Re: [PATCH] perf bench: Add atomic CAS benchmark

[Changbin Du]

Hello,
On Mon, Oct 05, 2026 at 03:30:46PM -0700, Namhyung Kim wrote:
> Hello,
> 
> On Wed, Sep 30, 2026 at 05:16:17PM +0800, Changbin Du wrote:
> > Add a new 'atomic' collection to perf bench for benchmarking
> > compare-and-swap (CAS) atomic operations with multi-threaded
> > contention testing.
> > 
> > The benchmark tests __atomic_compare_exchange_n operations
> > with configurable thread count and iteration count to measure
> > atomic contention effects.
> > 
> > Why this benchmark is needed:
> > - CAS operations are fundamental to lock-free algorithms and data
> >   structures. Understanding their performance characteristics under
> >   contention is critical for designing high-performance concurrent
> >   applications.
> > - The benchmark helps identify atomic operation latency and
> >   scalability issues across different thread counts, revealing
> >   contention patterns that are not visible in single-threaded tests.
> > - Useful for evaluating atomic implementation quality on different
> >   architectures and for regression testing after changes to atomic
> >   primitives or memory ordering.
> > 
> > Measurement methodology:
> > - Each thread starts a private timer (clock_gettime CLOCK_MONOTONIC)
> >   after synchronizing on a pthread_barrier, ensuring all threads
> >   begin simultaneously.
> > - Each thread performs a hot loop of atomic compare-and-swap on a
> >   shared u64 counter, incrementing from 0 to iterations.
> > - The shared counter is cache-line aligned (64 bytes) to isolate
> >   contention to the target cache line and avoid false sharing.
> > - The wall-clock time is measured as the max of all per-thread
> >   runtimes (the time for the slowest thread to finish).
> > - The first repeat is excluded from statistics as a warmup phase
> >   to avoid cache-cold effects.
> > Example usage:
> >   $ perf bench atomic cas --threads 2
> >   # Running 'atomic/cas' benchmark:
> > 
> >     Threads: 2, iterations/thread: 100000000, repeats: 10 (warmup: 1)
> >     Avg wall-clock time: 7365.480 msec  (stddev 66.014 msec)
> >     Total ops: 200,000,000
> >     Throughput total: 27,153,697 ops/sec
> >     Per-thread times and throughput (last repeat):
> >       fastest:  7510.031 msec  (13315525 ops/sec)
> >       slowest:  7581.678 msec  (13189692 ops/sec)
> >       avg:      7545.854 msec  (13252310 ops/sec)
> > 
> > Output fields explained:
> > - Threads: number of contending threads
> > - iterations/thread: CAS operations each thread performs
> > - repeats: number of test runs (first is warmup)
> > - Avg wall-clock time: mean time for all threads to complete
> > - stddev: standard deviation across repeats
> > - Total ops: threads x iterations/thread
> > - Throughput total: aggregate ops/sec across all threads
> > - Per-thread times: fastest/slowest/avg thread completion time
> > - Per-thread throughput: per-thread ops/sec (shows scheduling imbalance)
> 
> Thanks for the contribution!  I think it's very useful.
> Just a few suggestions.
> 
> 1. it'd be nice to add simple atomic_inc benchmark too.
> 2. it'd be nice to have an option to try other ordering requirements
>    than "relaxed".
> 
> Thanks,
> Namhyung

Thanks for the review!
1. Done in v2. The collection now provides atomic inc alongside
cas, measuring contended __atomic_fetch_add() throughput with the
same skeleton (barrier-synchronized start, wall-clock taken from the
slowest thread, warmup repeat). Note that its ops/sec is not
instruction-level comparable with cas — cas counts successful
compare-and-swaps only, not the retries and loads in between — which
the documentation now points out.

2. Adding memory-order variants is not necessary because ordering
has no semantic role in this benchmark. Memory ordering exists to
constrain the visibility of other memory operations relative to an
atomic access; its cost and effect only become meaningful in an
algorithm with additional accesses to order — for example, ordering
the initialization stores of a new node before publishing a pointer
with a release CAS. This benchmark, however, measures a single shared
counter with no other memory operations in the loop, so a stronger
ordering would not order anything of consequence: it would only
measure the marginal cost of the stronger instruction itself. Those
numbers would say nothing about how orderings behave in real
workloads, and would therefore add a configuration knob that invites
misleading comparisons rather than useful measurement.