Best Practice for Busy Waiting in Java

[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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