[SR-744] QOS is unimplemented on Linux

[drewcrawford]

Previous ID	SR-744
Radar	None
Original Reporter	@drewcrawford
Type	Bug
Status	Resolved
Resolution	Done

Environment

Linux x64
swift-DEVELOPMENT-SNAPSHOT-2016-02-08-a

Additional Detail from JIRA

Votes	0
Component/s	libdispatch
Labels	Bug, Linux
Assignee	None
Priority	Medium

md5: e80af79dce0d5276a9822844be18630c

Issue Description:

Darwin documentation for dispatch_get_global_queue says

* It is recommended to use quality of service class values to identify the
 * well-known global concurrent queues:
 *  - QOS_CLASS_USER_INTERACTIVE
 *  - QOS_CLASS_USER_INITIATED
 *  - QOS_CLASS_DEFAULT
 *  - QOS_CLASS_UTILITY
 *  - QOS_CLASS_BACKGROUND

This "preferred" code is not platform-independent, because

• These symbols are not available on Linux

• QOS is not implemented on Linux

I understand that this is not something going to happen in the immediate future, but I'm opening a bug to track this. I'm introducing a bunch of terrible Linux-specific QOS hacks as I port my Darwin code to Linux, and I need to link to a bug in my code comments that explains why the #if os(Linux) branch is so terrible.

[swift-ci]

Comment by David Grove (JIRA)

In the Swift 3 Dispatch overlay, Linux has the same QoS-based API as Darwin for getting the global queues. Since this issue was mostly about the API difference between Darwin and Linux in the Swift 2 APIs, I'm going to resolve this issue (even though Linux doesn't actually provide strong QoS guarantees, the cross-platform API difference is fixed).