[pve-devel] applied: [PATCH v2 common] tools: file_set_contents: use syswrite instead of print

[Thomas Lamprecht]

Am 30/09/2024 um 13:40 schrieb Filip Schauer:
> The use of `print` can be inefficient for writing larger files due to
> its default buffering in 8 KiB blocks.
> 
> This is especially problematic on `pmxcfs` where files are written in
> 4 KiB blocks due to the defaults of `libfuse2`. This leads to
> significant write amplification on files larger than 4 KiB.
> 
> Patch (fix #5728: pmxcfs: allow bigger writes than 4k for fuse) [1]
> addresses this by enabling `big_writes`, allowing up to 128 KiB blocks.
> But due to the use of `print` in `file_set_contents`, writes are still
> only buffered in 8 KiB blocks.
> 
> To further address this, this commit switches to using `syswrite`
> instead of `print` to mitigate the block size limit imposed by `print`.
> Combined with patch [1], file writes to `/etc/pve/` are now buffered in
> 128 KiB blocks.
> 
> The table below illustrates the drastic reduction in write
> amplification when writing files of different sizes to `/etc/pve/` using
> `file_set_contents`:
> 
>            print                big_writes+print     big_writes+syswrite
> file size  written     amplif.  written     amplif.  written    amplif.
>     1 KiB      48 KiB     48.0      45 KiB     45.0     41 KiB     41.0
>     2 KiB      48 KiB     24.0      45 KiB     22.5     62 KiB     31.0
>     4 KiB      82 KiB     20.5      80 KiB     20.0     73 KiB     18.3
>     8 KiB     121 KiB     15.1      90 KiB     11.3     89 KiB     11.1
>    16 KiB     217 KiB     13.6     146 KiB      9.1    113 KiB      7.1
>    32 KiB     506 KiB     15.8     314 KiB      9.8    158 KiB      4.9
>    64 KiB    1472 KiB     23.0     826 KiB     12.9    259 KiB      4.0
>   128 KiB    5585 KiB     43.6    3765 KiB     29.4    452 KiB      3.5
>   256 KiB   20424 KiB     79.8   10743 KiB     42.0   2351 KiB      9.2
>   512 KiB   86715 KiB    169.4   43650 KiB     85.3   3204 KiB      6.3
>  1024 KiB  369568 KiB    360.9  187496 KiB    183.1  15845 KiB     15.5
> 
> Since `file_set_contents` also performs a `rename` after writing, the
> following table shows the results when the file is written without
> renaming it afterwards:
> 
>            print                big_writes+print     big_writes+syswrite
> file size  written     amplif.  written     amplif.  written     amplif.
>     1 KiB      29 KiB     29.0      29 KiB     29.0     25 KiB      25.0
>     2 KiB      29 KiB     14.5      30 KiB     15.0     25 KiB      12.5
>     4 KiB      37 KiB      9.3      44 KiB     11.0     41 KiB      10.3
>     8 KiB      61 KiB      7.6      45 KiB      5.6     45 KiB       5.6
>    16 KiB     143 KiB      8.9      86 KiB      5.4     57 KiB       3.6
>    32 KiB     396 KiB     12.4     225 KiB      7.0     69 KiB       2.2
>    64 KiB    1281 KiB     20.0     673 KiB     10.5    105 KiB       1.6
>   128 KiB    4789 KiB     37.4    3478 KiB     27.2    169 KiB       1.3
>   256 KiB   18868 KiB     73.7    9976 KiB     39.0    572 KiB       2.2
>   512 KiB   79304 KiB    154.9   42714 KiB     83.4   2150 KiB       4.2
>  1024 KiB  347929 KiB    339.8  182483 KiB    178.2  11133 KiB      10.9
> 
> [1] https://lists.proxmox.com/pipermail/pve-devel/2024-September/065396.html
> 
> Signed-off-by: Filip Schauer <f.schauer at proxmox.com>
> ---
> Changes since v1:
> * Add benchmark results without rename to commit message
> * Fix "Wide character in syswrite" error by first encoding $data with print
> 
>  src/PVE/Tools.pm | 21 ++++++++++++++++++---
>  1 file changed, 18 insertions(+), 3 deletions(-)
> 
>

applied, and many thanks for the detailed benchmarks!