Comment 47 for bug 596327

Version: 1.0.0-rc1 (using KDE 4.3.4)
OS: Linux
Installed from: Ubuntu Packages

I had used 1.0.0-beta5 to set the ratings on thousands on images, but
when I checked the JPEG files, the metadata had not been updated in
most of them. In 1.0.0-rc1, if I change the metadata on a single image,
it seems to be written OK, so I don't know if this is a problem that
has been fixed.

Anyway, the issue now is that I have thousands of image files that are
missing their metadata. I got into this situation a year or two ago and
sync'd the image metadata to the database--unaware that the metadata was
missing--and lost thousands of tags. In order to fix the metadata, I'm
going through each album, checking that the metadata (from the database)
looks OK, selecting the images in the album, and then clicking the menu
item "Image" -> "Write Metadata to Selected Images".

This works fine; the metadata is updated correctly in the files. However,
if there are a large number of selected image files, the process becomes
extremely slow. There appears to be a non-linear relationship between the
number of files and the time it takes to write the metadata in RC1:

  39 files -> 2 seconds
  71 files -> 4 seconds
  87 files -> 9 seconds
  120 files -> 43 seconds
  145 files -> 38 seconds
  155 files -> 44 seconds
  276 files -> 132 seconds
  484 files -> 312 seconds
  1,200 files -> I went for lunch.

Throughout this time, digiKam uses about 70-90% CPU.

I have turned on the option to update the file modification time when
the metadata is written. This will make it easier for me to spot when
digiKam is not updating the files properly in the future. Turning off
this option makes a big difference. For example, I updated 120 files in
43 seconds and the same operation took 38 seconds when I simply repeated
it immediately, but when I turned off the file modification time update,
the third run took 15 seconds. Similarly, the album with 484 images took
312 seconds when file modification time stamps were updated and 87 seconds
when the time stamps were not updated.

Yesterday, when I was not really timing things, the end of the 1,200+
file update seemed to hang. However, when I went to the console and
looked at the timestamps on the files, I could see that about five files
would have their time stamps updated and then there would be a long wait
(maybe 30-60 seconds) with no apparent changes and then five more files
would be updated and then nothing for another while, and so on until it
eventually finished.

I can see about 30 images in the thumbnail view at one time. When I
select all images in an album of, say, 100 files and update the metadata,
the progress bar quickly hits about 20-30% and then pauses while all of
the thumbnail images disappear and, one-by-one, slowly appear again.
After that the progress bar slowly moves on to 100%.

Is digiKam re-scanning an album each time it detects a file time stamp
change, even when it is making those changes itself on another thread?
That would explain the kind of behaviour I see. Is this likely to have
a big effect on the performance of other things like batch processing?