How scheduler gets executed in vxworks

Scheduler can be invoked either synchronously or asynchronously or both. Scheduler is not a task, it is mere a function call invoked in the context of the interrupt or from the invoking task context
Asynchronous method
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When a BSP is configured, we also configure the hardware timer interrupt's which invokes the scheduler on its expiry. Together with this every time an interrupt occurs i.e. Ethernet interrupt, clock interrupt etc the scheduler would be called.
Synchronous method
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When ever the current executing task gets blocked the scheduler would be called. 
Note: Scheduler would be called each time a system call is called.