From: Andi Kleen <ak@linux.intel.com>
To: Ian Rogers <irogers@google.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
Ingo Molnar <mingo@redhat.com>,
Arnaldo Carvalho de Melo <acme@kernel.org>,
Mark Rutland <mark.rutland@arm.com>,
Alexander Shishkin <alexander.shishkin@linux.intel.com>,
Jiri Olsa <jolsa@redhat.com>, Namhyung Kim <namhyung@kernel.org>,
Thomas Gleixner <tglx@linutronix.de>,
linux-kernel@vger.kernel.org,
Stephane Eranian <eranian@google.com>
Subject: Re: [PATCH] perf bench: Add benchmark of find_next_bit
Date: Fri, 24 Jul 2020 07:45:03 -0700 [thread overview]
Message-ID: <20200724144503.GD1180481@tassilo.jf.intel.com> (raw)
In-Reply-To: <20200724071959.3110510-1-irogers@google.com>
On Fri, Jul 24, 2020 at 12:19:59AM -0700, Ian Rogers wrote:
> for_each_set_bit, or similar functions like for_each_cpu, may be hot
> within the kernel. If many bits were set then one could imagine on
> Intel a "bt" instruction with every bit may be faster than the function
> call and word length find_next_bit logic. Add a benchmark to measure
> this.
> This benchmark on AMD rome and Intel skylakex shows "bt" is not a good
> option except for very small bitmaps.
Small bitmaps is a common case in the kernel (e.g. cpu bitmaps)
But the current code isn't that great for small bitmaps. It always looks horrific
when I look at PT traces or brstackinsn, especially since it was optimized
purely for code size at some point.
Probably would be better to have different implementations for
different sizes.
-Andi
next prev parent reply other threads:[~2020-07-24 14:45 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-07-24 7:19 [PATCH] perf bench: Add benchmark of find_next_bit Ian Rogers
2020-07-24 14:45 ` Andi Kleen [this message]
2020-07-24 18:13 ` Ian Rogers
2020-07-28 11:51 ` Arnaldo Carvalho de Melo
2020-07-29 19:59 ` Arnaldo Carvalho de Melo
2020-07-29 20:44 ` Arnaldo Carvalho de Melo
2020-07-29 22:03 ` Ian Rogers
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