Memory Barriers for (Ubuntu) Linux (i686) Post: 302431048

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Special Forums UNIX and Linux Applications High Performance Computing Memory Barriers for (Ubuntu) Linux (i686) Post 302431048 by Corona688 on Sunday 20th of June 2010 09:14:36 PM

06-20-2010

Registered User

Quote:

Originally Posted by gorga

Let me put it this way, I have a list of jobs to do, as I progress through the list I find some jobs aren't ready. I can either block/sleep etc and wait for the job to become ready, or I can move onto the next task and execute that one instead.

...and when you don't assume best-case, you could be going through the next few thousand not-ready tasks. Meanwhile every idle worker's doing the same thing. I begin to understand why you're concerned about contention for memory.

Quote:

If I build my system with locks, I'll adhere to the first approach, whereas what I'm trying to achieve (aided by atomic flags and memory barriers) is the second.

But because it doesn't block, you'll be wasting time scanning the list anyway, time that could have been spent doing actual work. And since your system's as busy idle as it is when actually busy you'll have a difficult time guessing how much. If I read you correctly, the jobs are all tiny. How tiny? How much more work is it to do a job than to scan the list? (And don't assume it could never, ever become mostly empty, that's the goal, not the proof.) If they're even in the same ballpark, you're going to be wasting a worrying proportion of CPU time scanning your list.

Anyway, the queue needn't block like you're describing. Put jobs in the queue when they become ready, don't just stick them there in advance, that way threads won't block when picking up jobs unless you're actually out of jobs -- in which case you want them to block. If the queue's big enough and jobs can be added fast enough, things can run smoothly. You can also do other things to streamline the queue -- hand out jobs 16 at a time instead of one at a time, switch between multiple queues, etc.

Last edited by Corona688; 06-20-2010 at 10:33 PM..

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LEARN ABOUT POSIX

queuedefs

queuedefs(4)                                                       File Formats                                                       queuedefs(4)

NAME

       queuedefs - queue description file for at, batch, and cron

SYNOPSIS

       /etc/cron.d/queuedefs

DESCRIPTION

       The queuedefs file describes the characteristics of the queues managed by cron(1M). Each non-comment line in this file describes one queue.
       The format of the lines are as follows:

              q.[njobj][nicen][nwaitw]

       The fields in this line are:

       q        The name of the queue. a is the default queue for jobs started by at(1); b is the default queue for jobs  started  by  batch  (see
                at(1)); c is the default queue for jobs run from a crontab(1) file.

       njob     The  maximum  number of jobs that can be run simultaneously in that queue; if more than njob jobs are ready to run, only the first
                njob jobs will be run, and the others will be run as jobs that are currently running terminate.  The default value is  100.

       nice     The nice(1) value to give to all jobs in that queue that are not run with a user ID of super-user.  The default value is 2.

       nwait    The number of seconds to wait before rescheduling a job that was deferred because more than njob jobs were running in  that  job's
                queue, or because the system-wide limit of jobs executing has been reached.  The default value is 60.

       Lines beginning with # are comments, and are ignored.

EXAMPLES

       Example 1: A sample file.

       #
       #
       a.4j1n
       b.2j2n90w

       This  file  specifies that the a queue, for at jobs, can have up to 4 jobs running simultaneously; those jobs will be run with a nice value
       of 1.  As no nwait value was given, if a job cannot be run because too many other jobs are running cron will wait 60 seconds before  trying
       again to run it.

       The  b  queue, for batch(1) jobs, can have up to 2 jobs running simultaneously; those jobs will be run with a nice(1) value of 2.  If a job
       cannot be run because too many other jobs are running, cron(1M) will wait 90 seconds before trying again to run it. All  other  queues  can
       have up to 100 jobs running simultaneously; they will be run with a nice value of 2, and if a job cannot be run because too many other jobs
       are running cron will wait 60 seconds before trying again to run it.

FILES

       /etc/cron.d/queuedefs           queue description file for  at, batch, and  cron.

SEE ALSO

       at(1), crontab(1), nice(1), cron(1M)

SunOS 5.10                                                          1 Mar 1994                                                        queuedefs(4)