12-26-2002
The size of the LWP pool has a critical impact on the performance of the many-to-many model: if the number of LWPs in the pool is nearly equal to the number of threads, the implementation will act much like the one-to-one model. Conversely, if there are very few LWPs in the pool, the implementation will act like the many-to-one model.
Of particular concern is the risk of deadlock with an excessively small pool: one thread may block on a resource in the kernel and go to sleep, and by so doing block the LWP needed to run the resource-holder. To solve this problem, the threads package makes a minimal guarantee to the threads programmer: progress will always be made. This is implemented through the use of the SIGWAITING signal. When the kernel realizes that all of a process's LWPs are blocked at the kernel level, it drops a SIGWAITING on the process. Upon receipt of the signal, the user-level threads package decides whether or not to create a new LWP, on the basis of the number of runnable threads. The SIGWAITING mechanism makes no guarantees about optimal use of LWPs on a multiprocessor. Specifically, a process may have many more runnable user-level threads than it has LWPs, but it does not receive a SIGWAITING until all LWPs are blocked. Thus, even if there are processors available and work to be done, the SIGWAITING mechanism does not guarantee that there is a sufficient number of LWPs to run the user threads on the available processors. If the programmer wishes to use unbound threads and take advantage of all available processors, he or she is required to advise the library on the number of LWPs required.
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vxiod(1M) vxiod(1M)
NAME
vxiod - start, stop, and report on Veritas Volume Manager I/O threads
SYNOPSIS
vxiod
vxiod [-f ] [set count]
DESCRIPTION
The vxiod utility starts, stops, or reports on Veritas Volume Manager (VxVM) I/O kernel threads. An I/O thread provides a process context
for performing I/O in VxVM.
When the vxio module is loaded, 16 I/O threads are created, plus 2 threads per additional CPU for a system with more than 8 CPUs, up to a
maximum of 64 threads. At least one I/O thread must be running while the vxio module is loaded, and the number of I/O threads cannot be
forced to zero.
When invoked with no arguments, vxiod prints the current number of I/O threads to the standard output.
The number of threads that is required for handling I/O requests depends on the system load and usage. If volume recovery seems to proceed
more slowly at times, it may be possible to improve its performance by increasing the number of I/O threads up to a maximum of 64.
KEYWORDS
set When invoked with the set keyword, vxiod creates the number of I/O threads specified by count. If more volume I/O threads exist
than are specified by count, the excess processes terminate. If more than the maximum number(64) are specified, the requested
number is silently truncated to that maximum.
OPTIONS
-f This option has no effect from release 5.0 onward. The number of I/O threads cannot be reduced to zero.
EXIT CODES
The vxiod utility prints a diagnostic on the standard error, and exits if an error is encountered. If an I/O request occurs within a I/O
thread, the state of that I/O request is not reflected in the exit status for vxiod. Otherwise, vxiod returns a non-zero exit status on
error.
Usage errors result in an exit status of 1 and a usage message. If the requested number of threads cannot be created, the exit status is
2, and the number of threads that were successfully started is reported. If any other error occurs, the exit status is 3.
FILES
/dev/vx/iod The device used to report on and start volume I/O threads.
NOTES
Veritas Volume Manager I/O threads cannot be killed directly through the use of signals.
Depending on the operating system, VxVM I/O threads may not appear in the list of processes that is output by the ps command. The number
of I/O threads that is currently running can be determined by running vxiod.
SEE ALSO
fork(2), ps(1), vxconfigd(1M), vxdctl(1M), vxintro(1M), vxio(7), vxiod(7)
VxVM 5.0.31.1 24 Mar 2008 vxiod(1M)