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Diffstat (limited to 'qemu-options.hx')
| -rw-r--r-- | qemu-options.hx | 38 |
1 files changed, 38 insertions, 0 deletions
diff --git a/qemu-options.hx b/qemu-options.hx index 4cf7751..3ff8c4e 100644 --- a/qemu-options.hx +++ b/qemu-options.hx @@ -4916,6 +4916,44 @@ access 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 |