Sunday, October 2, 2016

The Sad State of Digital "Ownership"

I've been thinking a lot lately about how consumers of computer software have very little rights. EULAs and subscriber agreements seem to give businesses all of the power and the end users get little to nothing. This is actually quite depressing and I am surprised that, in 2016, we do not have more rights as digital consumers. It seems like the EFF or someone should be advocating harder for such rights and getting more precedents set. Richard Stallman has done a decent job overall, but comes off as an extremist (no offense to Mr. Stallman as I am an avid GPL fan!) There needs to be a middle ground when it comes to commercial software. What I would ultimately like to see is the following (as it pertains to game software):

  • I should be able to lend my copy of a video game to anyone I see fit just like we still can with physical media for consoles. Steam is almost there with Steam Family Sharing, but they lock your entire library when another person is playing one of your games. The lock should be on a per-game basis. The current per-library lock is far too restrictive to make sharing practical.

  • I should be able to purchase a copy of a game ONE TIME (for the same platform at the very least.) For example, if I purchase Mass Effect 3 for the PC from a store, the license should be valid not just within Origin, but on any PC distribution system such as Steam. Something that has also irked me in this regard is how Square Enix forces you to re-purchase the PC client of Final Fantasy: A Realm Reborn when you already purchased the software for a console.

  • Origin, PlayStation Network Store, Steam, Uplay, Xbox Live Marketplace, and the like can pull the plug on your account at *any* time for *any* reason. Blizzard and Steam do not even tell you why and may even refuse to assist you further. If you spent hundreds to thousands of dollars on games, you lose access to those titles. You should retain the right to use what you paid for despite losing access to the service. This opens up another can of worms: What happens when an MMORPG server goes away? Now you're left with a client that you likely paid for and now it can connect to nothing. You probably weren't refunded either (I'm looking at you, NCSoft.)

  • When I die, I would like to give my game collection to a family member. However, most of the aforementioned digital distribution services do not allow transfer of the account or titles to anyone else.

  • When you legitimately purchase a copy of a game, you should own that copy just like you own physical objects that you pay for such as automobiles, mobile devices, gardening equipment, your toothbrush, etc.

This stuff should anger and/or scare you because it totally stinks. The way things are currently, the virtual items you forked over money for can essentially disappear and you'll have nothing to show for it. I would like to point out that I love Steam. However, this is the biggest fault I find with their service... And it is a glaring one. If they resolved this issue, they would be nearly perfect. For now, I have no choice but to agree to their terms in order to continue accessing my library while having the fear in the back of my mind that my access could be revoked at any moment. I have slowed down on Steam purchases for this very reason.

I have found some hope though with a platform known as GOG. The current selection is nowhere near as large as Steam, but more titles are being added as time progresses. I've started using their Galaxy Client which behaves similarly to Steam. Their games are not tied to the service. Once you download them, they are yours forever. You can also re-download them, although you should make backups just in case the service ever ceases to exist. Lastly, there is GOG Connect which lets you import some of your Steam games to your GOG Library at no cost. You'll now have two separate copies of the game that exists within both libraries. The selection of games on Connect is quite small at the moment, but the amount is expected to grow as GOG develops better relationships with publishers. I highly recommend GOG to anyone upset by the current state of things or if you're just a PC gamer.

Our savior?
I usually do not complain publicly, but wanted to get all of these thoughts out there in the hopes that more people express their disappointment with the present software licensing and lack of digital consumer rights situation.

Sunday, December 20, 2015

Tactfully Handling Common Java Complaints

Java Sucks! ... Or Not?

So, your friends tell you Java blows chunks. They've either heard/read it elsewhere, had a bad history with slow-loading applets in 1996, or have personally worked with the language and loathe it. I'd like to dispel some of the arguments against Java in the modern age since I think it is a decent language. Let's start with some common things we hear or see about Java...

Java is Slow!

I remember loading applets on websites in Windows 95 and absolutely detesting the experience. Of course, this was on a 486 SX operating at 25 MHz with 4M of RAM and a 14.4Kbps modem. Java was also still in its infancy. Many people had similar experiences and the rancor spread. Unfortunately, such sentiment still exists today -- 20 years later. I am even guilty. A coworker in 2006 had an O'Reilly Java book he offered to lend me. I declined the offer and poked fun at him for even suggesting such a thing. Fast forward two years and I had become a Java fan. It was not until I was forced to learn the language in college that I developed an appreciation for it. Let's review some facts:
  1. Hardware has improved since the 1990s. Processors are faster and have more cache and registers. Bus speeds, disks, memory, and network access are also faster which improve program load times.
  2. The JVM has improved since the 1990s. HotSpot/JIT (just-in-time compilation), JNI (Java Native Interface), and other features have been added. There is also an array of garbage collection algorithms to choose from depending on your application.
With the convergence of #1 and #2 above, Java performance has come a long way. However, in many (not all) cases, C and C++ programs still handily beat Java in the performance department. Have a look at the matrix addition performance comparison below. The code is functionally equivalent in both the C and Java programs. Two one-dimensional arrays are filled with random numbers ranging from 0 to 65,535. The sum of the elements at the current index in both arrays are stored at the current position in the first array.
#include <stdlib.h> /* rand() and srand() */
#include <time.h>   /* time_t */

