At 08:24 AM 9/25/2009, Dan Nelson wrote:
In the last episode (Sep 25), Bennett Haselton said:
> I have a script that runs several times in the evening, and on
each run it
> adds several thousand entries to a table.
>
> On the first run, it adds the entries rather slowly. But then on all
> subsequent runs (usually about a minute or two later), the many
inserts go
> a lot faster. This is true regardless of how many entries are added by
> each run -- whether the first and second run both add 50,000 or
the first
> and second run both add 10,000, the first run goes slowly and the
second
> one goes fast. But by the following evening, the first run is back to
> going slowly again.
>
> It's as if in the minute or two following the first run of the script,
> MySQL catches its breath and realizes, hey, that table is getting
a lot of
> entries added to it, so it waves some magic dust so that the next
time I
> add a lot of entries, it goes a lot faster. (Hope I'm not losing
anybody
> with the technical terminology here.) Then by the next evening the
> optimization parameter has exp^W^W^W^W the fairy dust has worn off.
More likely, this is a relatively unused table, and the first batch of
inserts pulls most of the index and some of the table data into RAM, which
makes for much faster lookups on the next run. What do top and
iostat stats
show on both runs? I'd expect heavy disk usage and little CPU on
the first
run, and light disk and heavier CPU usage on the second.
That's interesting, I can look at that next time I try it. But if
that's the case, wouldn't the first run go slowly at first, but then
pick up speed once all of the indexes etc. have been pulled into
memory? Because that's not what I'm seeing -- if I insert 50,000 in
the first run, it's slow all the way through, but then the second
50,000 get inserted quickly.
-Bennett
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