[Linux] tweak process priority

在 debug vsync 的過程中，發現 CPU scheduling 對於 vsync accuracy 有影響，因此需要針對 vsync 所在的 process 修改其 process priority.
過程中發現利用 pthread 的 pthread_attr_t 並沒有辦法更動 priority，以下整理了網路上關於 process priority 的資料，目前猜測原因應該是：

Setting a priority under the default scheduling policy (SCHED_OTHER) isn't valid. The default Linux scheduling policy is SCHED_OTHER, which have no priority choice but a nice level to tweak inside the policy

由上面節錄的敘述中可以發現兩個陌生的東西："scheduling policy" 和 "nice value". 來認識他們：
All Linux threads have one of the following scheduling policies:

1. 'Normal' scheduling policies: ranging from 100 (highest priority) to 139 (lowest priority)
   

   SCHED_OTHER   the standard round-robin time-sharing policy;
   SCHED_BATCH   for "batch" style execution of processes; and
   SCHED_IDLE    for running very low priority background jobs.
   

2. Real-time scheduling policies: ranging from 1 (lowest priority) to 99 (higest priority)
   

   SCHED_FIFO    a first-in, first-out policy
   SCHED_RR      a round-robin policy
   

Real-time scheduling 有什麼特性呢，簡單理解應該就是擁有比 Normal scheduling 更不可被搶佔的特性:

SCHED_FIFO can't be preempted (context switched to another process) unless another process of higher priority shows up in the execution queue.

SCHED_RR can be preempted by a time quantum (delay given to a process to execute).

一般的 process 沒有特別指定 default 應該都是 SCHED_OTHER 所以沒辦法透過 pthread_attr_t 設定其 priority，不過還有另外一個方法就是使用其 nice value.

Conventional process's static priority = (120 + Nice value)

因為 Normal scheduling policies 的 priority range 是從 100 ~ 139，那麼從上面可以推得，nice value 的 range 是 -20(highest priority) ~ 19(lowest priority).
至於設定 nice value 的方式似乎不只一種，經過試驗，setpriority 這個 function 是有效的。

Ref

1. How to increase thread priority in pthreads?
2. how to increase the priority of a child pthread relative to the parent thread
3. Understanding Linux CPU scheduling priority

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