What is the difference between small memory model and large memory model

You can use the java.lang.instrument package.

Compile and put this class in a JAR:

import java.lang.instrument.Instrumentation;

public class ObjectSizeFetcher {
    private static Instrumentation instrumentation;

    public static void premain(String args, Instrumentation inst) {
        instrumentation = inst;
    }

    public static long getObjectSize(Object o) {
        return instrumentation.getObjectSize(o);
    }
}

Add the following to your MANIFEST.MF:

Premain-Class: ObjectSizeFetcher

Use the getObjectSize() method:

public class C {
    private int x;
    private int y;

    public static void main(String [] args) {
        System.out.println(ObjectSizeFetcher.getObjectSize(new C()));
    }
}

Invoke with:

java -javaagent:ObjectSizeFetcherAgent.jar C

Windows

Some of the above values are easily available from the appropriate Win32 API, I just list them here for completeness. Others, however, need to be obtained from the Performance Data Helper library (PDH), which is a bit "unintuitive" and takes a lot of painful trial and error to get to work. (At least it took me quite a while, perhaps I've been only a bit stupid...)

Note: for clarity all error checking has been omitted from the following code. Do check the return codes...!

• Total Virtual Memory:

  #include "windows.h"
  
  MEMORYSTATUSEX memInfo;
  memInfo.dwLength = sizeof(MEMORYSTATUSEX);
  GlobalMemoryStatusEx(&memInfo);
  DWORDLONG totalVirtualMem = memInfo.ullTotalPageFile;
  

  Note: The name "TotalPageFile" is a bit misleading here. In reality this parameter gives the "Virtual Memory Size", which is size of swap file plus installed RAM.

• Virtual Memory currently used:

  Same code as in "Total Virtual Memory" and then

   DWORDLONG virtualMemUsed = memInfo.ullTotalPageFile - memInfo.ullAvailPageFile;
  

• Virtual Memory currently used by current process:

  #include "windows.h"
  #include "psapi.h"
  
  PROCESS_MEMORY_COUNTERS_EX pmc;
  GetProcessMemoryInfo(GetCurrentProcess(), (PROCESS_MEMORY_COUNTERS*)&pmc, sizeof(pmc));
  SIZE_T virtualMemUsedByMe = pmc.PrivateUsage;
  

• Total Physical Memory (RAM):

  Same code as in "Total Virtual Memory" and then

  DWORDLONG totalPhysMem = memInfo.ullTotalPhys;
  

• Physical Memory currently used:

  Same code as in "Total Virtual Memory" and then

  DWORDLONG physMemUsed = memInfo.ullTotalPhys - memInfo.ullAvailPhys;
  

• Physical Memory currently used by current process:

  Same code as in "Virtual Memory currently used by current process" and then

  SIZE_T physMemUsedByMe = pmc.WorkingSetSize;
  

• CPU currently used:

  #include "TCHAR.h"
  #include "pdh.h"
  
  static PDH_HQUERY cpuQuery;
  static PDH_HCOUNTER cpuTotal;
  
  void init(){
      PdhOpenQuery(NULL, NULL, &cpuQuery);
      // You can also use L"\\Processor(*)\\% Processor Time" and get individual CPU values with PdhGetFormattedCounterArray()
      PdhAddEnglishCounter(cpuQuery, L"\\Processor(_Total)\\% Processor Time", NULL, &cpuTotal);
      PdhCollectQueryData(cpuQuery);
  }
  
  double getCurrentValue(){
      PDH_FMT_COUNTERVALUE counterVal;
  
      PdhCollectQueryData(cpuQuery);
      PdhGetFormattedCounterValue(cpuTotal, PDH_FMT_DOUBLE, NULL, &counterVal);
      return counterVal.doubleValue;
  }
  

• CPU currently used by current process:

