Διεργασίες

Κάθε διεργασία φυλάσσεται στον πίνακα διεργασιών με βάση τα παρακάτω πεδία:

struct  proc {
        TAILQ_ENTRY(proc) p_procq;      /* run/sleep queue. */
        LIST_ENTRY(proc) p_list;        /* List of all processes. */

        /* substructures: */
        struct  pcred *p_cred;          /* Process owner's identity. */
        struct  filedesc *p_fd;         /* Ptr to open files structure. */
        struct  pstats *p_stats;        /* Accounting/statistics (PROC ONLY). */
        struct  plimit *p_limit;        /* Process limits. */
        struct  vm_object *p_upages_obj;/* Upages object */
        struct  sigacts *p_sigacts;     /* Signal actions, state (PROC ONLY). */


        int     p_flag;                 /* P_* flags. */
        char    p_stat;                 /* S* process status. */
        char    p_pad1[3];

        pid_t   p_pid;                  /* Process identifier. */
        LIST_ENTRY(proc) p_hash;        /* Hash chain. */
        LIST_ENTRY(proc) p_pglist;      /* List of processes in pgrp. */
        struct  proc *p_pptr;           /* Pointer to parent process. */
        LIST_ENTRY(proc) p_sibling;     /* List of sibling processes. */
        LIST_HEAD(, proc) p_children;   /* Pointer to list of children. */

        struct callout_handle p_ithandle; /*
                                              * Callout handle for scheduling
                                              * p_realtimer.
                                              */

        pid_t   p_oppid;         /* Save parent pid during ptrace. XXX */
        int     p_dupfd;         /* Sideways return value from fdopen. XXX */

        struct  vmspace *p_vmspace;     /* Address space. */

        /* scheduling */
        u_int   p_estcpu;        /* Time averaged value of p_cpticks. */
        int     p_cpticks;       /* Ticks of cpu time. */
        fixpt_t p_pctcpu;        /* %cpu for this process during p_swtime */
        void    *p_wchan;        /* Sleep address. */
        const char *p_wmesg;     /* Reason for sleep. */
        u_int   p_swtime;        /* Time swapped in or out. */
        u_int   p_slptime;       /* Time since last blocked. */

        struct  itimerval p_realtimer;  /* Alarm timer. */
        struct  timeval p_rtime;        /* Real time. */
        u_quad_t p_uticks;              /* Statclock hits in user mode. */
        u_quad_t p_sticks;              /* Statclock hits in system mode. */
        u_quad_t p_iticks;              /* Statclock hits processing intr. */
        struct  timeval *p_sleepend;    /* Wake time for nanosleep & friends */

        int     p_traceflag;            /* Kernel trace points. */
        struct  vnode *p_tracep;        /* Trace to vnode. */

        int     p_siglist;              /* Signals arrived but not delivered. */

        struct  vnode *p_textvp;        /* Vnode of executable. */

        char    p_lock;                 /* Process lock (prevent swap) count. */
        char    p_oncpu;                /* Which cpu we are on */
        char    p_lastcpu;              /* Last cpu we were on */
        char    p_pad2;                 /* alignment */

        short   p_locks;                /* DEBUG: lockmgr count of held locks */
        short   p_simple_locks;         /* DEBUG: count of held simple locks */
        register_t p_retval[2];         /* syscall aux returns */

        sigset_t p_sigmask;     /* Current signal mask. */
        sigset_t p_sigignore;   /* Signals being ignored. */
        sigset_t p_sigcatch;    /* Signals being caught by user. */

        u_char  p_priority;     /* Process priority. */
        u_char  p_usrpri;       /* User-priority based on p_cpu and p_nice. */
        char    p_nice;         /* Process "nice" value. */
        char    p_comm[MAXCOMLEN+1];

        struct  pgrp *p_pgrp;   /* Pointer to process group. */

        struct  sysentvec *p_sysent; /* System call dispatch information. */

        struct  rtprio p_rtprio;        /* Realtime priority. */

        struct  user *p_addr;   /* Kernel virtual addr of u-area (PROC ONLY). */
        struct  mdproc p_md;    /* Any machine-dependent fields. */

        u_short p_xstat;        /* Exit status for wait; also stop signal. */
        u_short p_acflag;       /* Accounting flags. */
        struct  rusage *p_ru;   /* Exit information. XXX */

        int     p_nthreads;     /* number of threads (only in leader) */
        void    *p_aioinfo;     /* ASYNC I/O info */
        int     p_wakeup;       /* thread id */
        struct proc *p_peers;   
        struct proc *p_leader;
};

Κάθε διεργασία μπορεί να βρίσκεται σε μια από τις παρακάτω καταστάσεις:

#define SIDL    1               /* Process being created by fork. */
#define SRUN    2               /* Currently runnable. */
#define SSLEEP  3               /* Sleeping on an address. */
#define SSTOP   4               /* Process debugging or suspension. */
#define SZOMB   5               /* Awaiting collection by parent. */

Ο χρονοπρογραμματισμός των διεργασιών γίνεται με βάση πολλαπλές ουρές αναμονής.
Κάθε ουρά έχει διαφορετική προτεραιότητα.
Η αλγόριθμος επιλέγει πάντα τη διεργασία που μπορεί να εκτελεστεί από την ουρά με την υψηλότερη προτεραιότητα.
Η διεργασίες που βρίσκονται στην ίδια ουρά εκτελούνται κυκλικά.
Διεργασίες μετακινούνται από ουρά σε ουρά έτσι ώστε να επωφελούνται οι διαλογικές διεργασίες.