On Mon, Apr 26, 2021 at 1:24 AM Pure White <wu.purewh...@gmail.com> wrote:
>
> I'm trying to get time using `CLOCK_REALTIME_COARSE` and
> `CLOCK_MONOTONIC_COARSE` for performance reasons, and need to use vdso call
> by hand-written assembly code. That is, I want to reimplement `time.Now`
> using `CLOCK_REALTIME_COARSE` and `CLOCK_MONOTONIC_COARSE`.
>
> I referenced the code in runtime and found that there's an issue #20427
> indicates that I need to switch to g0 for vdso calls, so I tried two methods
> but neither is good.
>
> ## The first method
>
> The first method I tried is just copy the code in runtime and simply change
> the clockid, but this requires copying all the runtime type definations as
> well to make the compiler generate "go_asm.h" for me.
>
> The code runs well, but this is really ugly and unmaintainable as the type
> definations may change across different go versions.
>
> ## The second method
>
> The second method I tried is to link the `runtime.systemstack` and use it to
> do vdso calls:
> ```go
> //go:linkname systemstack runtime.systemstack
> //go:noescape
> func systemstack(fn func())
> ```
>
> My code is something like this:
>
> ```go
> // now calls vdso and is implemented in asm
> func now() (sec int64, nsec int32, mono int64)
>
> func Now() {
> var sec, mono int64
> var nsec int32
> systemstack(func() {
> sec, nsec, mono = now()
> })
> ... // logic copied from time.Now()
> }
> ```
>
> The code runs well without `-race`(test isn't enough), but I encountered
> fatal error under `-race` mode.
> For detailed information, I've filed an issue:
> https://github.com/golang/go/issues/45768.
>
> ## The right way?
>
> So I really want to know what is the right way to do vdso call outside
> runtime?
The right way is to use cgo. But probably the cost of making the cgo
call will overwhelm the speed advantage of using the coarse clocks.
Using go:linkname to call systemstack is completely unsupported, and
is even less maintainable than your first method.
I have to admit that I don't see a way to do this at all. What is the
application? How much difference will it make to use the coarse
timestamps?
Ian
--
You received this message because you are subscribed to the Google Groups
"golang-nuts" group.
To unsubscribe from this group and stop receiving emails from it, send an email
to golang-nuts+unsubscr...@googlegroups.com.
To view this discussion on the web visit
https://groups.google.com/d/msgid/golang-nuts/CAOyqgcWZUduYZDRfBZTcN3%2BrK%3DYCyt8fKAY2w2P%3DHHh--W1Tjg%40mail.gmail.com.