void fill_rand(int *arr, int length)
  int i;
  for(i = 0; i < length; i++)
    arr[i] = rand() % 65535 + 1;

void add_arr(int *arr1, int *arr2, int length)
  int i;
  for(i = 0; i < length; i++)
    arr1[i] += arr2[i];

int main()
  int arr1[5000];
  int arr2[5000];
  time_t t;
  fill_rand(arr1, 5000);
  fill_rand(arr2, 5000);
  add_arr(arr1, arr2, 5000);
  return 0;

import java.util.Random;

class Arr
  static void fillRand(int[] arr)
    Random rand = new Random();
    for(int i = 0; i < arr.length; i++)
      arr[i] = rand.nextInt(65535) + 1;

  static void addArr(int[] arr1, int[] arr2)
    for(int i = 0; i < arr1.length; i++)
      arr1[i] += arr2[i];

  public static void main(String[] args)
    int[] arr1 = new int[5000];
    int[] arr2 = new int[5000];
    addArr(arr1, arr2);

The C code executes in approximately 4ms. In comparison, the Java equivalent takes about 207ms. That is over fifty times longer than the C program. Why? Well, the JVM "warm-up" time needs to be considered. If we make the following changes to the main() method to disregard warm-up time, we get a more reasonable execution time of about 6ms:
    long startTime = System.nanoTime();
    int[] arr1 = new int[5000];
    int[] arr2 = new int[5000];
    addArr(arr1, arr2);
    long endTime = System.nanoTime();
    System.out.println((endTime - startTime) / 1000000); // get ms

That isn't so bad. In fact, that is where JIT shines for long-running and commonly-executed code. If we arbitrarily loop over the C program 100 times and sleep 1 second between iterations, the execution time will be similar for each iteration whereas the Java program should become faster (until a peak is achieved.) Modifying main() demonstrates this:
  public static void main(String[] args) throws InterruptedException
    for(int i = 0; i < 100; i++)
      long startTime = System.nanoTime();
      int[] arr1 = new int[5000];
      int[] arr2 = new int[5000];
      addArr(arr1, arr2);
      long endTime = System.nanoTime();
      System.out.println("Iteration " + i + ": " + (endTime - startTime) / 1000000 + "ms");

[lloyd@lindev ~]$ java Arr
Iteration 0: 6ms
Iteration 1: 3ms
Iteration 2: 3ms
Iteration 3: 3ms
Iteration 4: 3ms
Iteration 5: 3ms
Iteration 6: 3ms
Iteration 7: 2ms
Iteration 8: 2ms
Iteration 9: 2ms
Iteration 10: 2ms
Iteration 11: 2ms
Iteration 12: 1ms
Iteration 13: 1ms
Iteration 14: 1ms
Iteration 15: 1ms
Iteration 16: 1ms
Iteration 17: 1ms
Iteration 18: 1ms
Iteration 19: 0ms
Iteration 20: 0ms
Iteration 21: 0ms
Iteration 22: 0ms
Iteration 23: 0ms
Iteration 24: 0ms
Iteration 25: 0ms
Iteration 26: 0ms
Iteration 27: 0ms
Iteration 28: 0ms
Iteration 29: 0ms
Iteration 30: 0ms
Iteration 31: 0ms

The code within the loop body eventually executes faster than the equivalent C code due to JVM runtime optimizations. Even though the program reports a time of 0ms, it obviously still takes micro/nanoseconds to compute which are truncated off.

Java is Insecure!

As with most programs written in C/C++ (Apache HTTPD, ISC BIND, OpenSSL, etc.) there are vulnerabilities detected periodically for Java. These are primarily due to the potential dangers of inappropriate pointer use or from undersized buffers which allow overflows. The Java language itself features policies (via security manager) you can manipulate to effectively sandbox an application. This isolation limits what the program can do with resources such as disks and network access. Another thing to consider is that the JVM is an entire platform and is fairly sizable. The JVM needs to define types, abstract networking and GUI elements, and more. There is obviously an increased risk of bugs the more lines of code a program contains. For most Windows installations, Java even nags you when updates are available or takes care of updating itself automagically. In summary for this section, Java has admittedly had a large number of vulnerabilities over the years. Battling exploits is a part of life that IT folks must deal with. On the bright side, operating systems, web browsers, and Adobe Flash seem to have more vulnerabilities and keeping Java up-to-date is relatively easy.

Java is Bloated!

