[SR-12124] [NumberDate] CFGetSystemUptime for BSD and Windows

[benrimmington]

Previous ID	SR-12124
Radar	rdar://problem/59216628
Original Reporter	@benrimmington
Type	Improvement

Additional Detail from JIRA

Votes	0
Component/s	Foundation
Labels	Improvement
Assignee	None
Priority	Medium

md5: 69fb038c8490173be7492fcf9739e539

Issue Description:

Re: CFGetSystemUptime — use CLOCK_UPTIME for BSD, and QueryUnbiasedInterruptTimePrecise for Windows?

CLOCK_MONOTONIC is the correct choice for Linux and Cygwin, because it excludes system "downtime" on those platforms.

CLOCK_UPTIME is available on FreeBSD and OpenBSD, so it might be a better option for TARGET_OS_BSD.

GetTickCount64 may include system "downtime", and the result is in milliseconds, which might not be precise enough. For example, ProcessInfo.processInfo.systemUptime is currently used by swift-corelibs-xctest.

QueryUnbiasedInterruptTimePrecise excludes system "downtime", and the result is in 100-nanosecond units. It is also used by Cygwin and Dispatch for the same purpose.

[benrimmington]

Cc: @compnerd

[beccadax]

@swift-ci create

[benrimmington]

Cc: 3405691582 (JIRA User)

[swift-ci]

Comment by 3405691582 (JIRA)

👍

[swift-ci]

Comment by 3405691582 (JIRA)

Just something to think about since I brushed up against this recently: on OpenBSD at least, CLOCK_MONOTONIC and CLOCK_UPTIME are reasonably close, but a lot of Foundation uses MONOTONIC which Dispatch may interpret as UPTIME since it is trying to do stuff like clock inference (c.f. https://github.com/apple/swift-corelibs-libdispatch/blob/f13ea5dcc055e5d2d7c02e90d8c9907ca9dc72e1/src/shims/time.h#L257). I don't have any solutions or ideas for this right now, but it's something interesting to think about.