iOS GCD：DISPATCH_QUEUE_PRIORITY_BACKGROUND，文件中“磁盘I / O被限制”是什么意思？

Question

As the document says:

DISPATCH_QUEUE_PRIORITY_BACKGROUND Items dispatched to the queue will run at background priority, ie the queue will be scheduled for execution after all higher priority queues have been scheduled and the system will run items on this queue on a thread with background status as per setpriority(2) (ie disk I/O is throttled and the thread's scheduling priority is set to lowest value).

The last part of the document, what does "disk I/O is throttled" mean here?

Does it mean that tasks running at DISPATCH_QUEUE_PRIORITY_BACKGROUND level can't access disk?

Answer 1

What we can deduce from the documentation is that DISPATCH_QUEUE_PRIORITY_BACKGROUND is run on a thread with “background status as per setpriority(2) ”.

setpriority(2) has a parameter prio which can be set to either 0 or PRIO_DARWIN_BG . I assume this means that PRIO_DARWIN_BG is used and the documentation describes this as:

When a thread or process is in a background state the scheduling priority is set to the lowest value, disk IO is throttled (with behavior similar to using setiopolicy_np(3) to set a throttleable policy), and network IO is throttled for any sockets opened after going into background state. Any previously opened sockets are not affected.

setiopolicy_np(3) can set the thread I/O policy to IOPOL_IMPORTANT , IOPOL_STANDARD , IOPOL_UTILITY , IOPOL_THROTTLE , or IOPOL_PASSIVE . It describes the effect of throttled disk I/O as:

If a throttleable request occurs within a small time window of a request of higher priority, the thread that issued the throttleable I/O is forced to a sleep for a short period. (Both this window and the sleep period are dependent on the policy of the throttleable I/O.) This slows down the thread that issues the throttleable I/O so that higher-priority I/Os can complete with low-latency and receive a greater share of the disk bandwidth. Furthermore, an IMPORTANT I/O request may bypass a previously issued throttleable I/O request in kernel or driver queues and be sent to the device first. In some circumstances, very large throttleable I/O requests will be broken into smaller requests which are then issued serially.

Which basically means that reading and writing to disk may be slowed or delayed, if another thread with higher priority is also accessing the disk. So no, it does not prevent tasks running at DISPATCH_QUEUE_PRIORITY_BACKGROUND from accessing the disk.

Answer 2

throttling means it controls the rate of I/O based on available resources. On low memery condition it many reduce the number of I/O process, and slows down.