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From Miguel González Castaños <miguel_3_gonza...@yahoo.es>
Subject Re: Tomcat pausing and no java process
Date Tue, 12 Jun 2012 10:05:25 GMT
On 07/05/2012 23:13, Christopher Schultz wrote:
> -----BEGIN PGP SIGNED MESSAGE-----
> Hash: SHA1
>
> Miguel,
>
> On 5/5/12 5:41 AM, Miguel González Castaños wrote:
>>>> Yes, I have pinpointed several queries that took quite long
>>>> and specially subqueries. I forwarded this info to the
>>>> developer but she said it was alright. More than a second to
>>>> run a query seems to be a lot to me.
>>> That depends on what the query does. If it checks permissions to
>>> login, that's insane and you should get a new programmer. If it's
>>> a reporting query that gathers lots of data then 1 second might
>>> not be quite so far-fetched.
>> I have checked directly the slow query log to pinpoint the real
>> queries. I found many subqueries, which I believe is not a good
>> practice, am I right?
> There's nothing wrong with subqueries in general, it's just that the
> MySQL query optimizer appears to be particularly stupid when it comes
> to them, and you often end up with *DEPENDENT* subqueries which
> essentially issue N queries instead of only 2 queries (if the subquery
> for instance were to be independent, it could be evaluated first and
> then essentially substituted in the outer query). Only EXPLAIN can
> tell you what will happen with each query. Remember to use live data
> with EXPLAIN because the query optimizer is very sensitive to the data
> it will be scanning... if you have a test table with 5 records in it,
> you're going to get worthless EXPLAIN results.
Sorry I didn't reply to your email, but somehow it got lost in my inbox.

Well, in the end my client managed to get the developer onboard again 
and to rewrite the subqueries since they were not using any index at 
all. Let's see how that goes.

>
> Thread dumps can be very helpful as well. You might also want to look 
> at running 'sar' all the time to get CPU/IO/etc sampling to see if you 
> have any other kind of resource shortcoming. 

I have just installed sar yesterday and I'm downloading the reports 
through ksar remotely through SSH. Nice tool BTW.

Tomorrow we are probably getting many clients connecting to the webapp 
and we can get some conclusions.

I have just realized the server is running a 32 bits 1.5 JDK and the OS 
is a 64 bits linux machine. I'm wondering if using a JVM 1.7 for 64 bits 
could give us more room to have more people connected simultaneously.

Miguel







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