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

[Dominik Csapak <d.csapak@proxmox.com>]

On 10/14/24 10:42, Thomas Lamprecht wrote:
> 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@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!
> 

hi since you applied this, we probably also want to apply the pmxcfs patch?
(since without that, this patch does not really have an effect)

should i send a new version (as non rfc) with a better commit message for that
or is that not necessary from your POV ?


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