Best Practice for Busy Waiting in Java

[Markus KARG]

In general, I second do that. The point why I came up with a best 
practices document is that in many companies the average programmer is 
not an engineer, and would be happy to have some rules at hand. It must 
not be perfect, it must be just better than "empty" busy-wait.


Am 17.06.2025 um 11:33 schrieb Viktor Klang:
> I'll second what Alan said.
>
> What I've found to be true, over and over, is that every API needs its 
> own (broad) performance testing, since memory access patterns, 
> instructions-under-critical-section, and differences in scheduling 
> across different platforms all interact.
>
> It's worth having a look at how busy-waiting is implemented in 
> different java.util.concurrent constructs.
>
> Cheers,
> √
>
> *
> *
> *Viktor Klang*
> Software Architect, Java Platform Group
> Oracle
> ------------------------------------------------------------------------
> *From:* core-libs-dev <core-libs-dev-retn at openjdk.org> on behalf of 
> Alan Bateman <alan.bateman at oracle.com>
> *Sent:* Monday, 16 June 2025 17:23
> *To:* Markus KARG <markus at headcrashing.eu>
> *Cc:* core-libs-dev <core-libs-dev at openjdk.org>
> *Subject:* Re: Best Practice for Busy Waiting in Java
> On 16/06/2025 12:09, Markus KARG wrote:
> >
> >
> > In case you MUST use busy-wait, apply the following rules:
> >
> >
> > * NEVER have EMPTY busy-wait loops, but ALWAYS put Thread.onSpinWait()
> > into it. The performance drop is negligible but the CO2 footprint is
> > considerably smaller.
> >
> > * IF it is acceptable for the waiting thread to not have the
> > *absolute* maximum throughput, put Thread.yield() before
> > Thread.onSpinWait() in the busy-wait loop, so CPU cores are more
> > efficiently used.
> >
> > * Never use Thread.sleep() in busy-wait loops.
> >
> > * If possible, pin current thread to current CPU core to prevent
> > inefficient context switches.
> >
> > * ...more rules...
> >
> >
> > THAT is what my question is targeting! :-)
> >
>
> It may be possible to provide some guidelines but I don't think they can
> be turned into rules, e.g. the NEVER example is challenged by methods
> that do atomic increment/add/etc. as this read+CAS in a tight loop.
> There are also examples that of tight loops to CAS an object to the add
> it to a list. The examples of onSpinWait in the JDK might give you some
> ideas, e.g. using it in conjunction with a max spin count before timed
> back off or parking. One thing to add to your list is virtual threads
> where it may be better to park rather than Thread.yield.
>
> -Alan
>
>
>
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