https://bugs.kde.org/show_bug.cgi?id=524939
Bug ID: 524939
Summary: KIO local file copy is significantly slower with 512
KiB copy_file_range chunks
Classification: Frameworks and Libraries
Product: frameworks-kio
Version First 6.29.0
Reported In:
Platform: CachyOS
OS: Linux
Status: REPORTED
Severity: normal
Priority: NOR
Component: general
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected]
Target Milestone: ---
Description
When copying a large local file between two local HDDs using Dolphin/KIO, the
transfer speed is significantly lower than when using cp directly.
The affected file is a large UHD Blu-ray MKV (~56 GB). Both source and
destination are local filesystems:
Source: /mnt/HDD1
Destination: /mnt/HDD2
With Dolphin/KIO, the transfer speed is limited to approximately 72 MB/s,
resulting in a significantly longer copy time.
For comparison, the following command using GNU coreutils cp reaches the full
available disk-to-disk transfer speed:
strace -T -e trace=copy_file_range \
cp --reflink=auto \
/mnt/HDD1/test-file.mkv \
/mnt/HDD2/plik-test.mkv
The strace output shows that cp uses a very large copy_file_range() request:
copy_file_range(3, NULL, 4, NULL, 9223372035781033984, 0) = 56710777223
In contrast, Dolphin/KIO repeatedly calls copy_file_range() with a 512 KiB
size:
copy_file_range(44, NULL, 45, NULL, 524288, 0) = 524288
There are hundreds of these 512 KiB calls.
I tested the KIO source code and found:
static constexpr int s_maxIPCSize = 1024 * 512;
and:
copy_file_range(srcFile.handle(), nullptr,
destFile.handle(), nullptr,
s_maxIPCSize, 0);
I changed this to use an 8 MiB chunk specifically for copy_file_range():
static constexpr int s_copyFileRangeChunk = 8 * 1024 * 1024;
and:
copy_file_range(srcFile.handle(), nullptr,
destFile.handle(), nullptr,
s_copyFileRangeChunk, 0);
After rebuilding and installing the KIO file worker, Dolphin immediately
reached the full available transfer speed and the problem disappeared.
I also tested 16 MiB. Interestingly, 16 MiB was slower than 8 MiB on my
hardware, so 8 MiB appears to be a better value in this particular setup.
Expected behavior
Dolphin/KIO should provide transfer performance comparable to cp when copying
large files between local filesystems, rather than being limited by
unnecessarily small copy_file_range() requests.
Actual behavior
Large local file copies through Dolphin/KIO are limited to approximately 72
MB/s.
Version
KIO: 6.29.0
The issue is reproducible with large files copied between /mnt/HDD1 and
/mnt/HDD2.
The workaround is to increase the copy_file_range() chunk size used by the KIO
file worker from 512 KiB to 8 MiB.
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