[KPhotoAlbum] More on thumbnails

[Robert Krawitz]

On Fri, 18 May 2018 06:58:33 +0200, Tobias Leupold wrote:
> Nice work :-) Don't forget to create a branch and push your changes
> from time to time!

What are the testing etc. expectations?

> Am Freitag, 18. Mai 2018, 03:20:31 CEST schrieb Robert Krawitz:
>> There are some really serious problems in the thumbnail cache code:
>> 
>> 1) It misuses timers.  Timers can only be started and stopped in the
>>    same thread.  Even putting them all in the same function is not
>>    good enough, if that function can be called from different threads;
>>    with the multi-threaded nature of thumbnail building, calling
>>    ThumbnailCache::save() directly means it can be called from
>>    multiple threads.
>> 
>>    The practical impact is that the 1-second timer for thumbnail
>>    saving accomplished nothing, because the timer was seldom getting
>>    started (or stopped).
>> 
>>    The solution is to never call save() directly; always call it
>>    indirectly through emitting a signal.
>> 
>> 2) The thumbnail cache itself is saved very inefficiently; it's
>>    written in full to a temporary file, which is then copied (yes,
>>    copied, not renamed) to the real file.  Even with a big database,
>>    such as my own (270,303 images) that's probably not killing me with
>>    I/O, but it's a lot of work to be doing every 100 images.
>> 
>>    I'm changing it to do an incremental save where possible.  That
>>    technically violates the intent of the file format, which stores
>>    the most recently written file and offset as well as the image
>>    count, but those can be reconstituted on cache reload easily
>>    enough.
>> 
>>    Also, in order to do that, I had to add an additional list of
>>    unsaved files (auto-save list).  And there's no good reason for the
>>    data structure to be a QMap; a QHash is faster, and ordering
>>    doesn't matter in the cache.
>> 
>> 3) The save() routine wasn't protected by a mutex; with multiple
>>    threads processing thumbnails, I'm not sure how we didn't get a lot
>>    of data corruption in the index cache.
>> 
>> I'm working on all of these problems.

-- 
Robert Krawitz                                     <rlk at alum.mit.edu>

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