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time: use monotonic clock to measure elapsed time #12914

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tsuna opened this Issue Oct 13, 2015 · 154 comments

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tsuna commented Oct 13, 2015

Go's standard library doesn't provide any API to access a monotonic clock source, which means one can't reliably time how long an operation takes without copy-pasting or importing some low-level platform-dependent code such as the clock package at https://github.com/davecheney/junk.

This API doesn't necessarily need to return a time.Time, just returning a number of nanoseconds as a uint64 would be enough, kinda like Java's System.nanoTime().

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adg commented Oct 13, 2015

Do we have a monotonic clock source we can export?

If not, why should this be in the standard library and not just an
importable package?

On 13 October 2015 at 12:42, Benoit Sigoure notifications@github.com
wrote:

Go's standard library doesn't provide any API to access a monotonic clock
source, which means one can't reliably time how long an operation takes
without copy-pasting or importing some low-level platform-dependent code
such as the clock package at https://github.com/davecheney/junk.

This API doesn't necessarily need to return a time.Time, just returning a
number of nanoseconds as a uint64 would be enough, kinda like Java's
System.nanoTime()
http://docs.oracle.com/javase/8/docs/api/java/lang/System.html#nanoTime--
.


Reply to this email directly or view it on GitHub
#12914.

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tsuna commented Oct 13, 2015

Why should this be in the standard library: because it's a pretty fundamental thing, especially in a language geared towards systems programming. I see a lot of people timing how long operations take using time.Now() and subtracting two time.Time objects, and I always end up saying "this is not a correct way of measuring how much time elapsed" but don't have a clear alternative to propose.

Come on, even Java has it.

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adg commented Oct 13, 2015

I see a lot of people timing how long operations take using time.Now() and subtracting two time.Time objects, and I always end up saying "this is not a correct way of measuring how much time elapsed"

Got any concrete examples?

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tsuna commented Oct 13, 2015

Any concrete examples of ..? People using time.Now() or it being wrong because it doesn't take into account changes made to the system time?

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adg commented Oct 13, 2015

I suppose what I'm getting at is this: If there's a broad need for such a package, why isn't there one that people are already using?

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extemporalgenome commented Oct 13, 2015

This API doesn't necessarily need to return a time.Time, just returning a number of nanoseconds as a uint64 would be enough, kinda like Java's System.nanoTime().

time.Duration happens to represent a 64-bit quantity nanoseconds, and, unless there's some large problem with it, should be favored as the return type for any monotonic duration handling functions.

@rakyll rakyll added this to the Unplanned milestone Oct 13, 2015

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taruti commented Oct 21, 2015

I think there are two things:

A) A proper monotonic clock package (there are multiple 3rd party ones)

B) Measuring elapsed time on systems where system time may leap to either direction

B is simpler. Either ensure and document that time.Since works
even with clock shifts or define new API like:

type MeasureStart struct { ... }
func (*MeasureStart)Start()
func (*MeasureStart)Elapsed() time.Duration
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crawshaw commented Oct 23, 2015

On what platforms is time.Now() not monotonic? For linux/amd64, time.Now is implemented by runtime·nanotime, which is calling clock_gettime (via VDSO) with CLOCK_MONOTONIC. I see similar calls being used on linux/arm, openbsd, freebsd, etc.

Instead of a new interface, can we document time.Now as monotonic?

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ianlancetaylor commented Oct 23, 2015

@crawshaw time.Now is not monotonic on any platform that supports settimeofday. On amd64 GNU/Linux time.Now does not actually use CLOCK_MONOTONIC, it uses CLOCK_REALTIME. It's runtime.nanotime that uses CLOCK_MONOTONIC.

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ianlancetaylor commented Oct 23, 2015

Sorry, I somehow missed that you mentioned nanotime yourself. That's not what implements time.Now, though, that is implemented by time·now.

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crawshaw commented Oct 23, 2015

Ah right, thanks. I got confused by //go:linkname time_runtimeNano time.runtimeNano, which I misread as implementing time.Now

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alexbrainman commented Oct 23, 2015

time.Now() is monotonic on windows. We use undocumented way to fetch time, but it is monotonic as far as I know. @dvyukov to confirm.

Alex

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tsuna commented Oct 23, 2015

On Mon, Oct 12, 2015 at 9:07 PM, Andrew Gerrand wrote:

I suppose what I'm getting at is this: If there's a broad need for such a package, why isn't there one that people are already using?

