CS564-Programming Project 2 Buffer Manager Solved

The goal of the BadgerDB projects is to allow students in CS 564 to learn about the internals of a data processing engine. In this assignment, you will build a buffer manager, on top of an I/O layer that we provide.

Logistics. BadgerDB is coded in C++ and runs on the CS REL machines. Here are a few logistical points:

•    Platform: The stages will be compiled and tested on the CS department’s 64-bit Ubuntu. Linux machines called rockhopper. We will use the latest g++ compiler on those machines. You are free to develop on other platforms, but you must make sure that your project works with the official configuration. You can log on to the rockhopper pool remotely using the machines rockhopper-01.cs.wisc.edu through rockhopper-09.cs.wisc.edu. Since you are enrolled in a CS class, you can activate an account (if you already don’t have one) in the CSL through the following link: https://csl.cs.wisc.edu/services/user-accounts.

•    Warnings: One of the strengths of C++ is that it does compile time code checking (consequently reducing run-time errors). Try to take advantage of this by turning on as many compiler warnings as possible. The Makefile that we will supply will have -Wall on as default.

•    Tools: Always be on the lookout for tools that might simplify your job. Example: make for compiling and building your project, makedepend for automatically generating dependencies, perl for writing test scripts, valgrind for tracking down memory errors, gdb for debugging, and git/svn for version control. While we will not explicitly educate you about each of these, feel free to seek the TA’s advice.

•    Software Engineering: A large project such as this requires significant design effort. Spend some time thinking before you start writing code.

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THE BADGERDB I/O LAYER
 

The lowest layer of the BadgerDB database systems is the I/O layer. This layer allows the upper level of the system to create/destroy files, allocate/deallocate pages within a file and to read and write pages of a file. This layer consists of two classes: a file (class File) and a page (class Page) class. These classes use C++ exceptions to handle the occurrence of any unexpected event.

Implementation of the File class, the Page class, and the exception classes are provided to you. To start, you can copy BufMgr.tar.gz to your private workspace, and expand this tarball using the following command:

/bin/zcat bufmgr.tar.gz | /bin/tar -xvf -
The code has been adequately commented to help you with understanding how it does what it does. Please use Doxygen as shown below to generate documentation files on these files. Inside the bufmgr directory run the following command to generate documentation files: make doc. The doc files will be generated in docs directory. You can now open the docs/index.html file inside the browser and go through description of classes and their methods to better understand their implementation. Note that above, is shell prompt on Linux machines and hence not part of the command.

THE BADGERDB BUFFER MANAGER
 

A database buffer pool is an array of fixed-sized memory buffers called frames that are used to hold database pages (also called disk blocks) that have been read from disk into memory. A page is the unit of transfer between the disk and the buffer pool residing in main memory. Most modern database systems use a page size of at least 8,192 bytes. Another important thing to note is that a database page in memory is an exact copy of the corresponding page on disk when it is first read in. Once a page has been read from disk to the buffer pool, the DBMS software can update information stored on the page, causing the copy in the buffer pool to be different from the copy on disk. Such pages are termed dirty.

Since the database on disk itself is often larger than the amount of main memory that is available for the buffer pool, only a subset of the database pages fit in memory at any given time. The buffer manager is used to control which pages are memory resident. Whenever the buffer manager receives a request for a data page, the buffer manager checks to see if the requested page is already in the one of the frames that constitutes the buffer pool. If so, the buffer manager simply returns a pointer to the page. If not, the buffer manager frees a frame (possibly by writing to disk the page it contains if the page is dirty) and then reads in the requested page from disk into the frame that has been freed.

Before reading further you should first read the documentation that describes the I/O layer of BadgerDB so that you understand its capabilities (described on the previous page). In a nutshell the I/O layer provides an object-oriented interface to the Unix file with methods to open and close files and to read/write pages of a file. For now, the key thing you need to know is that opening a file (by passing in a character string name) returns an object of type File. This class has methods to read and write pages of the File.

You will use these methods to move pages between the disk and the buffer pool.

Buffer Replacement Policies and the Clock Algorithm.

There are many ways of deciding which page to replace when a free frame is needed. Commonly used policies in operating systems are FIFO, MRU and LRU. Even though LRU is one of the most commonly used policies it has high overhead and is not the best strategy to use in a number of common cases that occur in database systems. Instead, many systems use the clock algorithm that approximates LRU behavior and is much faster.

Figure 1 shows the conceptual layout of a buffer pool. Figure 2 illustrates the execution of the clock algorithm.

