linux-kernel - Re: [BUG?] bcachefs performance: read is way too slow when a file has no overwrite

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Message-ID: <3fcdf5b3.203d.19218ea100b.Coremail.00107082@163.com>
Date: Sun, 22 Sep 2024 16:47:25 +0800 (CST)
From: "David Wang" <00107082@....com>
To: kent.overstreet@...ux.dev
Cc: linux-bcachefs@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [BUG?] bcachefs performance: read is way too slow when a file
 has no overwrite


At 2024-09-22 16:31:48, "David Wang" <00107082@....com> wrote:
>>Hi, 
>>

>btree_io write pattern, collected from btree_node_write_endio, 
>is kind of uniform/flat distributed, not on block-friendly size
>boundaries (I think):
>	+---------+------------+
>	| sectors | percentage |
>	+---------+------------+
>	|    1    |   9.021%   |
>	|    3    |   1.440%   |
>	|    4    |   1.249%   |
>	|    2    |   1.157%   |
>	|    5    |   0.804%   |
>	|    6    |   0.409%   |
>	|    14   |   0.259%   |
>	|    15   |   0.253%   |
>	|    16   |   0.228%   |
>	|    7    |   0.226%   |
>	|    11   |   0.223%   |
>	|    10   |   0.223%   |
>	|    13   |   0.222%   |
>	|    9    |   0.213%   |
>	|    12   |   0.202%   |
>	|    41   |   0.194%   |
>	|    17   |   0.183%   |
>	|    8    |   0.182%   |
>	|    18   |   0.167%   |
>	|    20   |   0.167%   |
>	|    19   |   0.163%   |
>	|    21   |   0.160%   |
>	|   205   |   0.158%   |
>	|    22   |   0.145%   |
>	|    23   |   0.117%   |
>	|    24   |   0.093%   |
>	|    51   |   0.089%   |
>	|    25   |   0.080%   |
>	|   204   |   0.079%   |
>	+---------+------------+
>

Oops...wrong weight used to calculate percentage, it should be
+---------+------------+
| sectors | percentage |
+---------+------------+
|    1    |  45.105%   |
|    3    |   7.200%   |
|    4    |   6.244%   |
|    2    |   5.785%   |
|    5    |   4.018%   |
|    6    |   2.045%   |
|    14   |   1.296%   |
|    15   |   1.264%   |
|    16   |   1.141%   |
|    7    |   1.129%   |
|    11   |   1.117%   |
|    10   |   1.113%   |
|    13   |   1.111%   |
|    9    |   1.065%   |
|    12   |   1.011%   |
|    41   |   0.971%   |
|    17   |   0.913%   |
|    8    |   0.912%   |
|    18   |   0.836%   |
|    20   |   0.835%   |
|    19   |   0.812%   |
|    21   |   0.799%   |
|   205   |   0.791%   |
|    22   |   0.724%   |
|    23   |   0.587%   |
|    24   |   0.465%   |
|    51   |   0.443%   |
|    25   |   0.398%   |
|   204   |   0.396%   |
+---------+------------+


David