 * Method:    attach0
 * Signature: (Ljava/lang/String;Ljava/lang/String;)V
 */
JNIEXPORT void JNICALL Java_sun_jvm_hotspot_debugger_linux_LinuxDebuggerLocal_attach0__Ljava_lang_String_2Ljava_lang_String_2
  (JNIEnv *env, jobject this_obj, jstring execName, jstring coreName) {
  const char *execName_cstr;
  const char *coreName_cstr;
  jboolean isCopy;
  struct ps_prochandle* ph;

  execName_cstr = (*env)->GetStringUTFChars(env, execName, &isCopy);
  CHECK_EXCEPTION;
  coreName_cstr = (*env)->GetStringUTFChars(env, coreName, &isCopy);
  CHECK_EXCEPTION;

  verifyBitness(env, execName_cstr);
  CHECK_EXCEPTION;

  if ( (ph = Pgrab_core(execName_cstr, coreName_cstr)) == NULL) {
    (*env)->ReleaseStringUTFChars(env, execName, execName_cstr);
    (*env)->ReleaseStringUTFChars(env, coreName, coreName_cstr);
    THROW_NEW_DEBUGGER_EXCEPTION("Can't attach to the core file");
  }
  (*env)->SetLongField(env, this_obj, p_ps_prochandle_ID, (jlong)(intptr_t)ph);
  (*env)->ReleaseStringUTFChars(env, execName, execName_cstr);
  (*env)->ReleaseStringUTFChars(env, coreName, coreName_cstr);
  fillThreadsAndLoadObjects(env, this_obj, ph);
}

// attach to the process. One and only one exposed stuff
struct ps_prochandle* Pgrab(pid_t pid, char* err_buf, size_t err_buf_len) {
  struct ps_prochandle* ph = NULL;
  thread_info* thr = NULL;

  if ( (ph = (struct ps_prochandle*) calloc(1, sizeof(struct ps_prochandle))) == NULL) {
     snprintf(err_buf, err_buf_len, "can't allocate memory for ps_prochandle");
     print_debug("%s\n", err_buf);
     return NULL;
  }

  if (ptrace_attach(pid, err_buf, err_buf_len) != true) {
     free(ph);
     return NULL;
  }

  // initialize ps_prochandle
  ph->pid = pid;

  // initialize vtable
  ph->ops = &process_ops;

  // read library info and symbol tables, must do this before attaching threads,
  // as the symbols in the pthread library will be used to figure out
  // the list of threads within the same process.
  read_lib_info(ph);

  // read thread info
  read_thread_info(ph, add_new_thread);

  // attach to the threads
  thr = ph->threads;
  while (thr) {
     // don't attach to the main thread again
    if (ph->pid != thr->lwp_id && ptrace_attach(thr->lwp_id, err_buf, err_buf_len) != true) {
        // even if one attach fails, we get return NULL
        Prelease(ph);
        return NULL;
     }
     thr = thr->next;
  }
  return ph;
}

// attach to a process/thread specified by "pid"
static bool ptrace_attach(pid_t pid, char* err_buf, size_t err_buf_len) {
  if (ptrace(PTRACE_ATTACH, pid, NULL, NULL) < 0) {
    char buf[200];
    char* msg = strerror_r(errno, buf, sizeof(buf));
    snprintf(err_buf, err_buf_len, "ptrace(PTRACE_ATTACH, ..) failed for %d: %s", pid, msg);
    print_debug("%s\n", err_buf);
    return false;
  } else {
    return ptrace_waitpid(pid);
  }
}