JKH project..
From: Julian Elischer (julian_at_elischer.org)
Date: 04/12/04
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Date: Sun, 11 Apr 2004 15:51:45 -0700 (PDT) To: FreeBSD current users <current@freebsd.org>
Junior kernel hacker project..
the fork syscall has to check the new PID against all exixting pids..
here's the current code.. when teh PID-space starts becoming
"fragmented.." this starts to take "real time".
retry:
/*
* If the process ID prototype has wrapped around,
* restart somewhat above 0, as the low-numbered procs
* tend to include daemons that don't exit.
*/
if (trypid >= PID_MAX) {
trypid = trypid % PID_MAX;
if (trypid < 100)
trypid += 100;
pidchecked = 0;
}
if (trypid >= pidchecked) {
int doingzomb = 0;
pidchecked = PID_MAX;
/*
* Scan the active and zombie procs to check whether
this pid
* is in use. Remember the lowest pid that's greater
* than trypid, so we can avoid checking for a while.
*/
p2 = LIST_FIRST(&allproc);
again:
for (; p2 != NULL; p2 = LIST_NEXT(p2, p_list)) {
PROC_LOCK(p2);
while (p2->p_pid == trypid ||
(p2->p_pgrp != NULL &&
(p2->p_pgrp->pg_id == trypid ||
(p2->p_session != NULL &&
p2->p_session->s_sid == trypid)))) {
trypid++;
if (trypid >= pidchecked) {
PROC_UNLOCK(p2);
goto retry;
}
}
if (p2->p_pid > trypid && pidchecked >
p2->p_pid)
pidchecked = p2->p_pid;
if (p2->p_pgrp != NULL) {
if (p2->p_pgrp->pg_id > trypid &&
pidchecked > p2->p_pgrp->pg_id)
pidchecked = p2->p_pgrp->pg_id;
if (p2->p_session != NULL &&
p2->p_session->s_sid > trypid &&
pidchecked > p2->p_session->s_sid)
pidchecked =
p2->p_session->s_sid;
}
PROC_UNLOCK(p2);
}
if (!doingzomb) {
doingzomb = 1;
p2 = LIST_FIRST(&zombproc);
goto again;
}
}
with several thousand processes, forking a lot, this starts to take
a 'noticable' amount of time.
a small amount of time could be spent on trying to work out an
O(1) solution to this.
Possibly a small separate PID object that keeps reference counts
of how many procs refer to it in various ways, and lives in a
hash-table, and is freed when there are no more processes referring to
it.
Suggestions welcome.. :-)
julian
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