CODEWITHSUNDEEP
javavolatile
Coder 477
Coder 477

Reputation: 435

JAVA volatile variable read performance with LazySet

I am trying to Understand performance of volatile variable in JAVA.

I see https://brooker.co.za/blog/2012/09/10/volatile.html and it seems volatile reads are slow when there is a writer involved. I have not seen any more arguments or benchmarks mentioning the same.

How would AtomicReference lazySet affect volatile variable reads

Upvotes: 3

Views: 438

Answers (1)

pveentjer
pveentjer

Reputation: 11372

A regular read and volatile read from hardware x86 perspective are equally cheap. Volatile read requires acquire semantics which is provided by the tso memory model of x86. So both regular load and volatile load have acquire semantics and are equally cheap. On software level there is difference since volatile read prohibits many compiler optimizations.

A lazy set will not change the performance of the reader; just the performance of the writer. On X86 volatile write is a sequential consistent write; so a [StoreLoad] is needed and this requiring stopping any loads from being executed until the store buffer is drained. A lazySet aka orderedSet placed the store on the store buffer and then continues. So it won't stall the CPU. This is purely a writer concern; not a reader. So a reader will not go any faster or slower.

In your case: first determine if it is actual a problem. In most cases many other issues are playing and optimizing on this level makes code complex and introduces bugs. If it truly is a problem, I would be more focused on contention on the cache line than the overhead of reading/writing to a volatile variable.

Upvotes: 3

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