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Entry  17 Jan 2011, Andreas Suter, Bug Report, Problems with midas history SVN 4936 
    Reply  13 Feb 2011, Lee Pool, Bug Report, Problems with midas history SVN 4936 
       Reply  16 Feb 2011, Konstantin Olchanski, Bug Report, Problems with midas history SVN 4936 
          Reply  16 Feb 2011, Lee Pool, Bug Report, Problems with midas history SVN 4936 
             Reply  17 Feb 2011, Stefan Ritt, Bug Report, Problems with midas history SVN 4936 
    Reply  16 Feb 2011, Konstantin Olchanski, Bug Report, Problems with midas history SVN 4936 
       Reply  16 Feb 2011, Konstantin Olchanski, Bug Report, Problems with midas history SVN 4936 
Message ID: 752     Entry time: 17 Feb 2011     In reply to: 751
Author: Stefan Ritt 
Topic: Bug Report 
Subject: Problems with midas history SVN 4936 
> uhm, mine might be completely unrelated to this, but it just so happened that the rev.
> 4936 was one that was used in a recent experiment, in which there was complaints about
> the responsiveness of the history plots. The history plots would take up to 30 seconds
> to respond, if the run was about 30-40 minutes old. When the run is about  < 10 minutes
> old , the history plot was responsive to within 1-2 seconds.

Ah, that rings a bell. How big are your history files on disk? How much RAM do you have?

What I see in our experiment is that linux buffers everything I write to disk in a cache 
located in RAM. But this cache is limited, so after a certain time it's overwritten. Now 
this is handled by the OS, the application (mlogger in this case) has no influence on it. 
Let's say you write 5 MB/minute of history, and your cache is 50 MB large. Then after 10 
minutes you can still read the history data from the RAM cache which is ~10x faster than 
your disk. But your older history data (30-40 min) is flushed out of the cache and has to be 
re-read from disk. A typical symptom of this is that the first time you display this it 
takes maybe 30 seconds, but if you do a "reload" of your page it goes much faster. In that 
case the contents is cached again in RAM. If you observe this, you can almost be certain 
that you see th "too small RAM cache" problem. In that case just add RAM and things should 
run better (I use 16 GByte in my machine).

Best regards,

  Stefan
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