Re: [PATCH v2] mm/damon/ops-common: optimize damon_hot_score() using ilog2()

[SeongJae Park <sj@kernel.org>]

Hello Liew,


When you send a new version of patches, please send those as new thread, rather
than as a reply to the previous version.

On Sat, 21 Mar 2026 03:20:20 +0800 Liew Rui Yan <aethernet65535@gmail.com> wrote:

> The current implementation of damon_hot_score() uses a manual for-loop
> to calculate the value of 'age_in_log'. This can be efficiently replaced
> by ilog2(), which is semantically more appropriate for calculating the
> logarithmic value of age.
> 
> In a simulated-kernel-module performance test with 10,000,000 iterations,
> this optimization showed a significant reduction in latency (average
> latency reduced from ~12ns to ~1ns).
> 
> Test results from the simulated-kernel-module:
> - ilog2:
>     DAMON Perf Test: Starting 10000000 iterations
>     =============================================
>      Total Iterations : 10000000
>      Average Latency  : 1 ns
>      P95 Latency      : 41 ns
>      P99 Latency      : 41 ns
>     ---------------------------------------------
>      Range (ns)      | Count        | Percent
>     ---------------------------------------------
>      0-19            | 0            |      0%
>      20-39           | 2625000      |     26%
>      40-59           | 7374000      |     73%
>      60-79           | 0            |      0%
>      80-99           | 0            |      0%
>      100+            | 1000         |      0%
>     =============================================
> 
> - for-loop:
>     DAMON Perf Test: Starting 10000000 iterations
>     =============================================
>      Total Iterations : 10000000
>      Average Latency  : 12 ns
>      P95 Latency      : 51 ns
>      P99 Latency      : 60 ns
>     ---------------------------------------------
>      Range (ns)      | Count        | Percent
>     ---------------------------------------------
>      0-19            | 0            |      0%
>      20-39           | 0            |      0%
>      40-59           | 9862000      |     98%
>      60-79           | 135000       |      1%
>      80-99           | 1000         |      0%
>      100+            | 2000         |      0%
>     =============================================

Nice!

> 
> Full raw benchmark results can be found at [1].
> 
> [1] https://github.com/aethernet65535/damon-hot-score-fls-optimize/tree/master/result-raw
> 
> Signed-off-by: Liew Rui Yan <aethernet65535@gmail.com>

Reviewed-by: SeongJae Park <sj@kernel.org>

> ---
> Changes from v1:
> - Replace fls() with ilog2() per SeongJae Park's suggestion for better
>   semantic clarity.
> - Move performance benchmark results into the commit message and add 
>   comparison between for-loop and ilog2.

As I mentioned at the beginning, please send new versions as new thread from
the next time.  And, in the case, consider adding a link to the previous
versions.

[1] https://docs.kernel.org/process/submitting-patches.html#commentary


Thanks,
SJ

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