The intro of this article repeats the common assertion that
> ARM is the most relaxed, allowing significant hardware optimizations; and x86 is the most strict, enforcing a very strong coherency model that doesn’t allow a lot of room for optimization
but I've seen some compelling arguments that a relaxed model doesn't necessarily have much of a benefit, <a href="https://fgiesen.wordpress.com/2026/08/25/memory-ordering-in-cpus/#320ecacb-31dd-4a74-bd34-de2dbf46b1e0" rel="nofollow">https://fgiesen.wordpress.com/2026/08/25/memory-ordering-in-...
A recent study seemed to support Fabian's well written article: <a href="https://dl.acm.org/doi/epdf/10.1145/3779212.3790129" rel="nofollow">https://dl.acm.org/doi/epdf/10.1145/3779212.3790129
well it depends on your error bars. On many modern system you can have +/- 5% variation or more run to run just due to the non-determinism present in modern CPU architectures and operating systems (even things like room temperature, time of day, the number of environment variables, etc, can affect this). While you could maybe run a set of careful experiments to characterize and remove this, in my experience most researchers don't bother. So something as small as 3% would need a lot of convincing to me to make the argument that it is significant.
First, is a measured improvement actually real or just an artifact of noise?
Second, if it is real, is 3% anywhere close to the true value?
Third, if 3% is real, is it an important difference?
I'm just commenting on the third one. If 3% is real, it's important.
I'm inclined to believe there's a real improvement. They made a lot of different measurements. If the measured improvement was a result of noise, you'd expect a lot of variation an a lot of measurements where TSA was actually faster, and then 3% was the average of that variation. There was a lot of variation (expected, because they were measuring different things) but nearly all of them had TSO being either neutral or slower. Looking at their benchmark graphs, I see two (out of dozens) where TSO was faster.
As far as being close to the true value, these results suggest there is no single true value, as it depends on the workload. No surprise there.
pdw · · focus · HN ↗
> ARM is the most relaxed, allowing significant hardware optimizations; and x86 is the most strict, enforcing a very strong coherency model that doesn’t allow a lot of room for optimization
but I've seen some compelling arguments that a relaxed model doesn't necessarily have much of a benefit, <a href="https://fgiesen.wordpress.com/2026/08/25/memory-ordering-in-cpus/#320ecacb-31dd-4a74-bd34-de2dbf46b1e0" rel="nofollow">https://fgiesen.wordpress.com/2026/08/25/memory-ordering-in-...
Aissen · · focus · HN ↗
wat10000 · · focus · HN ↗
deater · · focus · HN ↗
wat10000 · · focus · HN ↗
First, is a measured improvement actually real or just an artifact of noise?
Second, if it is real, is 3% anywhere close to the true value?
Third, if 3% is real, is it an important difference?
I'm just commenting on the third one. If 3% is real, it's important.
I'm inclined to believe there's a real improvement. They made a lot of different measurements. If the measured improvement was a result of noise, you'd expect a lot of variation an a lot of measurements where TSA was actually faster, and then 3% was the average of that variation. There was a lot of variation (expected, because they were measuring different things) but nearly all of them had TSO being either neutral or slower. Looking at their benchmark graphs, I see two (out of dozens) where TSO was faster.
As far as being close to the true value, these results suggest there is no single true value, as it depends on the workload. No surprise there.