Java can handily consume a substantial amount of memory. I had 4M of RAM in 1994 or so. It is fairly common for users to have 8G or 16G these days. Of course, that is no reason to be wasteful. But, one must again consider that the JVM is an entire platform with a large set of features. The memory overhead that accompanies this should be expected. There are many things you can do to tune JVM memory usage. To put things into perspective, I am running a Tomcat instance, a Jetty instance, and one more JVM instance for a JRuby daemon on a virtual private server with 2G of memory (which is also running a plethora of other services) without breaking a sweat. Java also runs on many less-powerful mobile and embedded devices. To recap: Memory is plentiful and fairly cheap these days, the JVM can be tuned to use less memory, and don't be such a tightwad!

Why Java is Annoying

  • Java language lawyers who believe the JLS (Java Language Specification) is the only thing that matters. To them, memory addresses do not exist...
  • Unlike Ruby, everything is not an object (primitives like byte, short, int, long, etc.)
  • No explicit pointers
  • The library is too big
  • Calls to System.gc() are only suggestions that can be ignored
  • Cannot explicitly call deconstructors
  • Forced to put main() method in a class
  • Syntax can be very verbose/repetitive
  • No operator overloading
  • No multiple inheritance
  • It can be a hog unless you cap the heap
  • No native way to become a daemon or service
  • Others?

Why Java is Awesome

  • It picks up after you
  • Not having to deal with explicit pointers
  • Huge library
  • No operator overloading
  • No multiple inheritance
  • Portable networking and GUI code
  • Largely portable for most other things/compile once run anywhere
  • No need for sizeof
  • It's ubiquitous
  • Multithreading
  • Others?

Thursday, December 10, 2015

64-bit Linux Assembly Tutorial Part II


Welcome to the second installment of the 64-bit Linux assembly tutorial. If you have not yet read part one of this tutorial, you can do so here. If you have read it, I hope that you enjoyed it. We will be covering networking, debugging, optimizing, endianness, and analogous C code in this tutorial. Let us get to it!

Useful Links

The C Way

We are going to start by looking at how you create a network program in C. See Beej's Guide to Network Programming for more information. I am illustrating socket programming in a higher-level language to give you a better idea of the sequence of events that occur. In order to accept network connections in a C program (or assembly), you must take the following steps:
  1. Call socket() to obtain a file descriptor to be used for communication. We used file descriptors in the first tutorial (stdin/0 and stdout/1 specifically.)
  2. Call bind() to associate (or bind) the IP address of a network interface with the file descriptor returned by socket().
  3. Call listen() to make the file descriptor be receptive to incoming network connections.
  4. Call accept() to handle incoming network connections.
accept() returns a file descriptor for the client which you can use to send and receive data to and from the remote end. You can also call close() on the client file descriptor once you are done receiving or transmitting data. After putting it all together, it would look something like:
#include <stdio.h>        /* for printf() and puts() */
#include <stdlib.h>       /* for exit() and perror() */
#include <string.h>       /* for strlen() */
#include <sys/socket.h>   /* for AF_INET, SOCK_STREAM, and socket_t */
#include <netinet/in.h>   /* for INADDR_ANY and sockaddr_in */

#define PORT 9990         /* TCP port number to accept connections on */
#define BACKLOG 10        /* connection queue limit */

int main()
  /* server and connecting client file descriptors */
  int server_fd, client_fd;

  /* size of sockaddr_in structure */
  int addrlen;

  /* includes information for the server socket */
  struct sockaddr_in server_address;

  /* message we send to connecting clients */
  char *message = "Greetings!\n";

  /* socket() - returns a file descriptor we can use for our server
   * or -1 if there was a problem
   * Arguments:
   * AF_INET = address family Internet (for Internet addressing)
   * SOCK_STREAM = TCP (Transmission Control Protocol)
   * 0 = default protocol for this type of socket
  server_fd = socket(AF_INET, SOCK_STREAM, 0);

  /* Check for an error */
  if(server_fd == -1)
    perror("Unable to obtain a file descriptor for the server");

  server_address.sin_family = AF_INET;

  /* set the listen address to any/all available */
  server_address.sin_addr.s_addr = INADDR_ANY;

  /* The htons() function below deals with endian conversion which
   * we'll discuss later. This assignment sets the port number to
   * accept connections on. */
  server_address.sin_port = htons(PORT);

  /* bind() - binds the IP address to the server's file descriptor or
   * returns -1 if there was a problem */
  if(bind(server_fd, (struct sockaddr *)&server_address,
          sizeof(server_address)) == -1)
    perror("Unable to bind");

  /* listen() - listen for incoming connections */
  if(listen(server_fd, BACKLOG) == -1)
    puts("Failed to listen on server socket!");

  addrlen = sizeof(server_address);

  puts("Waiting for connections...");

  /* Infinite loop to accept connections forever */
    /* accept() - handle new client connections */
    client_fd = accept(server_fd, (struct sockaddr *)&server_address,
    if(client_fd == -1)
      perror("Unable to accept client connection");
    /* Send greeting to client and then disconnect them */
    send(client_fd, message, strlen(message), 0);

  return 0;

You should be able to copy and paste the above code into a text file.
Compile it with: gcc <file>.c -o network_example
After compiling the program, execute it with: ./network_example
If all went well, you should see something similar to below:
[lloyd@lindev ~]$ ./network_example
Waiting for connections...

