iothread: document -object iothread on man page

CN=laptop.example.com,O=Example Home,L=London,ST=London,C=GB

@end example

@item -object iothread,id=@var{id},poll-max-ns=@var{poll-max-ns},poll-grow=@var{poll-grow},poll-shrink=@var{poll-shrink}

Creates a dedicated event loop thread that devices can be assigned to.  This is

known as an IOThread.  By default device emulation happens in vCPU threads or

the main event loop thread.  This can become a scalability bottleneck.

IOThreads allow device emulation and I/O to run on other host CPUs.

The @option{id} parameter is a unique ID that will be used to reference this

IOThread from @option{-device ...,iothread=@var{id}}.  Multiple devices can be

assigned to an IOThread.  Note that not all devices support an

@option{iothread} parameter.

The @code{query-iothreads} QMP command lists IOThreads and reports their thread

IDs so that the user can configure host CPU pinning/affinity.

IOThreads use an adaptive polling algorithm to reduce event loop latency.

Instead of entering a blocking system call to monitor file descriptors and then

pay the cost of being woken up when an event occurs, the polling algorithm

spins waiting for events for a short time.  The algorithm's default parameters

are suitable for many cases but can be adjusted based on knowledge of the

workload and/or host device latency.

The @option{poll-max-ns} parameter is the maximum number of nanoseconds to busy

wait for events.  Polling can be disabled by setting this value to 0.

The @option{poll-grow} parameter is the multiplier used to increase the polling

time when the algorithm detects it is missing events due to not polling long

enough.

The @option{poll-shrink} parameter is the divisor used to decrease the polling

time when the algorithm detects it is spending too long polling without

encountering events.

The polling parameters can be modified at run-time using the @code{qom-set} command (where @code{iothread1} is the IOThread's @code{id}):

@example

(qemu) qom-set /objects/iothread1 poll-max-ns 100000

@end example

@end table