Negative caching

Steve Lamb grey at
Fri Jun 19 10:24:19 UTC 2009

Hal Burgiss wrote:
> I think I've figured out why recent Ubuntu has such abysmal performance. Very
> cleverly over a period of time, it takes all the free memory and uses it to
> store useless junk that you will never ever use. And then holds on to the
> useless stuff as long as it possibly can. And then you are forced to
> work out of swap.

     Uhm... no.  Simple as that.  Whatever your problem is has no relation to 
caching or buffers.  At least if it is you certainly have quite a further 
distance to go to prove it than what you've presented in your message.  The 
cached pages, for all intents an purposes, can be considered free.  The 
difference between a cached page and a free page is just a couple of bits in a 
memory map.

     Furthermore I can counter your anecdotal evidence of long running 
machines = swap death with two of my own:

{grey at teleute:~} free
              total       used       free     shared    buffers     cached
Mem:       2059048    2006968      52080          0      70812    1671312
-/+ buffers/cache:     264844    1794204
Swap:      2654200         92    2654108
{grey at teleute:~} uptime
  03:13:48 up 61 days,  4:49,  1 user,  load average: 0.01, 0.00, 0.00

{grey at olethros:~} free ; uptime
              total       used       free     shared    buffers     cached
Mem:        262312     258380       3932          0      13240      41160
-/+ buffers/cache:     203980      58332
Swap:       524280     155980     368300
  06:14:26 up 102 days, 14:27,  1 user,  load average: 0.00, 0.00, 0.00

     The first one is a real machine that I use on my home network for a 
firewall, router, file server and my "long-running, remote access X app" box. 
  IE, it has been running XFCE + KTorrent for those 61 days and it has 92Kb of 
swap used and is snappy to respond whenever I connect to it.

     The second is a leased VM which runs my http, ftp, smtp, pop, imap (and 
other) public facing services.  It does so with a whopping 256Mb of memory 
(hey, $25/month, ya gets what ya pays for!) and as you can see, an astounding 
155Mb of swap as well.  Even that machine is somewhat responsive when I remote 
into it.

     Your speed settings have to the result of something else going on because 
the caching that the kernel does, if it were the cause, would be evident to 
more people with heavier usage patterns than you.  It is the most commonly 
maligned function the kernel performs.  ;)

          Steve C. Lamb         | But who can decide what they dream
        PGP Key: 1FC01004       |      and dream I do

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