KSharedDataCache debugging

I was approached the other day by an Amarok developer who was receiving a lot of debug output from KImageCache (which uses my KSharedDataCache). When the cache was nearly full he was starting to receive a lot of messages about being “Unable to free up memory for” each entry.

He caught up with me on IRC and I pointed him to where the error message was from (which he had already found) and then explained what the logic was supposed to be inside of that function. Unfortunately I wasn’t able to be more helpful but I did leave with some tips about how I would debug it.

Probably half the time I get reports like this that’s where the matter is left. So I was surprised when I logged into IRC the next day that the dev was still looking into it! He had taken the steps I had suggested (including having to adapt it to his installation) and was able to verify that the KSharedDataCache was not finding free pages even though there should have been room available.

This meant that either the record-keeping was corrupted somewhere (which is bad), or that there was a problem in the function to find free pages. My contribution this day was limited to more debugging suggestions (pre-filling the cache to reproduce quickly), and more advice on what some auxiliary functions were doing. Oh, and the recommendation to check cache consistency (and when to do it) to flag immediately when the problem occurs.

And the next day, he had identified the problem and made a fix! As it turned out, I wasn’t searching the last n pages of the cache (where n is the number of pages needed to hold the entry to be inserted). So this problem could only occur when the cache was nearly full, or would be nearly full after the entry is added (since we defragment the cache first in this situation).

So thanks to Sam for sticking with the debugging! The fix will be in the next routine release of kdelibs (probably first week of July).

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