Open another terminal and issue: telnet localhost 9990
You should see something like the following:
[lloyd@lindev ~]$ telnet localhost 9990
Trying ::1...
telnet: connect to address ::1: Connection refused
Connected to localhost.
Escape character is '^]'.
Connection closed by foreign host.

You can read more about bind(), listen(), and accept() if you're interested. Next up, we're going to replicate the above C program in x86-64 assembly. Let's see how it looks...

The Assembly Way

[BITS 64]

; Description: 64-bit Linux TCP server
; Author: Lloyd Dilley
; Date: 04/02/2014

struc sockaddr_in
  .sin_family resw 1
  .sin_port resw 1
  .sin_address resd 1
  .sin_zero resq 1

section .bss
    istruc sockaddr_in
      at sockaddr_in.sin_family, resw 1
      at sockaddr_in.sin_port, resw 1
      at sockaddr_in.sin_address, resd 1
      at sockaddr_in.sin_zero, resq 1

section .data
  waiting:      db 'Waiting for connections...',0x0A
  waiting_len:  equ $-waiting
  greeting:     db 'Greetings!',0x0A
  greeting_len: equ $-greeting
  error:        db 'An error was encountered!',0x0A
  error_len:    equ $-error
  addr_len:     dq 16
    istruc sockaddr_in
      ; AF_INET
      at sockaddr_in.sin_family, dw 2
      ; TCP port 9990 (network byte order)
      at sockaddr_in.sin_port, dw 0x0627
      ; (network byte order)
      at sockaddr_in.sin_address, dd 0x0100007F
      at sockaddr_in.sin_zero, dq 0

section .text
global _start
  ; Get a file descriptor for sys_bind
  mov rax, 41           ; sys_socket
  mov rdi, 2            ; AF_INET
  mov rsi, 1            ; SOCK_STREAM
  mov rdx, 0            ; protocol
  mov r13, rax
  push rax              ; store return value (fd)
  test rax, rax         ; check if -1 was returned
  js exit_error

  ; Bind to a socket
  mov rax, 49           ; sys_bind
  pop rdi               ; file descriptor from sys_socket
  mov rbx, rdi          ; preserve server fd (rbx is saved across calls)
  mov rsi, sockaddr
  mov rdx, 16           ; size of sin_address is 16 bytes (64-bit address)
  push rax
  test rax, rax
  js exit_error

  ; Listen for connections
  mov rax, 50           ; sys_listen
  mov rdi, rbx          ; fd
  mov rsi, 10           ; backlog
  push rax
  test rax, rax
  js exit_error
  ; Notify user that we're ready to listen for incoming connections
  mov rax, 1            ; sys_write
  mov rdi, 1            ; file descriptor (1 is stdout)
  mov rsi, waiting
  mov rdx, waiting_len
  call accept

  ; Accept connections
  mov rax, 43           ; sys_accept
  mov rdi, rbx          ; fd
  mov rsi, peeraddr
  lea rdx, [addr_len]
  push rax
  test rax, rax
  js exit_error

  ; Send data
  mov rax, 1
  pop rdi               ; peer fd
  mov r15, rdi          ; preserve peer fd (r15 is saved across calls)
  mov rsi, greeting
  mov rdx, greeting_len
  push rax
  test rax, rax
  js exit_error

  ; Close peer socket
  mov rax, 3            ; sys_close
  mov rdi, r15          ; fd
  push rax
  test rax, rax
  js exit_error
  ;jz shutdown
  call accept           ; loop forever if preceding line is commented out

  ; Close server socket
  mov rax, 3
  mov rdi, rbx
  push rax
  test rax, rax
  js exit_error

  ; Exit normally
  mov rax, 60           ; sys_exit
  xor rdi, rdi          ; return code 0

  mov rax, 1
  mov rdi, 1
  mov rsi, error
  mov rdx, error_len

  mov rax, 60
  pop rdi               ; stored error code

Thank goodness for high-level languages, eh?
You can assemble and link just like you did from the first tutorial:
nasm -f elf64 -o network_example.o network_example.asm
ld -o network_example network_example.o

You can then execute the program and test it with telnet the same way you did with the C version. The functionality should be very similar.

Dissecting the Beast

NASM allows programmers to use structs, so we take advantage of this for better data organization. Just like in the C program, a sockaddr_in structure is defined. This is essentially a template which holds various data members. For review, the BSS section contains memory set aside for variable data during runtime. This makes sense considering it is not known what our connecting client source addresses and ports will be. And since we know what address and port to use on the server side, the information can be set in the data section as literals. I also touched on data types some in the first tutorial. The table below contains the types used in this program along with their sizes and examples.

