The combination of FLUSH STATUS and SHOW SESSSION STATUS can be used to see what happens while MySQL executes a query. First, run FLUSH STATUS to reset session status variables to zero.

mysql> FLUSH STATUS;

mysql> SELECT COUNT(*) FROM TABLE;

Procedure to add a swap file

You need to use dd command to create swapfile. Next you need to use mkswap command to set up a Linux swap area on a device or in a file.

a) Login as the root user

b) Type following command to create 512MB swap file (1024 * 512MB = 524288 block size):

# dd if=/dev/zero of=/swapfile1 bs=1024 count=524288

c) Set up a Linux swap area:

# mkswap /swapfile1

d) Activate /swapfile1 swap space immediately:

# swapon /swapfile1

e) To activate /swapfile1 after Linux system reboot, add entry to /etc/fstab file. Open this file using text editor such as vi:

# vi /etc/fstab

Append following line:

/swapfile1 swap swap defaults 0 0

So next time Linux comes up after reboot, it enables the new swap file for you automatically.

g) How do I verify swap is activated or not?

Simply use free command:

$ free -m

vi first  edit source file
then move your cursor to start of selection
ma       mark current position with letter a
then move your cursor to end of selection
y'a     yank to buffer x from current position to mark a
:e other  edit target file
move cursor to where you want the data
p       put from buffer x

Constructors

When you create a new instance (a new object) of a class using the new keyword, a constructor for that class is called. Constructors are used to initialize the instance variables (fields) of an object. Constructors are similar to methods, but with some important differences.

• Constructor name is class name. A constructors must have the same name as the class its in.
• Default constructor. If you don’t define a constructor for a class, a default parameterless constructor is automatically created by the compiler. The default constructor calls the default parent constructor (super()) and initializes all instance variables to default value (zero for numeric types, null for object references, and false for booleans).
• Default constructor is created only if there are no constructors. If you define any constructor for your class, no default constructor is automatically created.
• Differences between methods and constructors.
  • There is no return type given in a constructor signature (header). The value is this object itself so there is no need to indicate a return value.
  • There is no return statement in the body of the constructor.
  • The first line of a constructor must either be a call on another constructor in the same class (using this), or a call on the superclass constructor (using super). If the first line is neither of these, the compiler automatically inserts a call to the parameterless super class constructor.

  These differences in syntax between a constructor and method are sometimes hard to see when looking at the source. It would have been better to have had a keyword to clearly mark constructors as some languages do.

• this(...) – Calls another constructor in same class. Often a constructor with few parameters will call a constructor with more parameters, giving default values for the missing parameters. Use this to call other constructors in the same class.
• super(...). Use super to call a constructor in a parent class. Calling the constructor for the superclass must be the first statement in the body of a constructor. If you are satisfied with the default constructor in the superclass, there is no need to make a call to it because it will be supplied automatically.

To copy a block of text between files execute the commands:

It is possible that mysqld could use up to
key_buffer_size + (read_buffer_size + sort_buffer_size)*max_connections .

strace -q -f -c -p

strace -f verbose=all -e write=all -o /tmp/strace.log -p [pid]

strace -T -t -q -f -e trace=file,open,close,read -o /tmp/prod.strace.log -p <oms or ui pid>

-f traces all child processes as they are created byt he currently traced process as a result of the fork() system call.

-e is a qualifying expression which modifies which events to trace or how to race them

verbose=all dereferences structures for all system calls

write=all performs a full hexadecimal and ASCII dump of all the data written to all file descriptors

-o output file

-p process id to trace

Replace -p [pid] with [command] to trace a specific command.

Kernels 2.6.16 and newer provide a mechanism to have the kernel drop the page cache and/or inode and dentry caches on command, which can help free up a lot of memory. Now you can throw away that script that allocated a ton of memory just to get rid of the cache…

To use /proc/sys/vm/drop_caches, just echo a number to it.

To free pagecache:

# echo 1 > /proc/sys/vm/drop_caches

To free dentries and inodes:

# echo 2 > /proc/sys/vm/drop_caches

To free pagecache, dentries and inodes:

echo 3 > /proc/sys/vm/drop_caches

As this is a non-destructive operation and dirty objects are not freeable, the user should run “sync” first!

tmpfs is supported by the Linux kernel from version 2.4 and up. tmpfs (previously known as shmfs) distinguishes itself from the Linux ramdisk device by allocating memory dynamically and by allowing less-used pages to be moved onto swap space. RAMFS, in contrast, does not make use of virtual memory (which can be an advantage or disadvantage). In addition, MFS and some older versions of ramfs did not grow and shrink dynamically and instead used a fixed amount of memory at all times.

Usage of tmpfs for example is “mount -t tmpfs -o size=1G,nr_inodes=10k,mode=0700 tmpfs /space” which will allow up to 1 GiB in RAM/swap with 10240 inodes and only accessible by the owner of the directory /space. The filesystem’s maximum size can also be changed on-the-fly, like “mount -o remount,size=2G /space”.

AND

A bitwise AND takes two binary representations of equal length and performs the logical AND operation on each pair of corresponding bits. In each pair, the result is 1 if the first bit is 1 AND the second bit is 1. Otherwise, the result is 0. For example:

0101
AND 0011
= 0001


XOR

A bitwise exclusive or takes two bit patterns of equal length and performs the logical XOR operation on each pair of corresponding bits. The result in each position is 1 if the two bits are different, and 0 if they are the same. For example:

0101
XOR 0011
= 0110

In Java, all integer types are signed, and the “<<” and “>>” operators perform arithmetic shifts. Java adds the operator “>>>” to perform logical right shifts, but since the logical and arithmetic left-shift operations are identical, there is no “<<<” operator in Java. These general rules are affected in several ways by the default type promotions; for example, since the eight-bit type byte is promoted to int in shift-expressions,^[2] the expression “b >>> 2” effectively performs an arithmetic shift of the byte value b instead of a logical shift. Such effects can be mitigated by judicious use of casts or bitmasks; for example, “(b & 0xFF) >>> 2” effectively results in a logical shift.