* [pve-devel] [PATCH common] tools: file_set_contents: use syswrite instead of print
@ 2024-09-24 14:33 Filip Schauer
2024-09-25 8:39 ` Dietmar Maurer
2024-09-25 14:37 ` Dominik Csapak
0 siblings, 2 replies; 5+ messages in thread
From: Filip Schauer @ 2024-09-24 14:33 UTC (permalink / raw)
To: pve-devel
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
data 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
[1] https://lists.proxmox.com/pipermail/pve-devel/2024-September/065396.html
Signed-off-by: Filip Schauer <f.schauer@proxmox.com>
---
src/PVE/Tools.pm | 13 ++++++++++---
1 file changed, 10 insertions(+), 3 deletions(-)
diff --git a/src/PVE/Tools.pm b/src/PVE/Tools.pm
index bd305bd..0edf166 100644
--- a/src/PVE/Tools.pm
+++ b/src/PVE/Tools.pm
@@ -285,10 +285,17 @@ sub file_set_contents {
}
die "unable to open file '$tmpname' - $!\n" if !$fh;
- binmode($fh, ":encoding(UTF-8)") if $force_utf8;
+ $data = encode("utf8", $data) if $force_utf8;
- die "unable to write '$tmpname' - $!\n" unless print $fh $data;
- die "closing file '$tmpname' failed - $!\n" unless close $fh;
+ my $offset = 0;
+ my $len = length($data);
+
+ while ($offset < $len) {
+ $offset += syswrite($fh, $data, $len - $offset, $offset)
+ or die "unable to write '$tmpname' - $!\n";
+ }
+
+ close $fh or die "closing file '$tmpname' failed - $!\n";
};
my $err = $@;
--
2.39.2
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^ permalink raw reply [flat|nested] 5+ messages in thread* Re: [pve-devel] [PATCH common] tools: file_set_contents: use syswrite instead of print
2024-09-24 14:33 [pve-devel] [PATCH common] tools: file_set_contents: use syswrite instead of print Filip Schauer
@ 2024-09-25 8:39 ` Dietmar Maurer
2024-09-25 12:54 ` Filip Schauer
2024-09-25 14:37 ` Dominik Csapak
1 sibling, 1 reply; 5+ messages in thread
From: Dietmar Maurer @ 2024-09-25 8:39 UTC (permalink / raw)
To: Proxmox VE development discussion, Filip Schauer
Please can you add the column showing write amplification using dd instead of file_set_contents, so that we can also see the minimal write amplf. from sqlite.
> 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
> data 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
>
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^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: [pve-devel] [PATCH common] tools: file_set_contents: use syswrite instead of print
2024-09-25 8:39 ` Dietmar Maurer
@ 2024-09-25 12:54 ` Filip Schauer
0 siblings, 0 replies; 5+ messages in thread
From: Filip Schauer @ 2024-09-25 12:54 UTC (permalink / raw)
To: Dietmar Maurer, Proxmox VE development discussion
I am appending the following to the commit message:
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
On 25/09/2024 10:39, Dietmar Maurer wrote:
> Please can you add the column showing write amplification using dd instead of file_set_contents, so that we can also see the minimal write amplf. from sqlite.
>
>> 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
>> data 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
>>
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^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: [pve-devel] [PATCH common] tools: file_set_contents: use syswrite instead of print
2024-09-24 14:33 [pve-devel] [PATCH common] tools: file_set_contents: use syswrite instead of print Filip Schauer
2024-09-25 8:39 ` Dietmar Maurer
@ 2024-09-25 14:37 ` Dominik Csapak
2024-09-30 11:38 ` Filip Schauer
1 sibling, 1 reply; 5+ messages in thread
From: Dominik Csapak @ 2024-09-25 14:37 UTC (permalink / raw)
To: Proxmox VE development discussion, Filip Schauer
hi,
as we already talked off list, this may be an issue when we do have a string
that is marked as utf8 (e.g. api parameters that contain codepoints > 127)
if such a parameter reach the syswrite it will fail, so we should probably
use is_utf8 (or something better?) to detect if a string is utf8 and encode
it before passing it to syswrite.
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^ permalink raw reply [flat|nested] 5+ messages in thread
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2024-09-24 14:33 [pve-devel] [PATCH common] tools: file_set_contents: use syswrite instead of print Filip Schauer
2024-09-25 8:39 ` Dietmar Maurer
2024-09-25 12:54 ` Filip Schauer
2024-09-25 14:37 ` Dominik Csapak
2024-09-30 11:38 ` Filip Schauer
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