Because people incorrectly assume that time only moves forward? It's an extremely common mistake that even at Google we were trying to hammer in people's head. When you see the large impact that things such as leap seconds have (including at Google), it's easy to see this is a well known problem but also a recurring problem.

How is it that a language geared towards systems programming that comes with such a rich standard library offers no standard way of measuring time correctly?

Look at gRPC, every single use of time.Now() there is buggy because it implicitly assumes that time only moves forward and at a constant pace, neither of which is true for CLOCK_REALTIME. Pretty much any distributed systems code that uses time.Now() other than for printing the current time is subtly buggy. And there is a lot of it. And it's there chiefly because there is no alternative in the standard library to get monotonic time so people don't even think about it.

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rsc commented Oct 24, 2015

How is it that a language geared towards systems programming that comes with such a rich standard library offers no standard way of measuring time correctly?

I expect that if you care that much about time you will run your system clocks correctly. On a well-run system, time does only move forward and at a constant (enough) pace.

When you see the large impact that things such as leap seconds have (including at Google), it's easy to see this is a well known problem but also a recurring problem.

I don't understand this comment. Leap seconds don't exist at Google.

It is true that if you use time.Now to measure time during a leap smear then your measurements may be off by approximately 0.001%. But time will still be monotonic.

@rsc rsc changed the title runtime: Add a new API to access a monotonic clock source runtime: time: expose monotonic clock source Oct 24, 2015

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dvyukov commented Oct 24, 2015

I expect that if you care that much about time you will run your system clocks correctly. On a well-run system, time does only move forward and at a constant (enough) pace.

How about programs that run on client machines? Or server software that you sells to clients?

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dvyukov commented Oct 24, 2015

time.Now() is monotonic on windows. We use undocumented way to fetch time, but it is monotonic as far as I know. @dvyukov to confirm.

I don't think we return monotonic time from time.Now on windows. This would be a bug, because time.Now should return system time which can be changed by user.
The shared page contains InterruptTime, this time is monotonic and we use it for runtime.nanotime; and SystemTime, this time is not monotonic and we use for time.Now.

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alexbrainman commented Oct 24, 2015

What Dmitry said.

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tsuna commented Oct 25, 2015

On Fri, Oct 23, 2015 at 9:50 PM, Russ Cox wrote:

I don't understand this comment. Leap seconds don't exist at Google.

I was working just down the hall from where you were in B43 in 2008, when the leap second smearing scheme was devised, because everybody remembered all too well how all hell broke loose for the previous leap second.

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driskell commented Feb 21, 2016

I'm not quite understanding, why the time and net package can get access to a monotonic clock, through runtime package, but nothing else can. Surely it would be beneficial to allow others to write packages that have similar capabilities to the time and net standard libraries? It seems the protections of the monotonic clock are currently limited to the standard library and inaccessible to all other libraries.

I can understand the arguments about clocks should never go backwards but the fact that time and net is already using monotonic time, in my opinion, means that discussion was had a long time ago and the argument for monotonic time won out. It's unfortunate it's been kept inaccessible to everyone else.

Is it feasible and would it be acceptable to expose a runtime.NanoTime? That name is likely misleading though but it seems reasonable to me. (https://github.com/golang/go/blob/master/src/runtime/time.go#L296)

Arista-Jenkins pushed a commit to aristanetworks/goarista that referenced this issue May 2, 2016

atime: New package to provide access to a fast monotonic clock source.
This package fills in the gap of the Go standard library discussed in
golang/go#12914.

Change-Id: Icefcb17a11a7061be369318b50ca0cd55c53e7da
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tsuna commented May 2, 2016

For those who need a solution to this problem, here's a way to expose Go's own runtime.nanotime(), which avoids the hassle of reimplementing an Nth solution that is fast & cross-platform: aristanetworks/goarista@46272bf

@rsc rsc changed the title proposal: time: use monotonic clock to measure elapsed time time: use monotonic clock to measure elapsed time Feb 3, 2017

@rsc rsc modified the milestones: Go1.9, Proposal Feb 3, 2017

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cespare commented Feb 3, 2017

Looks like the current code on master uses 1885-2157 as the range of wall times when monotonic time is present.

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rsc commented Feb 3, 2017

OK, this is checked in for Go 1.9. I moved the internal epoch to 1885 (max year 2157) to avoid any possible problem with NTP-epoch-derived times. I also adjusted the design doc to reflect this change and some minor encoding changes.

Thanks for the constructive conversation everyone.

