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Shared_ptr<T>: the (not always) atomic reference counted smart pointer (2019)

(snf.github.io)
51 points klaussilveira | 1 comments | 27 Aug 25 14:34 UTC | HN request time: 0s | source
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sesuximo ◴[31 Aug 25 12:36 UTC] No.45082719[source]▶
Why is the atomic version slower? Is it slower on modern x86?
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eptcyka ◴[31 Aug 25 12:50 UTC] No.45082778[source]▶
Atomic write operations force a cache line flush and can wait until the memory is updated. Atomic reads have to be read from memory or a shared cache. Atomics are slow because memory is slow.
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1. nly ◴[31 Aug 25 14:55 UTC] No.45083625[source]▶
This isn't true.

Atomic operations work inside the confines of the cache coherence protocol. Nothing has to be flushed to main memory, or even a lower cache

An atomic operation does something more along the lines of emitting an invalidation, putting the cache line in to an exclusive state, and then ignores find and invalidation requests from other cores while it operates.