Type Size Example
resb/db 1 byte (8 bits) A keyboard character such as the letter 'c'
resw/dw 2 bytes (16 bits) -- also called a "word" A network port with a maximum value of 65,535
resd/dd 4 bytes (32 bits) -- also called a "double word" An IPv4 address such as
resq/dq 8 bytes (64 bits) -- also called a "quad word" A "long long" in C/C++ or represents a decimal number (float)

An "octa word" (128 bits) is also worth mentioning, but is not used in this program. These are used for scientific calculations, graphics, IPv6 addresses, globally unique IDs (GUIDs), etc. The dX variety are initialized and the 'd' stands for "data". So, db is "data byte" and dw is "data word". The resX assortment is used for reserving space for uninitialized data. resb would be "reserve byte" and resq is "reserve quad" for example. The "at" macro gets at each field and sets it with the specified data. "struc" and "endstruc" define a structure. "istruc" and "iend" declare an instance of a structure. You can see in the code how to refer to an instance by using a label (peeraddr for example.)

In the text section (code), you should be able to get an idea of what is going on with the comments. The format is the same as the program from the first tutorial. It is all a matter of putting bullets (data) in certain positions (registers) of a revolver and then pulling the trigger with syscall. That is an analogy I like to use anyway. Again, you can refer to Ryan A. Chapman's 64-bit Linux system call table for reference. sys_bind, sys_listen, sys_accept, and other calls are all present there.

Ten Little Endians

Endianness (name originates from Gulliver's Travels) refers to the way data is arranged in memory in the context of hardware architectures. I bring this up because we needed to call htons() (short data from host to network order) in our C program on the network port. We also needed to convert the loopback IP address and TCP port number to network byte order in the assembly program.

x86/x86-64 are considered little-endian architectures whereas SPARC is big endian. Some processors, such as PPC, can handle both modes and are referred to as bi-endian. What does this mean exactly? Well, on little-endian machines, the most-significant byte (MSB) is stored at the highest memory address. The least-significant byte (LSB) is stored at the lowest address. Big endian is the reverse of this. An example would be storing three bytes that make up the word "BEEF". Using the ASCII values for each letter in hexadecimal: 'B' is 0x42, 'E' is 0x45, and 'F' is 0x46. On a big-endian system, the arrangement of bytes would appear as: 42 45 45 46. However, on a little-endian system, they would appear as: 46 45 45 42. Obviously, debugging is easier on a big-endian system since data is still easily readable by humans. Meanwhile, little endian has the advantage of programmers being able to determine if a number is even or odd by looking at its LSB.

Due to these differences, the need for a common format for data being transmitted over a network was clear. Big endian or network byte order was decided on for this purpose. How can we convert? The easiest method is to use a calculator in programmer mode. Windows calculator supports this mode. The TCP port number 9990 in decimal is 2706 in hex. Since 0x27 is the most significant part, it goes in the right-most slot. 0x06 goes on the left resulting in 0x0627. This is similar for the IP address. Each octet of must be converted to hex. This yields 7F 00 00 01. Again, 127 or 0x7F is the most significant part, so it goes on the far right (lowest memory address.) You end up with 0x0100007F.

A Closer Look

You can use gdb or valgrind to debug of course, but this section is more about tracing program execution to demonstrate what is going on from an OS perspective with system calls. If you have strace installed, issue:
strace -f ./network_example

You can actually see each system call from the assembly program and what arguments populate each function such as source port for peer address. Try connecting with telnet with the trace still running and you can see write() and close() being called. Have a look:
[lloyd@lindev ~]$ strace -f ./network_example
execve("./network_example", ["./network_example"], [/* 26 vars */]) = 0
bind(3, {sa_family=AF_INET, sin_port=htons(9990), sin_addr=inet_addr("")}, 16) = 0
listen(3, 10)                           = 0
write(1, "Waiting for connections...\n", 27Waiting for connections...
) = 27
accept(3, {sa_family=AF_INET, sin_port=htons(47944), sin_addr=inet_addr("")}, [16]) = 4
write(4, "Greetings!\n", 11)            = 11
close(4)                                = 0
accept(3, {sa_family=AF_INET, sin_port=htons(47946), sin_addr=inet_addr("")}, [16]) = 4
write(4, "Greetings!\n", 11)            = 11
close(4)                                = 0
accept(3, ^CProcess 27238 detached

You can see from above that the server is assigned a file descriptor of 3 and the client is 4. 11 is the length of the greeting sent to the client. sin_port and sin_addr from accept() contain the connecting client's source IP address and port. Pretty slick, huh?

Compacting A Compact Program

As you can see, the size difference between the assembly program and the C program is significant. The functionally-equivalent C program is over 4 times as large:
[lloyd@lindev ~]$ ls -lah network_example_*
-rwxr-xr-x. 1 lloyd linux_users 2.1K Dec 10 03:51 network_example_asm
-rwxr-xr-x. 1 lloyd linux_users 8.9K Dec 10 04:06 network_example_c

Let's see if we can squeeze both of these binaries a bit more...
[lloyd@lindev ~]$ strip -s network_example_*
[lloyd@lindev ~]$ ls -lah network_example_*
-rwxr-xr-x. 1 lloyd linux_users  888 Dec 10 04:28 network_example_asm
-rwxr-xr-x. 1 lloyd linux_users 6.2K Dec 10 04:28 network_example_c

Even after shaving off symbols from both binaries, the C program is now over 6 times larger than the assembly program. The assembly program isn't even 1K. This is a testament to assembly's efficiency. Yay for assembly!


