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I have a server with 64 GiB RAM running 32 bit Debian GNU/Linux v7.9:

# uname -a
Linux srv1 3.2.0-4-686-pae #1 SMP Debian 3.2.68-1+deb7u5 i686 GNU/Linux
# free -m
             total       used       free     shared    buffers     cached
Mem:         64203       8352      55850          0        134       5861
-/+ buffers/cache:       2356      61847
Swap:         8582          0       8582

The problem is that the memory stays unused while it can be used for caching and thus improving system performance.

I have another server with the same hardware which runs 64 bit Debian GNU/Linux v8.1 and does NOT have such problem:

# uname -a
Linux srv2 3.16.0-4-amd64 #1 SMP Debian 3.16.7-ckt11-1 (2015-05-24) x86_64 GNU/Linux
# free -m
         total       used       free     shared    buffers     cached
Mem:         64513      62750       1763         45      10235      42048
-/+ buffers/cache:      10466      54047
Swap:         8582         39       8543

The first server is actually busier than the second, so the problem is not that it does not have enough data to cache. Also the first server is not that busy in order the caches to be expired very quickly due to high memory demands from the userspace processes.

Can you please give me an idea why the memory on the first server stays unused ?

Some additional notes:

  • The servers run stock Debian kernels
  • I've not done custom /proc/sys/vm modifications
  • The servers are used for the same purpose and run the same software
  • The problem is also present on more than one server
  • The biggest cache size I've seen on these servers is about 7-8 GiB
Delian Krustev
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    Why on earth are you running a 32-bit kernel?! – Michael Hampton Nov 23 '15 at 13:37
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    To point in the same direction as Michael but maybe a little bit more helpful: Are there any reasons why you don't use a 64-bit kernel on the first machine? This would solve all your problems without any complicated custom configurations. PAE is possible BUT one single process can only use up to 4GB RAM - maybe there lies your problem(?) – Cpt.Bloodner Nov 23 '15 at 14:05
  • 4 GiB per process is perfectly fine - the system does not run software which requires more. As you can see the total amount of used memory by userspace is just over 2 GB. And I don't want to go in the direction of arguing of 32bits vs 64bits. The question is about 32 bits. – Delian Krustev Nov 23 '15 at 21:51

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