 

Figure 1: Structure of the Buffer Manager

In Figure 1, each square box corresponds to a frame in the buffer pool. Assume that the buffer pool contains numBufs frames, numbered 0 to numBufs-1. Conceptually, all the frames in the buffer pool are arranged in a circular list. Associated with each frame is a bit termed the refbit. Each time a page in the buffer pool is accessed (via a readPage() call to the buffer manager) the refbit of the corresponding frame is set to true. At any point in time the clock hand (an integer whose value is between 0 and numBufs - 1) is advanced (using modular arithmetic so that it does not go past numBufs - 1) in a clockwise fashion.

For each frame that the clockhand goes past, the refbit is examined and then cleared. If the bit had been set, the corresponding frame has been referenced "recently" and is not replaced. On the other hand, if the refbit is false, the page is selected for replacement (assuming it is not pinned - pinned pages are discussed below). If the selected buffer frame is dirty (ie. it has been modified), the page currently occupying the frame is written back to disk. Otherwise the frame is just cleared and a new page from disk is read in to that location. The details of the algorithm is given below.

 

Figure 2: The Clock Replacement Algorithm

The Structure of the Buffer Manager.

The BadgerDB buffer manager uses three C++ classes: BufMgr, BufDesc and BufHashTbl. There is only one instance of the BufMgr class. A key component of this class is the actual buffer pool which consists of an array of numBufs frames, each the size of a database page. In addition to this array, the BufMgr instance also contains an array of numBufs instances of the BufDesc class that is used to describe the state of each frame in the buffer pool. A hash table is used to keep track of the pages that are currently resident in the buffer pool. This hash table is implemented by an instance of the BufHashTbl class. This instance is a private data member of the BufMgr class. These classes are described in detail below.

The BufHashTbl Class. The BufHashTbl class is used to map file and page numbers to buffer pool frames and is implemented using chained bucket hashing. We have provided an implementation of this class for your use.

struct          hashBucket      {

File * f i l e ; // pointer to a f i l e object (more on this below ) PageId pageNo ; // page number within a f i l e

FrameId frameNo ; // frame number of page in the buffer pool hashBucket * next ; // next bucket in the chain

} ;
Here is the definition of the hash table.

class         BufHashTbl

{

private :

            hashBucket **      ht ;     //      pointer      to      actual      hash      table

           int      HTSIZE ;

int hash ( const File * f i l e , const PageId pageNo) ; //returns a value between 0 and HTSIZE−1 public :

              BufHashTbl ( const        int       htSize ) ;       //       constructor

             ~BufHashTbl ( ) ;       //       destructor

//  insert     entry      into        hash       table      mapping ( f i l e , pageNo)  to           frameNo void     insert ( const        File *      f i l e ,     const      int          pageNo ,               const      int     frameNo) ;

            // Check      i f      ( f i l e , pageNo)      is       currently       in    the      buffer      pool     ( ie .     in

// the hash table . I f so , return the corresponding frame number in frameNo void lookup ( const File * f i l e , const int pageNo , int& frameNo) ;

            // remove       entry       obtained     by     hashing       ( f i l e , pageNo)      from    hash      table .

           void      remove ( const       File *      f i l e ,     const      int      pageNo) ;

} ;
The BufDesc Class. The BufDesc class is used to keep track of the state of each frame in the buffer pool. It is defined as follows.

First notice that all attributes of the BufDesc class are private and that the BufMgr class is defined to be a friend. While this may seem strange, this approach restricts access to BufDesc’s private variables to only the BufMgr class. The alternative (making everything public) opens up access too far.

The purpose of most of the attributes of the BufDesc class should be pretty obvious. The dirty bit, if true indicates that the page is dirty (i.e. has been updated) and thus must be written to disk before the frame is used to hold another page. The pinCnt indicates how many times the page has been pinned. The refbit is used by the clock algorithm. The valid bit is used to indicate whether the frame contains a valid page. You do not HAVE to implement any methods in this class. However you are free to augment it in any way if you wish to do so.

class BufDesc { friend class BufMgr ;

private :

             File *     f i l e ;          //      pointer      to    f i l e      object

               PageId pageNo ;           // page       within      f i l e

FrameId frameNo ; // buffer pool frame number int pinCnt ; // number of times this page has been pinned bool dirty ; // true i f dirty ; false otherwise
 
             bool      valid ;
//      true     i f    page    is      valid
 
             bool     r e f b i t ;
      //     true     i f     this      buffer      frame    been            referenced
recently
             void           Clear ( ) ;
      //     i n i t i a l i z e      buffer      frame
 
             void        Set ( File *
f i l e P t r ,           PageId pageNum) ;        //set          BufDesc member
variable
values void    Print ( )
        //Print        values      of member        variables
 
