On May 10, 2007, at 11:08 PM, Juri Wichanow wrote:
For "create index.." in large database : "pragma
default_cache_size = 2000;"
For "select ..." -- "pragma default_cache_size = 12000000;"
Hmm, quite interesting.
I would like to share my naive observations, which led me to believe
the cache_size was important (although I am now beginning to doubt
this, see below).
First we run the indexing on server 1 - I am essentially the only
user, but we only have 2GB of ram. I see (this is using top as is
everything else I say about performance) that very quickly, the
memory usage caps out and the cpu usage drops. My conclusion - an I/O
bottleneck. So I increase cache_size and surely - the program runs
longer before it seems to bottleneck. So I switch to another server
(about the same speed, 16GB ram, unknown I/O performance compared to
the first one), and increase the cache_size dramatically,
hypothesizing that I could just have the entire db in ram (I know
think I understand that I will still have to do I/O when writing the
index). And surely I see no drop in CPU usage over the five days the
program runs before the server gets rebooted. At this point
(yesterday) I was convinced that the new server was dramatically
better. Nevertheless I had never terminated my old runs on server 1.
And lo and behold, yesterday I accidently discovered they were
finished in around 7-8 days. This is something that makes no sense to
me, but it could have something to do with how top measures cpu usage
or whatever. Or I could have made a mistaken observation.
Well, I am curious now, so I am going to time this carefully on
various servers (and report my results back to the list of course).
Kasper
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