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pc 0.3-2008-04-02 (Default branch)
04-03-2008
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aio_proc_threads(5) File Formats Manual aio_proc_threads(5) NAME
aio_proc_threads - maximum number of process threads allowed in AIO pool VALUES
Failsafe Default Allowed values Recommended values DESCRIPTION
The implementation of POSIX AIO on HP-UX uses kernel threads to perform I/Os to filesystems that do not directly support true asynchronous I/O. (This distinction is transparent to the user.) The kernel threads are organized into worker-thread pools (called AIO thread pools) created on a per-process basis. Since a thread pool mechanism for I/Os introduces a variety of trade-offs concerning utilization of CPU time vs. I/O resources, four dynamic tunables are available to customize the behavior of this thread pool: aio_proc_threads(5), aio_proc_thread_pct(5), aio_req_per_thread(5), and aio_monitor_run_sec(5). Please see individual manpages for details on each of these tunables. The tunable specifies, on a per-process basis, the maximum number of process threads that can be used by the POSIX AIO system as kernel threads for issuing I/Os. This tunable interacts with in the following way: the maximum number of threads used for AIO will be the smaller of the two values defined by the two tunables, i.e.: This allows the number of AIO threads to vary dynamically with but to always be bound by an absolute limit of Who Is Expected to Change This Tunable? System administrators that run applications requiring heavy usage of POSIX AIO to filesystems. Restrictions on Changing This tunable is dynamic. Changes to to this tunable take effect immediately for new processes started after the change. They also impact existing processes, but the speed with which the changes propagate to running processes is determined by the tunable When Should the Value of This Tunable Be Raised? should be raised for applications that do not use very many threads for their own work, but desire high performance from the POSIX AIO sub- system. What Are the Side Effects of Raising the Value of This Tunable? Some applications that use POSIX AIO but also require a large number of threads may find that they are unable to create new threads. (This could happen if the POSIX AIO thread pool ends up using too many of a process' allowable threads.) In addition, using a larger number of kernel threads might lead to increased CPU utilization. When Should the Value of This Tunable Be Lowered? should be lowered when POSIX AIO performance is acceptable but applications using POSIX AIO are seeing errors when trying to create new threads for other work. What Are the Side Effects of Lowering the Value of This Tunable? By ultimately reducing the number of threads available to handle POSIX AIO requests, overall I/O throughput of the POSIX AIO subsystem could be reduced. What Other Tunables Should Be Changed at the Same Time as This One? interacts with this tunable by setting a limit on the number of AIO threads based on a percentage of the maximum number of allowable process threads. This allows the AIO thread pools to respond dynamically to changes in defines the desired relationship between the number of POSIX AIO kernel threads and the number of I/Os to be serviced. defines how often (in seconds) the AIO thread mechanism will monitor itself for adherence to the constraints defined by the tunables above. WARNINGS
All HP-UX kernel tunable parameters are release specific. This parameter may be removed or have its meaning changed in future releases of HP-UX. Installation of optional kernel software, from HP or other vendors, may cause changes to tunable parameter values. After installation, some tunable parameters may no longer be at the default or recommended values. For information about the effects of installation on tun- able values, consult the documentation for the kernel software being installed. For information about optional kernel software that was factory installed on your system, see at AUTHOR
was developed by HP. SEE ALSO
kctune(1M), sam(1M), gettune(2), settune(2), aio_proc_thread_pct(5), aio_req_per_thread(5), aio_monitor_run_sec(5). Tunable Kernel Parameters aio_proc_threads(5)