I apologize for the delay between the first tutorial and this one. Better late than never, right? I hope people still find this information useful. If you have any questions or feedback, please drop me a line in the comments and I would be happy to reply.

Tuesday, September 2, 2014

JVM Memory Tuning and Analysis

The Java Virtual Machine (JVM) can be tuned to meet your program's memory needs. I will be discussing the various memory options along with how you can monitor the status of your Java programs.

Two Mounds

Let's start by looking at the various types of memory that Java uses. As you may already know, Java manages memory for you automatically (much like C# and Ruby do) via a process called garbage collection. There are two areas of memory in Java just like in C and C++. These areas are referred to as the stack and the heap. The stack is where local primitive variables, local object references (just the memory address/location of object data and not the actual object contents itself), and sometimes objects go (using a compile-time optimization as of Java 6 called escape analysis.) Also note that method parameters are considered local to the method where they are declared. When a code block or method goes out of a scope, the local variables go away. Any local reference variables pointing to objects are also gone. When there are no more references pointing to a particular object, the object is garbage collected and that location can be used again by another object. Since Java emphasizes concurrent programming, it is also worth mentioning that each thread has its own stack.

The heap typically contains the bulk of data for a Java program. Objects and their static and instance members reside on the heap. Have a look at the program below to help illustrate this information.

class Example
  public static int total = 0; // heap
  protected int a;             // heap
  protected int b;             // heap

  public static String sumOf(int a, int b) // primitives 'a' and 'b' live on the stack
    String c = "The sum is: " + (a + b); // reference 'c' lives on the stack
                                         // but, "The sum is: ..." lives on the heap
    total += a + b;
    return c;

  public static void main(String[] args)
    Example example = new Example();

    example.a = 5;  // example reference is on the stack and value '5' is on the heap
    example.b = 10; // example reference is on the stack and value '10' is on the heap

    System.out.println(sumOf(example.a, example.b));

Remember though that objects _may_ be allocated on the stack at times depending on what the compiler decides.

Analyzing Java Processes

You can get an idea of how much total memory (stack + heap) your program is consuming using Task Manager, ps, /proc, pmap, top, Activity Monitor, and the like. The Java Development Kit (JDK) actually contains some useful utilities for looking at running Java programs. These utilities live under the $JAVA_HOME/bin directory. First off, there's jps. This command displays running Java processes. It provides the PID (process ID) and name of the program. Below is an example of its output.

ldilley@develop:~$ jps
9515 Jps
9464 LinPot
ldilley@develop:~$ ps -ef | grep -v grep | grep 9464
ldilley   9464  9217  0 03:46 pts/0    00:00:00 java me.dilley.linpot.LinPot

Now that we have the PID of a running Java process, let's investigate it some more using the jmap utility. To use jmap, grab the PID of a running Java process you want to attach to. You can easily find running Java processes with jps as shown above.

ldilley@develop:~$ jmap -heap 9464
Attaching to process ID 9464, please wait...
Debugger attached successfully.
Server compiler detected.
JVM version is 24.65-b04

using thread-local object allocation.
Parallel GC with 2 thread(s)

Heap Configuration:
   MinHeapFreeRatio = 0
   MaxHeapFreeRatio = 100
   MaxHeapSize      = 698351616 (666.0MB)
   NewSize          = 1310720 (1.25MB)
   MaxNewSize       = 17592186044415 MB
   OldSize          = 5439488 (5.1875MB)
   NewRatio         = 2
   SurvivorRatio    = 8
   PermSize         = 21757952 (20.75MB)
   MaxPermSize      = 174063616 (166.0MB)
   G1HeapRegionSize = 0 (0.0MB)

Heap Usage:
PS Young Generation
Eden Space:
   capacity = 8912896 (8.5MB)
   used     = 1434848 (1.368377685546875MB)
   free     = 7478048 (7.131622314453125MB)
   16.098561006433822% used
From Space:
   capacity = 1048576 (1.0MB)
   used     = 0 (0.0MB)
   free     = 1048576 (1.0MB)
   0.0% used
To Space:
   capacity = 1048576 (1.0MB)
   used     = 0 (0.0MB)
   free     = 1048576 (1.0MB)
   0.0% used
PS Old Generation
   capacity = 21495808 (20.5MB)
   used     = 0 (0.0MB)
   free     = 21495808 (20.5MB)
   0.0% used
PS Perm Generation
   capacity = 22020096 (21.0MB)
   used     = 4385528 (4.182365417480469MB)
   free     = 17634568 (16.81763458251953MB)
   19.916025797526043% used

1131 interned Strings occupying 76888 bytes.

jmap is handy since it provides a detailed view of the heap. Don't worry if this output looks unintelligible. We shall decipher it together. We'll skip the thread-local object allocation setting since it is outside the scope of this post. It is enabled by default and can be toggled with the -XX:UseTLAB option. You can read more about it here if you are interested.

