Copyright © Curt Hill Concurrent Execution An Overview for Database

Copyright © Curt Hill MultiTasking Multitasking is the apparent execution of programs in parallel This supports parallel execution usually under one or more of these names –Tasks –Processes –Threads This can be accomplished with a single processor or with several processors

Copyright © Curt Hill Definitions Process –An executable program to the OS –Sole possession of code and variable memory –Must communicate through the OS Thread –Independent execution within a process –Share code and variables within a process These work in Win32 and UNIX Tasks can be either

Copyright © Curt Hill Single Processor UniProcessing Single CPU, but apparent multiple tasks Permissive –Any system call allows the current task to be suspended and another started Preemptive –A task is suspended when it makes a system call that could require waiting –A time slice occurs

Copyright © Curt Hill Multiple Processors MultiProcessing Real multiprocessing involves multiple CPUs Multiple CPUs can be executing different jobs They may also be in the same job, if it allows A DBMS is usually multi-threaded

Copyright © Curt Hill Off to the Races Most programming languages are Sequential There are a number of kinds of errors that can occur when there are two simultaneously executing tasks that share anything When different results may be obtained by the same code and data because of time dependencies, this is called a race

Granularity At the program level we are concerned with instruction granularity –On uniprocessors individual instructions are usually atomic –No guarantees of any sort with multiprocessors At the database level we are concerned with transaction granularity –Since a transaction generally involves multiple database accesses and hundreds of machine instructions even fewer guarantees Copyright © Curt Hill

Race Example Consider two transactions Transaction A has –Read row x of table y –Add 2 to field z –Write row x of table y Transaction B has –Read row x of table y –Add 5 to field z –Write row x of table y

Copyright © Curt Hill Racing Form Read x of y Add 2 to z Write x of y Read x of y Add 5 to z Write x of y There are three possible outcomes. It all depends on where a time slice occurs.

Copyright © Curt Hill Race Results If transactions are serialized z will have 7 added to it If A starts and is interrupted, then the add of two is kept and the add of five is lost If B starts first and is interrupted, then the add of five is kept and the add of two is lost Without precautions we could end with a add of 2, 5 or 7

Query Race This gets complicated when multiple updates must occur as a unit Consider the airline reservation problem I want to book a flight from: Fargo to Minneapolis Minneapolis to Atlanta Atlanta to Pensacola Each leg reservation is competing with many others and I must have them as a unit Copyright © Curt Hill

How to solve? At the machine instruction level there are several techniques –Critical sections –Mutexes –Semaphores All of these involve some kind of lock Locks come in two flavors: –Shared –Exclusive Copyright © Curt Hill

Lock Types An exclusive lock only allows one thread to have control –Usually used for write or update Any others that attempt are forced to wait A shared lock is usually used for reading Any number of items can share but one exclusive lock prevents all shared or is forced to wait until all shared locks are released Copyright © Curt Hill

Waiting If a lock prohibits entry the task must wait Two kinds of wait: –Busy wait - spin in a loop –Idle wait - suspended until later This wait time keeps a multi CPU system from reaching its true potential, especially while in the dispatcher

Copyright © Curt Hill Locked Solution Lock y Read x of y Add 2 to z Write x of y Unlock y Lock y Read x of y Add 5 to z Write x of y Unlock y Always update z to 7 larger

Lock Granularity In the above example an exclusive lock was applied to the entire table Databases have considerable variance as to what may be lockable: Tables Pages Records Fields The finer the granularity the less conflicts there will be –Thus better performance –However, much more bookkeeping Copyright © Curt Hill

Wait Problems A critical section guarded by a semaphore is a special case of resource allocation In order for a task to complete it has to ask for and receive a resource –If it cannot, it is forced to wait This however allows for other problems

Copyright © Curt Hill Deadlock Situation where tasks cannot get the resources they need Task A has resource 1 and is waiting for resource 2 Task B has resource 2 and is waiting for resource 1 Both are waiting for a situation that cannot be resolved

Copyright © Curt Hill The Dining Philosophers Problem h

Copyright © Curt Hill The Dining Philosophers Problem There is a round table with five philosophers, five plates of spaghetti and five forks A philosopher always picks up the right fork, then the left, then eats If all five reach out their right hand at the same time all the forks are gone then there is a deadlock

Copyright © Curt Hill Traffic Deadlock

Traffic Deadlock Again Car A –Holds resource 1, which is street in front of West street –Wants resource 2, which is street in front of East street Car B –Holds resource 2, which is street in front of East street –Wants resource 1, which is street in front of West street In order to turn, each needs both It can get much more complicated Copyright © Curt Hill

Deadlock Is it a programmer problem or a DBMS problem? Both.

Copyright © Curt Hill Avoiding Deadlock A DBMS should be able to detect a deadlock Usual solution is to abort transactions until something can proceed Ability to reclaim a resource also required Programmer should avoid situations that might lead to it: Do not wait for a resource while holding a resource