[Zope-DB] Re: SQL Relay

Matthew T. Kromer matt at zope.com
Fri Oct 31 16:06:19 EST 2003

Keep in mind that there is a hidden limiter in Zope, being the number 
of ZODB threads available.  This thread count is different than -t, and 
you can't get concurrent threads needing ZODB objects higher than the 
number of ZODB threads.  If you have REALLY LONG running queries, you 
usually have to bump up both -t and the number of ZODB threads.  Newer 
zopes may do this with -t, I can't recall; it used to be you had to go 
in and modify the default storage mount parameters.

On Friday, October 31, 2003, at 12:14 PM, Kent Hoxsey wrote:

>> So if I understand you correctly, both queries are doing SELECTs only 
>> ?!
> Correct. There is no indication that the blocking we see in Zope is 
> due to any
> sort of database issue. Intuitively, the 3-tier design we are using, 
> with Zope
> in as the tier 2 appserver, should allow us to continue to serve 
> content pages
> even while the database server processes long-running sql or complex 
> analytics.
>>> To re-state the problem: Zope will not serve an index_html request 
>>> once I start
>>> a second ZSQL method running.
>> I've never seen such a problem with Zope before
> I believe this is probably because very few people have any need to 
> struggle with
> such slow and ineffective SQL as I am saddled with. The problem is 
> much harder
> to see when executing efficient SQL, since the ZSQL method returns 
> quickly enough
> that the blocking of new requests appears to simply be a load factor.
>> so this must
>> be related to the settings on your database or the database interface
>> you are using, e.g. there might be calls to C APIs that the underlying
>> Oracle interface blocks when running the query and that the interface
>> does not release the global Python interpreter lock for.
> That was my initial theory, prior to talking with Umberto. I now 
> believe
> this hypothesis has been disproven. First fact: Umberto experienced the
> same behavior using Firebird, a database with no commonality with 
> Oracle
> whatsoever. Second fact (or rather, observation): if the underlying 
> libraries
> were causing the blocking, I would expect the behavior to be 
> thread-specific.
> If that were the case, I would expect to be able to process more 
> concurrent
> queries by increasing the number of threads running in Zope. This is 
> not
> the case.
>> I'd suggest you connect to the Zope process using gdb or a
>> similar debugger and check where the threads are spending all
>> their time.
> That sounds like an excellent idea. I believe that is what I will do 
> today.
>> If there's something we could do in the mxODBC Zope DA to help
>> prevent the problem, we'd be most happy to do so.
> Cool. If it's DA related, that would resolve a number of issues for 
> me, and
> would probably help Umberto as well.
> Kent
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