Taking Out the Trash

Next comes the garbage collection method. My Java 7 JVM is using parallel garbage collection by default. This method uses multiple threads to perform collection. It can be explicitly enabled with the -XX:+UseParallelGC option. The JVM also supports "Mark Sweep Compact Garbage Collection" which is a serial collection method recommended for systems with a single processor or for applications with small amounts of data. -XX:+UseSerialGC is used to enable it. Lastly, -XX:+UseConcMarkSweepGC enables the concurrent collector. This collection method aims to increase application response time by minimizing pausing. If concurrent collection incurs significant pausing, you can enable incremental mode with -Xincgc which should shorten the pauses.

To summarize:

Serial Garbage Collection
  • Enabled with -XX:+UseSerialGC
  • Also called "Mark Sweep Compact Garbage Collection"
  • Useful on systems with a single processor
  • Useful when object data does not exceed 100MB
Parallel Garbage Collection
  • Enabled with -XX:+UseParallelGC
  • Is similar to the serial method, but uses multiple threads for collection
  • Useful on systems with multiple processors
  • Number of collector threads can be controlled with -XX:ParallelGCThreads=<number>
  • The number of collector threads should be equal to the number of system cores
Concurrent Garbage Collection
  • Enabled with -XX:+UseConcMarkSweepGC
  • Useful on systems with multiple processors
  • Useful for low-latency applications such as 3D games
  • Can be told to collect incrementally with -Xincgc which lessens pausing that may occur during collection
  • -Xincgc is the equivalent of specifying -XX:+UseConcMarkSweepGC and -XX:+CMSIncrementalMode
  • Is a good default to use
You can monitor garbage collection statistics to see which algorithm works best for your program by enabling the following JVM options:
  1. -verbose:gc
  2. -XX:+PrintGCDetails
  3. -XX:+PrintGCTimeStamps
I'd also like to bring up compaction. Just like filesystems on a hard disk can become fragmented, so too can the heap. You may already know that some data structures such as arrays are allocated in memory sequentially. Let's say for the sake of simplicity that there is 1 simple 1-byte object on the heap followed by a sequential array of 1,000 1-kilobyte elements and then 1 other simple 1-byte object. What happens when the array is garbage collected and a new larger array needs room on the heap? It is not a very efficient use of space to have many gaps. Compaction mitigates this issue by squeezing data closer together.

Digging Through the Heap

As if things weren't complicated enough, Java breaks up the heap into other segments where objects reside. These segments are called generations. Different implementations of the JVM may use different terms to describe these areas. However, they are all generational. This means that objects essentially age. When an object is created, it is spawned in the young/new/Eden space. One exception is that if the object is rather large, it will go directly to the tenured/old generation which typically is larger in size. If all references to the object go away, it is marked for collection. If the object survives, it grows older and is moved to the tenured area. And as you guessed, if an object is geriatric, it will live in the retirement housing area known as the old generation space. "PS" in this context means "parallel scavenge" and simply reflects the garbage collection algorithm employed. If you change from parallel collection to serial or concurrent, the number of areas and names may change.

I have not yet touched on the permanent generation area, but I will do so in the summary of heap areas below.

  • Part of the young generation
  • Where new objects are freshly allocated (unless they are too large -- then they go directly to the tenured/old generation)
  • Used for quick allocation
Survivor (or From Space and To Space)
  • Also part of the young generation
  • Objects that survive Eden/New space collection become survivors
  • The "From" and "To" space roles are swapped once one area has been collected and cleared (one is usually empty)
  • Objects that make it past the survivor phase retire to the tenured/old space to live out their days (or minutes/hours)
  • This area is not as frequently collected as the Eden/New space
  • Objects that are deemed too large skip the young generation altogether and go here
  • The permanent generation
  • Loaded class information lives here such as reflective information about classes (metadata) and interned strings (until Java 7 at least)
The bit about interned strings provides details of how many string objects have been added to the string intern pool. This is an optimization that stores unique instances of string literals thus reducing memory usage.

Bending the JVM to Your Will

You might be wondering why we did not cover the heap configuration section of the jmap output yet. You may also be pondering when and why you would want to change the way the JVM handles memory and how. Wonder and ponder no more. Read on for the respective JVM options and the situations in which you would modify them.

