10-30-2014
Often cache is just reloaded from the lower, slower layers on the new core, and eventually snooped empty on the old core when the data is modified. This means that while a different core may be available at an instant, it is better to wait a bit for the old core, which may not be 100% busy in the longer term. Of course, some caches cache keyed on virtual addresses, not physical ones, and may be flushed when other processes use the core. For them, dispatching multiple threads of the same process in succession reduces cache flushing. So, while you have asked for concurrent threads, that might actually be made less true in the fine by the system.
Hyperthreading is only for same-process threads, as they share the same virtual space. It is a nice way to increase use of CPU resources, with some added delay when threads' needs collide. It is an interesting alternate direction to the trend in modern CPU design to do speculative operations that are 50% or more a waste of the resource, but speed the critical thread. I find it reminiscent of the old Honeywell-800, where the CPU ran instructions of up to 8 threads more or less in rotation. (If you loaded the accumulator, it did not 'hunt', so many programmers used the accumulator as a register to hog the CPU and speed their thread.) I used to fix this stuff, before it crawled inside a chip!
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CORE(5) BSD File Formats Manual CORE(5)
NAME
core -- memory image file format
SYNOPSIS
#include <sys/param.h>
DESCRIPTION
A small number of signals which cause abnormal termination of a process also cause a record of the process's in-core state to be written to
disk for later examination by one of the available debuggers. (See sigaction(2).) This memory image is written to a file named by default
core.pid, where pid is the process ID of the process, in the /cores directory, provided the terminated process had write permission in the
directory, and the directory existed.
The maximum size of a core file is limited by setrlimit(2). Files which would be larger than the limit are not created.
The core file consists of the Mach-O(5) header as described in the <mach-o/loader.h> file. The remainder of the core file consists of vari-
ous sections described in the Mach-O(5) header.
NOTE
Core dumps are disabled by default under Darwin/Mac OS X. To re-enable core dumps, a privileged user must do one of the following
* Edit /etc/launchd.conf or $HOME/.launchd.conf and add a line specifying the limit limit core unlimited
* A privileged user can also enable cores with launchctl limit core unlimited
* A privileged user can also enable core files by using ulimit(1) or limit(1) depending upon the shell.
SEE ALSO
gdb(1), setrlimit(2), sigaction(2), Mach-O(5), launchd.conf(5), launchd.plist(5), sysctl(8)
HISTORY
A core file format appeared in Version 6 AT&T UNIX.
BSD
June 26, 2008 BSD