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Exercicio 1 – Sistemas Operacionais II – INE5424 – UFSC

Trabalho 1 – EPOS
Idle-waiting Thread Joining
– Este trabalho tinha como objetivo alterar o modo que era feito o “join” em uma Thread. Sendo que o mesmo inicialmente era feito da seguinte maneira:
[code lang=”c”] while(_state != FINISHING)
yield();
[/code]

O mesmo consiste basicamente, em verificar o estado da thread que se esta dando join e caso o estado nao seja FINISHING dar a vez a outro processo. Esta solucao, acarreta no consumo excessivo de recursos ja que o mesmo sempre retorna para a fila de _ready. E eh executado novamente quando o escalonador escolher esta thread. O trabalho pede que seja modificado de modo que o consumo de recursos nao seja excessivo.

Arquivos Modificados:
– thread.cc
– Thread.h

Solucao:
     Foi criada uma variável adicional em threads.h, _waiting_for_me, que é um ponteiro para uma thread. Quando o método “join” é chamado, a thread que está executando é colocada no _waiting_for_me da thread chamada e é suspensa. Se a thread já finalizou ou alguém já chamou “join” nela, nada é feito. No método “exit”, caso _waiting_for_me esteja preenchida, é chamado o método “resume” na thread referenciada.
     Pensamos em colocar uma lista de referências para várias threads poderem dar “join” na mesma thread, porém vimos na documentação do POSIX que isso era considerado um erro lógico, e achamos que acarretaria um consumo de recursos adicional desnecessário, pois não seria possível usar o _link já existente na thread, já que ela teria de estar tanto na lista de suspensos quanto na lista _waiting_for_me da outra thread.

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