Path: utzoo!utgpu!jarvis.csri.toronto.edu!csri.toronto.edu!moraes
Newsgroups: comp.sys.sgi
From: moraes@csri.toronto.edu (Mark Moraes)
Subject: semaphores vs locks?
Message-ID: <1989Sep5.185510.7836@jarvis.csri.toronto.edu>
Organization: University of Toronto, CSRI
Date: 5 Sep 89 22:55:11 GMT

On an Iris4d multiprocessor, what is the difference between a semaphore
and a lock? I couldn't find any specific mention of the difference in
the documentation.  As far as I can tell, semaphores have queues in
which threads wait till the semaphore is freed, whereas locks are
continuously retried (assuming ussetlock) with some delay.

For a system with hardware locks, which is faster/cheaper - a
semaphore or a lock? Does a lock consume much cpu time while spinning?
Is the cost of going to sleep and awakening on a semaphore high? Does
anyone have any numbers on these?

Is there a section of the manual summarizing the parallel facilities
available? (Suggestion: Perhaps a future release of the documentation
could group manual pages by subsection and alphabetically - i.e. all
manual pages for 3P together, rather than alphabetically across all
subsections) The following is a list I've created, with some attempt
to order them in 'recommended reading order'. Have I missed anything?

usinit, _utrace (3P)    - semaphore and lock initialization routine
usconfig (3P)           - semaphore and lock configuration operations

taskblock, taskunblock, tasksetblockcnt (3P) - routines to block/unblock tasks
taskcreate (3P)         - create a new task
taskdestroy (3P)        - destroy a task
taskctl (3P)            - operations on a task

barrier, new_barrier, free_barrier (3P) - barrier functions

usnewlock (3P)          - allocates and initializes a lock
usinitlock (3P)         - initializes a lock
ussetlock, uscsetlock, uswsetlock, ustestlock, usunsetlock (3P) - lock routines
usfreelock (3P)         - free a lock
usctllock (3P)          - lock control operations

usnewsema (3P)          - allocates and initializes a semaphore
usinitsema (3P)         - initializes a semaphore
uspsema (3P)            - attempt to acquire a semaphore
usvsema (3P)            - frees a resource to a semaphore
ustestsema (3P)         - return the value of a semaphore
uscpsema (3P)           - attempts to acquire a semaphore, and fails if not possible
usfreesema (3P)         - free a semaphore
usctlsema (3P)          - semaphore control operations

usmalloc, usfree, usrealloc (3P) - user shared memory allocator
acreate, adelete, amalloc, afree, arealloc, acalloc, amallopt, amallinfo (3P) - arbitrary arena main memory allocator

pcreate: pcreatel, pcreatev, pcreateve, pcreatelp, pcreatevp (3P) - create a process

m_fork, m_kill_procs, m_set_procs, m_get_numprocs, m_get_myid, m_next, m_lock, m_unlock, (3P) - parallel programming primitives

sproc (2)               - create a new share group process
blockproc, unblockproc, setblockproccnt (2) - routines to block/unblock processes

Thanks.