what is PCTFREE , PCTUSED and FREELIST in oracle

Before discussing with PCTFREE and PCTUSED we need to understand the structure of a database block.
Overview of Data Blocks

Oracle manages the storage space in the datafiles of a database in units called data blocks. A data block is the smallest unit of data used by a database. In contrast, at the physical, operating system level, all data is stored in bytes. Each operating system has a block size. Oracle requests data in multiples of Oracle data blocks, not operating system blocks. The Oracle data block format is similar regardless of whether the data block contains table, index, or clustered data. Below illustrates the format of a data block.

Header (Common and Variable)

The header contains general block information, such as the block address and the type of segment (for example, data or index).

Table Directory

This portion of the data block contains information about the table having rows in this block.

Row Directory

This portion of the data block contains information about the actual rows in the block (including addresses for each row piece in the row data area).

After the space has been allocated in the row directory of a data block's overhead, this space is not reclaimed when the row is deleted. Therefore, a block that is currently empty but had up to 50 rows at one time continues to have 100 bytes allocated in the header for the row directory. Oracle reuses this space only when new rows are inserted in the block.

Overhead

The data block header, table directory, and row directory are referred to collectively as overhead. Some block overhead is fixed in size; the total block overhead size is variable. On average, the fixed and variable portions of data block overhead total 84 to 107 bytes.

Row Data

This portion of the data block contains table or index data. Rows can span blocks.

Free space

Free space is allocated for insertion of new rows and for updates to rows that require additional space (for example, when a trailing null is updated to a nonnull value).

In data blocks allocated for the data segment of a table or cluster, or for the index segment of an index, free space can also hold transaction entries. A transaction entry is required in a block for each INSERT, UPDATE, DELETE, and SELECT...FOR UPDATE statement accessing one or more rows in the block. The space required for transaction entries is operating system dependent; however, transaction entries in most operating systems require approximately 23 bytes.

The PCTFREE parameter 

The PCTFREE parameter sets the minimum percentage of a data block to be reserved as free space for possible updates to rows that already exist in that block. For example, assume that you specify the following parameter within a CREATE TABLE statement:

PCTFREE 20 

This states that 20% of each data block in this table's data segment be kept free and available for possible updates to the existing rows already within each block. New rows can be added to the row data area, and corresponding information can be added to the variable portions of the overhead area, until the row data and overhead total 80% of the total block size.Below figure illustrates PCTFREE.

The PCTUSED Parameter

The PCTUSED parameter sets the minimum percentage of a block that can be used for row data plus overhead before new rows are added to the block. After a data block is filled to the limit determined by PCTFREE, Oracle considers the block unavailable for the insertion of new rows until the percentage of that block falls beneath the parameter PCTUSED. Until this value is achieved, Oracle uses the free space of the data block only for updates to rows already contained in the data block. For example, assume that you specify the following parameter in a CREATE TABLE statement:

PCTUSED 40 

In this case, a data block used for this table's data segment is considered unavailable for the insertion of any new rows until the amount of used space in the block falls to 39% or less (assuming that the block's used space has previously reached PCTFREE). Until reaching this value oracle utilize the free space for the updation of existing rows .Below figure illustrates this.

How PCTFREE and PCTUSED work together

PCTFREE and PCTUSED work together to optimize the use of space in the data blocks of the extents within a data segment. Below figure illustrates the interaction of these two parameters.

In a newly allocated data block, the space available for inserts is the block size minus the sum of the block overhead and free space (PCTFREE). Updates to existing data can use any available space in the block. Therefore, updates can use the space once it falls below PCTFREE, the space reserved for updates but not accessible to inserts.

A FREELIST is where Oracle keeps tracks of blocks under the high-water mark for an object. Each will have at least one FREELIST associated with it. As blocks are used, they will be placed or taken off the FREELIST as needed. It is important to note that only blocks under the high-water mark of an object will be found on the FREELIST. The blocks that remain above the high-water mark will be used only when the FREELISTS are empty. In this fashion, Oracle postpones increasing the high-water mark for an object until it must.An object may have more than one FREELIST. If you anticipate heavy insert or update activity on an object by many concurrent users, configuring more than one FREELIST can make a major positive impact on performance (at the cost of possible additional storage). Individual sessions will be assigned to different FREELISTS, and when they need space, they will not contend with each other.

For each data and index segment, Oracle maintains one or more free lists—lists of data blocks that have been allocated for that segment's extents and have free space greater than PCTFREE. These blocks are available for inserts. When you issue an INSERT statement, Oracle checks a free list of the table for the first available data block and uses it if possible. If the free space in that block is not large enough to accommodate the INSERT statement, and the block is at least PCTUSED, then Oracle takes the block off the free list. Multiple free lists for each segment can reduce contention for free lists when concurrent inserts take place.

After you issue a DELETE or UPDATE statement, Oracle processes the statement and checks to see if the space being used in the block is now less than PCTUSED. If it is, then the block goes to the beginning of the transaction free list, and it is the first of the available blocks to be used in that transaction. When the transaction commits, free space in the block becomes available for other transactions.

An object may have more than one FREELIST. If you anticipate heavy insert or update activity on an object by many concurrent users, configuring more than one FREELIST can make a major positive impact on performance (at the cost of possible additional storage). Individual sessions will be assigned to different FREELISTS, and when they need space, they will not contend with each other.  Every segment will have 1 freelist by default .If there is only one FREELIST, only one transaction at a time may review and modify this list—they would have to wait for each other. Multiple FREELISTS and FREELIST GROUPS serve the purpose of increasing concurrency in such a case, as the transactions may each be looking at different lists and not contending with each other.