RFR(s): 8214180: Need better granularity for sleeping

[David Holmes]

Hi Robbin,

On 22/11/2018 12:06 am, Robbin Ehn wrote:
> Hi all, please review.
> 
> naked_short_sleep is to coarse grained on contemporary hardware/os:es.
> 1 ms as minimum when we can complete an entire safe-point in 0.5 ms is a 
> very long time.
> Sleeping a very short time instead yielding have several uses-cases.

So you factored out os::naked_short_sleep into os_posix.cpp for use by 
all platforms except Windows. That seems fine. Solaris is already linked 
with -lrt so use of nanosleep should be fine there.

You added os::naked_short_nanosleep, defined in os_posix.cpp, to use 
nanosleep. Also fine.

Question: have you actually measured the observable minimum sleep time 
on different OS? (And it can even vary depending on hardware).

For Windows you create and use a WaitableTimer. That does not seem okay. 
That seems extremely heavyweight. The time taken to create and use the 
timer might be longer than what you intended to sleep for! And again 
there is the issue of the actual accuracy of the timer even if you can 
specify nanosecond times. I'm also unclear about the time value passed 
to the timer - the docs state it is supposed to be expressed in 100ns 
increments, and it's unclear if that also applies to the relative form ??

> Here I add it SpinYield to get much smother back-off delay curve.
> Which means it will be usable in more places.

Seems okay - assuming a 1 microsecond sleep time is achieveable.

Thanks,
David
-----

> CR:
> https://bugs.openjdk.java.net/browse/JDK-8214180
> 
> Webrev:
> http://cr.openjdk.java.net/~rehn/8214180/webrev/
> 
> Passes t1-3.
> 
> Thanks, Robbin