linux-kernel - Re: [PATCH v7 4/5] rust: add find_bit_benchmark

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Message-ID: <CACQBu=VkFT5yDuDz098L+S+tGvtGHMvm4FaZ6p3sr9VCp88jww@mail.gmail.com>
Date: Fri, 25 Apr 2025 18:17:59 +0200
From: Burak Emir <bqe@...gle.com>
To: Boqun Feng <boqun.feng@...il.com>
Cc: Yury Norov <yury.norov@...il.com>, Rasmus Villemoes <linux@...musvillemoes.dk>, 
	Viresh Kumar <viresh.kumar@...aro.org>, Miguel Ojeda <ojeda@...nel.org>, 
	Alex Gaynor <alex.gaynor@...il.com>, Gary Guo <gary@...yguo.net>, 
	Björn Roy Baron <bjorn3_gh@...tonmail.com>, 
	Benno Lossin <benno.lossin@...ton.me>, Andreas Hindborg <a.hindborg@...nel.org>, 
	Alice Ryhl <aliceryhl@...gle.com>, Trevor Gross <tmgross@...ch.edu>, rust-for-linux@...r.kernel.org, 
	linux-kernel@...r.kernel.org, Rong Xu <xur@...gle.com>
Subject: Re: [PATCH v7 4/5] rust: add find_bit_benchmark_rust module.

On Fri, Apr 25, 2025 at 3:45 PM Boqun Feng <boqun.feng@...il.com> wrote:
>
> On Fri, Apr 25, 2025 at 02:20:13PM +0200, Burak Emir wrote:
> > On Fri, Apr 25, 2025 at 12:31 AM Boqun Feng <boqun.feng@...il.com> wrote:
> > >
> > > On Thu, Apr 24, 2025 at 09:48:17AM -0700, Boqun Feng wrote:
> > > > On Thu, Apr 24, 2025 at 06:45:33PM +0200, Burak Emir wrote:
> > > > > On Wed, Apr 23, 2025 at 6:56 PM Yury Norov <yury.norov@...il.com> wrote:
> > > > > > So? Can you show your numbers?
> > > > >
> > > > > For now, I only have numbers that may not be very interesting:
> > > > >
> > > > > - for find_next_bit,  find_next_zero_bit and find_next_zero_bit (sparse):
> > > > >   22 ns/iteration in C, 32 ns/iteration in Rust.
> > > > >
> > > > > - for sparse find_next_bit (sparse):
> > > > >   60 ns/iteration in C, 70 ns/iteration in Rust.
> > > > >
> > > > > This is a VM running nested in a VM. More importantly: the C helper
> > > > > method is not inlined.
> > > > > So we are likely measuring the overhead (plus the extra bounds checking).

Alice and I discussed that it may be better to do away with the extra
bounds check.
Micro benchmark, for the upcoming v8 that has the bounds check removed
(and the test changed to >=, as requested):

[] Start testing find_bit() with random-filled bitmap
[] find_next_bit:                 3598165 ns, 164282 iterations
[] find_next_zero_bit:            3626186 ns, 163399 iterations
[] Start testing find_bit() with sparse bitmap
[] find_next_bit:                   40865 ns,    656 iterations
[] find_next_zero_bit:            7100039 ns, 327025 iterations
[] find_bit_benchmark_rust_module: Start testing find_bit() Rust with
random-filled bitmap
[] find_bit_benchmark_rust_module: next_bit:
4572086 ns, 164112 iterations
[] find_bit_benchmark_rust_module: next_zero_bit:
4582930 ns, 163569 iterations
[] find_bit_benchmark_rust_module: Start testing find_bit() Rust with
sparse bitmap
[] find_bit_benchmark_rust_module: next_bit:
42622 ns,    655 iterations
[] find_bit_benchmark_rust_module: next_zero_bit:
8835122 ns, 327026 iterations

cheers,
Burak