Memory Barriers for (Ubuntu) Linux (i686) Post: 302430449

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Special Forums UNIX and Linux Applications High Performance Computing Memory Barriers for (Ubuntu) Linux (i686) Post 302430449 by gorga on Thursday 17th of June 2010 03:48:05 PM

06-17-2010

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Quote:

Originally Posted by Corona688

Small world, how so? Smilie

Noting your help on the Programming forum too!

Quote:

I think you're overreacting... Any memory I/O monopolizes the bus*, LOCK just guarantees one instruction gets two ops in a row.

Are you suggesting then, that if I used such an instruction relatively frequently (say once in a loop of maybe a 100 execution statements, per core), I shouldn't notice a significant drop in throughput of the application?

Quote:

I'm not sure LOCK XCGH even forces a real memory fetch anymore(might be simple to test, try to get back to you on that.)

You'd expect that each core accessing the XCHG variable though would have to get the value from memory though as soon as it accessed it, otherwise what use would CMPXCHG be? Not sure about this area to be honest, (but I read that these atomic operations do create a memory barrier so a core cannot execute instructions either side of said barrier out of order).

Quote:

Lastly, if you're doing no mutexing, what are you doing instead -- polling? That's not going to be more efficient, untold amounts of CPU will be expended on what amounts to a while(1) loop.

What I'm building is a thread-pool with n pthreads equal to the number of cores (so I am using pthreads). The pthreads continually execute a list of "lightweight tasks". The user can create tasks and send "messages" between them. (If you've ever used Erlang, something similar to the abstraction provided there but in my case using C).

The pthreads occasionally check the "value" of a task "state", when they reach that task in the queue, therefore if the "state" isn't "ready" they simply move on to the next task (hence the pthread has more work to do and isn't polling continuously). You see what this means, as long as a pthread "eventually" discovers a task is "ready" that's okay, even if it's not asap. It seems like a lock would be unnecessary here then, but a pthread shouldn't detect that the task state is "ready" before its other data members have been updated (hence the need for a memory barrier).

If using these atomic operations isn't going to impact throughput, then great they solve the problem, but even that seems like overkill when I only need to ensure that a handful of statements are executed in a certain order.

gorga

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ubuntu-dev-tools

ubuntu-dev-tools(5)						File Formats Manual					       ubuntu-dev-tools(5)

NAME

       ubuntu-dev-tools - Configuration for the ubuntu-dev-tools package.

DESCRIPTION

       The  ubuntu-dev-tools  package  is  similar in scope to the devscripts(1) package, providing a collection of scripts which may be of use to
       Ubuntu and Debian developers or others wishing to build Debian packages.

       Some of these scripts have options which may be configured  on  a  system-wide  and  per-user  basis.   These  options  are  configured	in
       devscripts.conf(5).

       All variables are described in the script's manpages. Package-wide variables begin with "UBUNTUTOOLS" and are listed below.

       Every script which reads the configuration files can be forced to ignore them by using the --no-conf command-line option.

ENVIRONMENT

       All ubuntu-dev-tools configuration variables can be set (and overridden) by setting them in the environment (unlike devscripts).

       In addition, several scripts use the following environment variables:

       UBUMAIL
	      Overrides  DEBEMAIL  and	DEBFULLNAME  when  the	target	is  clearly  Ubuntu.   Can  either  contain an e-mail address or Full Name
	      <email@example.org>.

       DEBEMAIL, DEBFULLNAME
	      As in devscripts(1).

PACKAGE-WIDE VARIABLES
       The currently recognised package-wide variables are:

       UBUNTUTOOLS_BUILDER
	      This specifies the preferred test-builder, one of pbuilder (default), sbuild, pbuilder-dist.

       UBUNTUTOOLS_DEBIAN_MIRROR
	      The preferred Debian archive mirror.  Should be of the form http://ftp.debian.org/debian (no trailing slash).  If not specified, the
	      master will be used.

       UBUNTUTOOLS_DEBSEC_MIRROR
	      The  preferred Debian security archive mirror.  Should be of the form http://security.debian.org (no trailing slash).  If not speci-
	      fied, the master will be used.

       UBUNTUTOOLS_UBUNTU_MIRROR
	      The preferred Ubuntu archive mirror.  Should be of the form http://archive.ubuntu.com/ubuntu (no trailing slash).  If not specified,
	      the master will be used.

       UBUNTUTOOLS_UBUNTU_PORTS_MIRROR
	      The preferred Ubuntu archive mirror.  Should be of the form http://ports.ubuntu.com (no trailing slash).	If not specified, the mas-
	      ter will be used.

       UBUNTUTOOLS_LPINSTANCE
	      The launchpad instance to communicate with. e.g. production (default) or staging.

       UBUNTUTOOLS_MIRROR_FALLBACK
	      Whether or not to fall-back to the master archive mirror.  This is usually the desired behaviour, as mirrors can	lag  the  masters.
	      If on a private network with only a local mirror, you may want to set this to no.  One of yes (default) or no.

       UBUNTUTOOLS_UPDATE_BUILDER
	      Whether or not to update the test-builder before each test build.  One of yes or no (default).

       UBUNTUTOOLS_WORKDIR
	      The directory to use for preparing source packages etc.  When unset, defaults to a directory in /tmp/ named after the script.

SEE ALSO

       devscripts(1), devscripts.conf(5)

AUTHORS

       This manpage was written by Stefano Rivera <stefanor@ubuntu.com>.

ubuntu-dev-tools						 December 19 2010					       ubuntu-dev-tools(5)

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