Sets the thread stack size. Even if you have not created threads explicitly, your program is a single thread. If you have a large call stack or are making heavy use of recursion, you will want to increase this value. You'll know when you start receiving StackOverflowError messages.
Example which sets the stack size to 2 megabytes: java -Xss2M someClass

-Xms<number>[M/G] and -Xmx<number>[M/G]
Sets the minimum and maximum heap size. If you are loading a significant amount of objects in memory and see "java.lang.OutOfMemoryError: Java heap space", you'll want to increase these values.
Example which sets the initial heap to 512M and maximum heap size to 4G:
java -Xms512M -Xmx4G someClass

Sets the total size of the young generation.  the size of -Xmx is usually a decent value. If you have frequent allocation of a large number of objects, you should consider increasing this value.
Example: java -Xmx3G -Xmn1G someClass

-XX:PermSize=<number>[M/G] and -XX:MaxPermSize=<number>[M/G]
Sets the initial and maximum permanent generation sizes. If you are loading a large number of classes, you may need to bump these values up. If you see "java.lang.OutOfMemoryError: PermGen space", it is an indication that you need to raise at least MaxPermSize.
Example: java -XX:PermSize=64M -XX:MaxPermSize=256M someClass

-XX:NewSize=<number>[M/G] and -XX:MaxNewSize=<number>[M/G]
Sets the initial and maximum sizes for the young generation. Again, MaxNewSize should be ⅓ the size of -Xmx. By default, MaxNewSize is extremely large. This is where new objects go.
Example: java -XX:NewSize=512M -XX:MaxNewSize=1G someClass

Explicitly sets the old generation size.
Example: java -XX:OldSize=1G someClass

Sets the ratio of space allocated to the young and old generations. For example, setting this option to a value of 2 would mean that the maximum old generation space would be twice as large as the maximum young generation space. So, if MaxNewSize was 1G, the old generation space (OldSize) would be 2G.
Example of the old generation set to three times the size of the young generation:
java -XX:NewRatio=3 someClass

-XX:MinFreeHeapRatio=<number> and -XX:MaxFreeHeapRatio=<number>
Number is a value from 0 to 100 which represents a percentage. When less than MinFreeHeapRatio of the heap is available, the heap grows up to the -Xmx value. When MaxFreeHeapRatio is reached, compaction occurs. In other words, if MaxFreeHeapRatio is 90%, compaction occurs at 10% occupancy. When -Xms == -Xmx, MinFreeHeapRatio has no effect. Many recommend setting MinFreeHeapRatio to 40% and MaxFreeHeapRatio to 70%.
Example: java -XX:MinFreeHeapRatio=40 -XX:MaxFreeHeapRatio=70 someClass

This option sets the ratio between the From/To space (which are always equal in size) and Eden. So, if SurvivorRatio is equal to 5, Eden is 5 times the size of the From/To space. The total parts would be 5 + 1 (From) + 1 (To) = 7.
Example: java -XX:SurvivorRatio=10 someClass


I hope this journey has been beneficial to you. Feel free to leave any feedback or questions. Happy JVM tweaking!

Saturday, April 19, 2014

Introducing MineStat - A Minecraft Server Status Checker

My daughter and I have been playing Minecraft for the past year or so. After configuring a server of our own, I wanted a way to check its status. I initially wrote a Ruby class thinking our website would be a Rails application. I decided to port the code to Java and use JSP instead since I already had a Tomcat server running. I cleaned up the code this evening and figured I would release the source in case it could be of use to anyone else.

The project can be found on GitHub at You can use the software to whip up a monitoring script that periodically polls multiple Minecraft servers or allow visitors of your website to see the status of your Minecraft server. Examples for C#, Java, PHP, and Ruby are below for demonstration purposes.

C# Example:

using System;

class Example
  public static void Main()
    MineStat ms = new MineStat("", 25565);
    Console.WriteLine("Minecraft server status of {0} on port {1}:", ms.GetAddress(), ms.GetPort());
      Console.WriteLine("Server is online running version {0} with {1} out of {2} players.", ms.GetVersion(), ms.GetCurrentPlayers(), ms.GetMaximumPlayers());
      Console.WriteLine("Message of the day: {0}", ms.GetMotd());
      Console.WriteLine("Server is offline!");

Java Example:

import org.devux.MineStat;  
class Example  
  public static void main(String[] args)  
    MineStat ms = new MineStat("", 25565);    
    System.out.println("Minecraft server status of " + ms.getAddress() + " on port " + ms.getPort() + ":");  
      System.out.println("Server is online running version " + ms.getVersion() + " with " + ms.getCurrentPlayers() + " out of " + ms.getMaximumPlayers() + " players.");  
      System.out.println("Message of the day: " + ms.getMotd());  
      System.out.println("Server is offline!");  

PHP Example:


$ms = new MineStat("", 25565);
printf("Minecraft server status of %s on port %s:<br>", $ms->get_address(), $ms->get_port());
  printf("Server is online running version %s with %s out of %s players.<br>", $ms->get_version(), $ms->get_current_players(), $ms->get_max_players());
  printf("Message of the day: %s<br>", $ms->get_motd());
  printf("Server is offline!<br>");

Ruby Example:

require_relative 'minestat'

ms ="", 25565)  
puts "Minecraft server status of #{ms.address} on port #{ms.port}:"
  puts "Server is online running version #{ms.version} with #{ms.current_players} out of #{ms.max_players} players."
  puts "Message of the day: #{ms.motd}"
  puts "Server is offline!"