Django: same filesystem based caching works in Windows, not in Linux

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http://stackoverflow.com – I'm willing to cache most of the processing results for the site homepage, namely a data dict summarizing numbers of database objects (resources per category). Tried filesystem cache at low level (cache.set, cache.get), since my virtual server hasn't much memory. This works well on the Windows development server, but doesn't work in production on the Ubuntu server: I can see that the contents of the cache directory are continually updated, but they are not used. How can I identify the problem? Why the API call "cache.get(key, None)" seems to return always None? Below is the cache configurati (HowTos)