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HP-UX Reference > A
aio_proc_thread_pct(5)Tunable Kernel ParametersHP-UX 11i Version 3: February 2007 |
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NAMEaio_proc_thread_pct — percentage of all process threads allowed in AIO pool DESCRIPTIONThe 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 aio_proc_thread_pct specifies, on a per-process basis, the percentage of threads that can be used by the POSIX AIO system as kernel threads for issuing I/Os. The percentage is taken as a percentage of max_thread_proc, which is the upper bound on the number of threads a process may have. This tunable interacts with aio_proc_threads 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.: MIN (aio_proc_threads, aio_proc_thread_pct * max_thread_proc) This allows the number of AIO threads to vary dynamically with max_thread_proc, but to always be bound by an absolute limit of aio_proc_threads. Who Is Expected to Change This Tunable?System administrators that run applications requiring heavy usage of POSIX AIO to filesystems. Restrictions on ChangingThis tunables 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 aio_monitor_run_sec. When Should the Value of This Tunable Be Raised?aio_proc_thread_pct should be raised for applications that do not use very many threads for their own work, but desire high performance from the POSIX AIO subsystem. 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, because 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?aio_proc_thread_pct 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?aio_proc_threads interacts with this tunable by setting a strict limit on the number of threads that can be used for POSIX AIO. This allows a hard limit to be imposed, regardless of what value max_thread_proc happens to take. aio_req_per_thread defines the desired relationship between the number of POSIX AIO kernel threads and the number of I/Os to be serviced. aio_monitor_run_sec defines how often (in seconds) the AIO thread mechanism will monitor itself for adherence to the constraints defined by the tunables above. WARNINGSAll 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 tunable values, consult the documentation for the kernel software being installed. For information about optional kernel software that was factory installed on your system, see HP-UX Release Notes at http://docs.hp.com. |
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