Re: [PATCH] fs.h: Optimize file struct to prevent false sharing

[Christian Brauner]

On Thu, Jun 01, 2023 at 05:24:00AM -0400, chenzhiyin wrote:
> In the syscall test of UnixBench, performance regression occurred due
> to false sharing.
> 
> The lock and atomic members, including file::f_lock, file::f_count and
> file::f_pos_lock are highly contended and frequently updated in the
> high-concurrency test scenarios. perf c2c indentified one affected
> read access, file::f_op.
> To prevent false sharing, the layout of file struct is changed as
> following
> (A) f_lock, f_count and f_pos_lock are put together to share the same
> cache line.
> (B) The read mostly members, including f_path, f_inode, f_op are put
> into a separate cache line.
> (C) f_mode is put together with f_count, since they are used frequently
>  at the same time.
> Due to '__randomize_layout' attribute of file struct, the updated layout
> only can be effective when CONFIG_RANDSTRUCT_NONE is 'y'.
> 
> The optimization has been validated in the syscall test of UnixBench.
> performance gain is 30~50%. Furthermore, to confirm the optimization
> effectiveness on the other codes path, the results of fsdisk, fsbuffer
> and fstime are also shown.
> 
> Here are the detailed test results of unixbench.
> 
> Command: numactl -C 3-18 ./Run -c 16 syscall fsbuffer fstime fsdisk
> 
> Without Patch
> ------------------------------------------------------------------------
> File Copy 1024 bufsize 2000 maxblocks   875052.1 KBps  (30.0 s, 2 samples)
> File Copy 256 bufsize 500 maxblocks     235484.0 KBps  (30.0 s, 2 samples)
> File Copy 4096 bufsize 8000 maxblocks  2815153.5 KBps  (30.0 s, 2 samples)
> System Call Overhead                   5772268.3 lps   (10.0 s, 7 samples)
> 
> System Benchmarks Partial Index         BASELINE       RESULT    INDEX
> File Copy 1024 bufsize 2000 maxblocks     3960.0     875052.1   2209.7
> File Copy 256 bufsize 500 maxblocks       1655.0     235484.0   1422.9
> File Copy 4096 bufsize 8000 maxblocks     5800.0    2815153.5   4853.7
> System Call Overhead                     15000.0    5772268.3   3848.2
>                                                               ========
> System Benchmarks Index Score (Partial Only)                    2768.3
> 
> With Patch
> ------------------------------------------------------------------------
> File Copy 1024 bufsize 2000 maxblocks  1009977.2 KBps  (30.0 s, 2 samples)
> File Copy 256 bufsize 500 maxblocks     264765.9 KBps  (30.0 s, 2 samples)
> File Copy 4096 bufsize 8000 maxblocks  3052236.0 KBps  (30.0 s, 2 samples)
> System Call Overhead                   8237404.4 lps   (10.0 s, 7 samples)
> 
> System Benchmarks Partial Index         BASELINE       RESULT    INDEX
> File Copy 1024 bufsize 2000 maxblocks     3960.0    1009977.2   2550.4
> File Copy 256 bufsize 500 maxblocks       1655.0     264765.9   1599.8
> File Copy 4096 bufsize 8000 maxblocks     5800.0    3052236.0   5262.5
> System Call Overhead                     15000.0    8237404.4   5491.6
>                                                               ========
> System Benchmarks Index Score (Partial Only)                    3295.3
> 
> Signed-off-by: chenzhiyin <zhiyin.chen@xxxxxxxxx>
> ---

Dave had some more concerns and perf analysis requests for this. So this
will be put on hold until these are addressed.