BufDesc ( ) ;

} ;
//Constructor
 
The BufMgr Class. The BufMgr class is the heart of the buffer manager. This is where you write your code for this assignment.

class       BufMgr

{

private :

           FrameId       clockHand ;         //     clock     hand    for      clock       algorithm

            BufHashTbl        * hashTable ;          // hash       table      mapping      ( File ,      page )    to         frame number

BufDesc * bufDescTable ; // BufDesc objects , one per frame std : : uint32_t numBufs ; // Number of frames in the buffer pool BufStats bufStats ; // S t a t i s t i c s about buffer pool usage

// allocate a free frame using the clock algorithm void allocBuf ( FrameId & frame ) ;

void             advanceClock ( ) ;           //Advance       clock     to     next     frame     in    the     buffer      pool

public :

           Page     * bufPool ;                //      actual       buffer      pool

              BufMgr( std : : uint32_t          bufs ) ;      //       Constructor

           ~BufMgr ( ) ;                     //       Destructor

void readPage ( File * f i l e , const PageId PageNo , Page*& page ) ; void unPinPage ( File * f i l e , const PageId PageNo , const bool dirty ) ; void allocPage ( File * f i l e , PageId& PageNo , Page*& page ) ; void disposePage ( File * f i l e , const PageId pageNo) ; void flushFile ( const File * f i l e ) ;

} ;
This class is defined as follows:

BufMgr(const int bufs)
This is the class constructor. Allocates an array for the buffer pool with bufs page frames and a corresponding BufDesc table. The way things are set up all frames will be in the clear state when the buffer pool is allocated. The hash table will also start out in an empty state. We have provided the constructor.

~BufMgr()
Flushes out all dirty pages and deallocates the buffer pool and the BufDesc table. void advanceClock()

Advance clock to next frame in the buffer pool. void allocBuf(FrameId& frame)

Allocates a free frame using the clock algorithm; if necessary, writing a dirty page back to disk. Throws BufferExceededException if all buffer frames are pinned. This private method will get called by the readPage() and allocPage() methods described below. Make sure that if the buffer frame allocated has a valid page in it, you remove the appropriate entry from the hash table.

void readPage(File* file, const PageId PageNo, Page*& page)
First check whether the page is already in the buffer pool by invoking the lookup() method, which may throw HashNotFoundException when page is not in the buffer pool, on the hashtable to get a frame number. There are two cases to be handled depending on the outcome of the lookup() call:

•    Case 1: Page is not in the buffer pool. Call allocBuf() to allocate a buffer frame and then call the method file-readPage() to read the page from disk into the buffer pool frame. Next, insert the page into the hashtable. Finally, invoke Set() on the frame to set it up properly. Set() will leave the pinCnt for the page set to 1. Return a pointer to the frame containing the page via the page parameter.

•    Case 2: Page is in the buffer pool. In this case set the appropriate refbit, increment the pinCnt for the page, and then return a pointer to the frame containing the page via the page parameter.

void unPinPage(File* file, const PageId PageNo, const bool dirty)
Decrements the pinCnt of the frame containing (file, PageNo) and, if dirty == true, sets the dirty bit. Throws PAGENOTPINNED if the pin count is already 0. Does nothing if page is not found in the hash table lookup.

void allocPage(File* file, PageId& PageNo, Page*& page)
The first step in this method is to to allocate an empty page in the specified file by invoking the file-allocatePage() method. This method will return a newly allocated page. Then allocBuf() is called to obtain a buffer pool frame. Next, an entry is inserted into the hash table and Set() is invoked on the frame to set it up properly. The method returns both the page number of the newly allocated page to the caller via the pageNo parameter and a pointer to the buffer frame allocated for the page via the page parameter.

void disposePage(File* file, const PageId pageNo)
This method deletes a particular page from file. Before deleting the page from file, it makes sure that if the page to be deleted is allocated a frame in the buffer pool, that frame is freed and correspondingly entry from hash table is also removed. void flushFile(File* file)

Should scan bufTable for pages belonging to the file. For each page encountered it should: (a) if the page is dirty, call file-writePage() to flush the page to disk and then set the dirty bit for the page to false, (b) remove the page from the hashtable (whether the page is clean or dirty) and (c) invoke the Clear() method of BufDesc for the page frame.

Throws PagePinnedException if some page of the file is pinned. Throws BadBufferException if an invalid page belonging to the file is encountered.