aio_proc_threads(5)

NAME

aio_proc_threads — maximum number of process threads allowed in AIO pool

VALUES

Failsafe

256

Default

1024

Allowed values

4-2048

Recommended values

32-2048

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 aio_proc_threads 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 aio_proc_thread_pct 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 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 aio_monitor_run_sec.

When Should the Value of This Tunable Be Raised?

aio_proc_threads 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. (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?

aio_proc_threads 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_thread_pct 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 max_thread_proc.

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.

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 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.

AUTHOR

aio_proc_threads was developed by HP.