Re: [RFC PATCH v4 3/4] hazptr: Implement Hazard Pointers

[Boqun Feng <boqun.feng@gmail.com>]

On Thu, Dec 18, 2025 at 12:35:18PM -0500, Mathieu Desnoyers wrote:
[...]
> > Could you utilize this[1] to see a
> > comparison of the reader-side performance against RCU/SRCU?
> 
> Good point ! Let's see.
> 
> On a AMD 2x EPYC 9654 96-Core Processor with 192 cores,
> hyperthreading disabled,
> CONFIG_PREEMPT=y,
> CONFIG_PREEMPT_RCU=y,
> CONFIG_PREEMPT_HAZPTR=y.
> 
> scale_type                 ns
> -----------------------
> hazptr-smp-mb             13.1   <- this implementation
> hazptr-barrier            11.5   <- replace smp_mb() on acquire with barrier(), requires IPIs on synchronize.
> hazptr-smp-mb-hlist       12.7   <- replace per-task hp context and per-cpu overflow lists by hlist.
> rcu                       17.0

Hmm.. now looking back, how is it possible that hazptr is faster than
RCU on the reader-side? Because a grace period was happening and
triggered rcu_read_unlock_special()? This is actualy more interesting.

Regards,
Boqun

> srcu                      20.0
> srcu-fast                  1.5
> rcu-tasks                  0.0
> rcu-trace                  1.7
> refcnt                  1148.0
> rwlock                  1190.0
> rwsem                   4199.3
> lock                   41070.6
> lock-irq               46176.3
> acqrel                     1.1
> 
> So only srcu-fast, rcu-tasks, rcu-trace and a plain acqrel
> appear to beat hazptr read-side performance.
> 
> [...]
> 
[...]