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High Performance Computing
Memory Barriers for (Ubuntu) Linux (i686)
Post 302430438 by Corona688 on Thursday 17th of June 2010 03:00:49 PM
06-17-2010
Small world, how so? 
Well, yes. It has to synchronize somehow. One way or another you must interrupt other cores with this change in status, or they may never know.
Wow, those are nice.
I think you're overreacting... Any memory I/O monopolizes the bus*, LOCK just guarantees one instruction gets two ops in a row.
Also. The original 8088 has precisely one instruction worth of cache, so locking the bus stalls it instantly... The huge caches, multiple independent memory buses, and cache communication systems in recent NUMA systems usually let cores keep going or find something else to do. 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.)
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.
I really think pthreads is still what you're looking for. They've made it as fast as they know how, significantly changing the kernel to accommodate it.
* Exceptions exist for very special-purpose memory chips like video RAM.
Quote:
Quote:
In that case I could just use one of GCC's built-in atomic operations like "__sync_fetch_ and_ add" or "__sync_bool_compare_and_swap" as described here...
Atomic Builtins - Using the GNU Compiler Collection (GCC)
Atomic Builtins - Using the GNU Compiler Collection (GCC)
Quote:
The thing with these is they use the asm op-code "lock", which issues a hardware lock on the data-bus effectively locking every other process out of memory.
Also. The original 8088 has precisely one instruction worth of cache, so locking the bus stalls it instantly... The huge caches, multiple independent memory buses, and cache communication systems in recent NUMA systems usually let cores keep going or find something else to do. 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.)
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.
I really think pthreads is still what you're looking for. They've made it as fast as they know how, significantly changing the kernel to accommodate it.
* Exceptions exist for very special-purpose memory chips like video RAM.
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LEARN ABOUT DEBIAN
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)