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orivej commented Feb 3, 2017

  1. The merged implementation changes the meaning of time.Now().Sub(old) when now and old are separated by system suspend and resume: it used to calculate real time difference and now that difference is reduced by the duration the system was suspended. Shouldn't time use monotonic real time source when available, and if so, can runtime.nanotime be redefined to read from that source, or should there be a new clock specifically for time.Now() to read from?

  2. Is there an explanation for + 1 in nanotime() - startNano + 1 in the design doc? If it is needed, why startNano is not reduced by 1 instead?

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rsc commented Feb 3, 2017

@orivej,

  1. Whether time.Since(start) includes "system suspended" time depends on the underlying OS's monotonic clock, but in general it's tricky to add that time into the clock since you can't take it back if you later find out you are wrong. My preference here would be to do the same thing on all operating systems, whatever that is. My guess is that all operating systems can provide the current semantics but that not all can provide the "include suspended time" semantics.

    It seems to me that if you want to include time while the computer was off, you're really talking about wall times, so the fix would be to flip into wall mode by recording 'start := time.Now().AddDate(0, 0, 0)', putting up with the fact that time.Since(start) might then return negative elapsed times.

  2. The +1 is just to make sure that tests never see a 0 monotonic time, since I used GetMono(&t) == 0 to say that there's no monotonic time. It's only for testing, and even in that context it's probably unnecessary, but also harmless.

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gopherbot commented Feb 5, 2017

CL https://golang.org/cl/36334 mentions this issue.

gopherbot pushed a commit that referenced this issue Feb 5, 2017

time: Fix typo in Time.String() description.
Refs #12914.

Change-Id: Iadac4cbef70db6a95b47f86eaffcfc63bfdb8e90
Reviewed-on: https://go-review.googlesource.com/36334
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
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neild commented Mar 1, 2017

I'm looking at some code which assumes that times truncated to the same time are equal. Simplified:

func main() {
        t1 := time.Now().Truncate(1 * time.Second)
        t2 := time.Now().Truncate(1 * time.Second)
        h1, m1, s1 := t1.Clock()
        h2, m2, s2 := t2.Clock()
        if h1 == h2 && m1 == m2 && s1 == s2 {
                if !t1.Equal(t2) {
                        fmt.Printf("BUG: times truncated to the same second, but are not equal:\n%v\n%v\n", t1, t2)
                }
        }
}

The actual code is mapping times to windows; it assumes that all times within a given window will truncate to the same time.

This doesn't seem unreasonable. Should Truncate discard the monotonic time component?

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dmitshur commented Mar 1, 2017

@neild A relevant open issue is #18991.

dmitshur added a commit to google/go-github that referenced this issue Mar 1, 2017

Modify time first, round after (and strip additional precision).
I've noticed that TestDo_rateLimit_noNetworkCall test fails on Go
master with the following error:

	--- FAIL: TestDo_rateLimit_noNetworkCall (0.00s)
		github_test.go:549: rateLimitErr rate reset = 2017-02-25 01:49:01 +0000 UTC, want 2017-02-25 01:49:01 +0000 UTC m=+60.206993003

(Source: https://travis-ci.org/google/go-github/jobs/205180544.)

This is due to changes in Go 1.9 to perform monotonic elapsed time
measurements. See golang/go#12914 and
https://golang.org/design/12914-monotonic for full details.

Make the test pass by modifying time first, round after, and strip
the monotonic clock reading by using AddDate(0, 0, 0). Doing that
eliminates the unwanted additional monotonic precision from wanted
time, making the expected time equality true.

borkmann added a commit to cilium/cilium that referenced this issue May 27, 2017

pkg, bpf: add api to return monotonic clock
This adds an API that we can use to compare values against bpf_ktime_get_ns()
BPF helper for the timeouts. Looks like golang doesn't offer such an interface,
so we need to add a cgo version right here. Probably also better to not rely
on go internals should they once change their clock source internally, etc.

Related: golang/go#12914

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

tgraf added a commit to cilium/cilium that referenced this issue May 27, 2017

pkg, bpf: add api to return monotonic clock
This adds an API that we can use to compare values against bpf_ktime_get_ns()
BPF helper for the timeouts. Looks like golang doesn't offer such an interface,
so we need to add a cgo version right here. Probably also better to not rely
on go internals should they once change their clock source internally, etc.

Related: golang/go#12914

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

@golang golang locked and limited conversation to collaborators Mar 1, 2018

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