  #include "windows.h"
  
  static ULARGE_INTEGER lastCPU, lastSysCPU, lastUserCPU;
  static int numProcessors;
  static HANDLE self;
  
  void init(){
      SYSTEM_INFO sysInfo;
      FILETIME ftime, fsys, fuser;
  
      GetSystemInfo(&sysInfo);
      numProcessors = sysInfo.dwNumberOfProcessors;
  
      GetSystemTimeAsFileTime(&ftime);
      memcpy(&lastCPU, &ftime, sizeof(FILETIME));
  
      self = GetCurrentProcess();
      GetProcessTimes(self, &ftime, &ftime, &fsys, &fuser);
      memcpy(&lastSysCPU, &fsys, sizeof(FILETIME));
      memcpy(&lastUserCPU, &fuser, sizeof(FILETIME));
  }
  
  double getCurrentValue(){
      FILETIME ftime, fsys, fuser;
      ULARGE_INTEGER now, sys, user;
      double percent;
  
      GetSystemTimeAsFileTime(&ftime);
      memcpy(&now, &ftime, sizeof(FILETIME));
  
      GetProcessTimes(self, &ftime, &ftime, &fsys, &fuser);
      memcpy(&sys, &fsys, sizeof(FILETIME));
      memcpy(&user, &fuser, sizeof(FILETIME));
      percent = (sys.QuadPart - lastSysCPU.QuadPart) +
          (user.QuadPart - lastUserCPU.QuadPart);
      percent /= (now.QuadPart - lastCPU.QuadPart);
      percent /= numProcessors;
      lastCPU = now;
      lastUserCPU = user;
      lastSysCPU = sys;
  
      return percent * 100;
  }
  

Linux

On Linux the choice that seemed obvious at first was to use the POSIX APIs like getrusage() etc. I spent some time trying to get this to work, but never got meaningful values. When I finally checked the kernel sources themselves, I found out that apparently these APIs are not yet completely implemented as of Linux kernel 2.6!?

In the end I got all values via a combination of reading the pseudo-filesystem /proc and kernel calls.

• Total Virtual Memory:

  #include "sys/types.h"
  #include "sys/sysinfo.h"
  
  struct sysinfo memInfo;
  
  sysinfo (&memInfo);
  long long totalVirtualMem = memInfo.totalram;
  //Add other values in next statement to avoid int overflow on right hand side...
  totalVirtualMem += memInfo.totalswap;
  totalVirtualMem *= memInfo.mem_unit;
  

• Virtual Memory currently used:

  Same code as in "Total Virtual Memory" and then

  long long virtualMemUsed = memInfo.totalram - memInfo.freeram;
  //Add other values in next statement to avoid int overflow on right hand side...
  virtualMemUsed += memInfo.totalswap - memInfo.freeswap;
  virtualMemUsed *= memInfo.mem_unit;
  

• Virtual Memory currently used by current process:

  #include "stdlib.h"
  #include "stdio.h"
  #include "string.h"
  
  int parseLine(char* line){
      // This assumes that a digit will be found and the line ends in " Kb".
      int i = strlen(line);
      const char* p = line;
      while (*p <'0' || *p > '9') p++;
      line[i-3] = '\0';
      i = atoi(p);
      return i;
  }
  
  int getValue(){ //Note: this value is in KB!
      FILE* file = fopen("/proc/self/status", "r");
      int result = -1;
      char line[128];
  
      while (fgets(line, 128, file) != NULL){
          if (strncmp(line, "VmSize:", 7) == 0){
              result = parseLine(line);
              break;
          }
      }
      fclose(file);
      return result;
  }
  

• Total Physical Memory (RAM):

  Same code as in "Total Virtual Memory" and then

  long long totalPhysMem = memInfo.totalram;
  //Multiply in next statement to avoid int overflow on right hand side...
  totalPhysMem *= memInfo.mem_unit;
  

• Physical Memory currently used:

  Same code as in "Total Virtual Memory" and then

  long long physMemUsed = memInfo.totalram - memInfo.freeram;
  //Multiply in next statement to avoid int overflow on right hand side...
  physMemUsed *= memInfo.mem_unit;
  

• Physical Memory currently used by current process:

  Change getValue() in "Virtual Memory currently used by current process" as follows:

  int getValue(){ //Note: this value is in KB!
      FILE* file = fopen("/proc/self/status", "r");
      int result = -1;
      char line[128];
  
      while (fgets(line, 128, file) != NULL){
          if (strncmp(line, "VmRSS:", 6) == 0){
              result = parseLine(line);
              break;
          }
      }
      fclose(file);
      return result;
  }
  

• CPU currently used:

  #include "stdlib.h"
  #include "stdio.h"
  #include "string.h"
  
  static unsigned long long lastTotalUser, lastTotalUserLow, lastTotalSys, lastTotalIdle;
  
  void init(){
      FILE* file = fopen("/proc/stat", "r");
      fscanf(file, "cpu %llu %llu %llu %llu", &lastTotalUser, &lastTotalUserLow,
          &lastTotalSys, &lastTotalIdle);
      fclose(file);
  }
  
  double getCurrentValue(){
      double percent;
      FILE* file;
      unsigned long long totalUser, totalUserLow, totalSys, totalIdle, total;
  
      file = fopen("/proc/stat", "r");
      fscanf(file, "cpu %llu %llu %llu %llu", &totalUser, &totalUserLow,
          &totalSys, &totalIdle);
      fclose(file);
  
      if (totalUser < lastTotalUser || totalUserLow < lastTotalUserLow ||
          totalSys < lastTotalSys || totalIdle < lastTotalIdle){
          //Overflow detection. Just skip this value.
          percent = -1.0;
      }
      else{
          total = (totalUser - lastTotalUser) + (totalUserLow - lastTotalUserLow) +
              (totalSys - lastTotalSys);
          percent = total;
          total += (totalIdle - lastTotalIdle);
          percent /= total;
          percent *= 100;
      }
  
      lastTotalUser = totalUser;
      lastTotalUserLow = totalUserLow;
      lastTotalSys = totalSys;
      lastTotalIdle = totalIdle;
  
      return percent;
  }
  

• CPU currently used by current process:

  #include "stdlib.h"
  #include "stdio.h"
  #include "string.h"
  #include "sys/times.h"
  #include "sys/vtimes.h"
  
  static clock_t lastCPU, lastSysCPU, lastUserCPU;
  static int numProcessors;
  
  void init(){
      FILE* file;
      struct tms timeSample;
      char line[128];
  
      lastCPU = times(&timeSample);
      lastSysCPU = timeSample.tms_stime;
      lastUserCPU = timeSample.tms_utime;
  
      file = fopen("/proc/cpuinfo", "r");
      numProcessors = 0;
      while(fgets(line, 128, file) != NULL){
          if (strncmp(line, "processor", 9) == 0) numProcessors++;
      }
      fclose(file);
  }
  
  double getCurrentValue(){
      struct tms timeSample;
      clock_t now;
      double percent;
  
      now = times(&timeSample);
      if (now <= lastCPU || timeSample.tms_stime < lastSysCPU ||
          timeSample.tms_utime < lastUserCPU){
          //Overflow detection. Just skip this value.
          percent = -1.0;
      }
      else{
          percent = (timeSample.tms_stime - lastSysCPU) +
              (timeSample.tms_utime - lastUserCPU);
          percent /= (now - lastCPU);
          percent /= numProcessors;
          percent *= 100;
      }
      lastCPU = now;
      lastSysCPU = timeSample.tms_stime;
      lastUserCPU = timeSample.tms_utime;
  
      return percent;
  }
  

TODO: Other Platforms

I would assume, that some of the Linux code also works for the Unixes, except for the parts that read the /proc pseudo-filesystem. Perhaps on Unix these parts can be replaced by getrusage() and similar functions?