I read the article you referred to. It seems to me that this warning (i.e.
the possibility of returning a pointer to an uninitialized object) does not
apply to the case where you are assigning to a local (i.e.. stack) variable
and not some instance variable (i.e. shared memory) and putting that value
in a Map. The example in the article was testing/assigning a static member
variable that could be pointing to an uninitialized object. There is no
possibility of this happening if the test is made on a local stack variable.
Am I incorrect in this interpretation? Or do I still miss something?
For example if the code in the article had been changed to the following,
there would have been no issue:
public static Disfunctional_singleton get_instance()
{
Disfunctional_singleton instance = the_instance;
if( instance == null )
synchronized( Disfunctional_singleton.class )
{
instance = the_instance;
if(instance == null ) {
instance = new Disfunctional_singleton();
new_instance = instance;
}
}
return instance;
}
In any case, I am rather shocked by the analysis! I am going to have to look
through all my singleton code now! But I don't think that the "double check"
lock pattern should be thrown away entirely. Thanks for the info. Regards,
--mike
-----Original Message-----
From: Vadim Gritsenko [mailto:[EMAIL PROTECTED]]
Sent: Monday, August 20, 2001 10:28 AM
To: [EMAIL PROTECTED]
Subject: RE: [C2] sitemap creation threading issue ?
According to
http://www.javaworld.com/javaworld/jw-02-2001/jw-0209-toolbox.html
this technique does not work, especially on SMP systems.
Vadim
> -----Original Message-----
> From: Michael McKibben [mailto:[EMAIL PROTECTED]]
> Sent: Monday, August 20, 2001 12:20 PM
> To: '[EMAIL PROTECTED]'
> Subject: RE: [C2] sitemap creation threading issue ?
>
>
> I briefly glanced at the patch and one thing you can do to optimize the
> syncronization is to apply the "double check" lock pattern. This will
allow
> you to remove the synchronized keyword from the method. For example:
>
> Handler sitemapHandler = (Handler)sitemaps.get(source);
> if (sitemapHandler != null) {
> ...
> }
>
> Remove synchronized from the method and change body to:
>
> Handler sitemapHandler = (Handler)sitemaps.get(source);
> if (sitemapHandler == null) {
> synchronized (sitemaps) {
> sitemapHandler = (Handler)sitemaps.get(source);
> if (sitemapHandler == null) {
> // create handler, put in the Map ...
> sitemaps.put(...)
> }
> }
> }
>
> Notice that threads only get synchronized in the case where a new handler
> has not yet been created, not every call into the method!
>
> Regards,
>
> --mike
>
> -----Original Message-----
> From: Morrison, John [mailto:[EMAIL PROTECTED]]
> Sent: Monday, August 20, 2001 5:47 AM
> To: '[EMAIL PROTECTED]'
> Subject: RE: [C2] sitemap creation threading issue ?
>
>
> Hi all, esp Marcus,
>
> I too have been running Load tests, I also noticed that 'something' was
> happening regularly to slow the response. I hadn't gotten round to
checking
> the code though (the testing machines are isolated - 0 access from/to my
dev
> station).
>
> I saw your patch and rejoiced ;) but it's taken the response of some of my
> pages from a couple of hundred milliseconds to over 5! I've not yet
looked
> at _what_ your patch did, but have you seen a similar increase in time?
>
> J.
>
> > -----Original Message-----
> > From: Carsten Ziegeler [mailto:[EMAIL PROTECTED]]
> > Sent: Monday, 20 August 2001 7:57 am
> > To: [EMAIL PROTECTED]
> > Subject: AW: [C2] sitemap creation threading issue ?
> >
> >
> > Hi Marcus,
> >
> > thanks for your patch. I applied it, please cross check :-)
> >
> > Carsten
> >
> > > -----Ursprungliche Nachricht-----
> > > Von: Marcus Crafter [mailto:[EMAIL PROTECTED]]
> > > Gesendet: Samstag, 18. August 2001 22:41
> > > An: Cocoon Developers Mailing List
> > > Betreff: [C2] sitemap creation threading issue ?
> > >
> > >
> > > Hi *,
> > >
> > > Hope all is well.
> > >
> > > Michael and I have spent the day testing our C2 application with
> > > LoadRunner and have potentially uncovered a threading
> > problem during
> > > sitemap creation.
> > >
> > > We're not experts with the code but from our understanding the
> > > following is happening, please let us know if we are
> > right/wrong:
> > >
> > > There seems to be a problem with the getHandler()
> > method, located in
> > > the sitemap Manager class (line 154). getHandler() attempts
> > > to access a
> > > sitemap handler object for each request for processing. If
> > > the handler
> > > object is not available it creates one, causing the
> > sitemap to be
> > > generated.
> > >
> > > We've noticed under load, that many handler objects are
> > created for
> > > the same sitemap. This is because getHandler() does not
> > protect the
> > > following lines:
> > >
> > > Handler sitemapHandler = (Handler)sitemaps.get(source);
> > >
> > > and
> > >
> > > sitemaps.put(source, sitemapHandler);
> > >
> > > as a critical area.
> > >
> > > If multiple concurrent threads pass through
> > getHandler() which are
> > > requests for resources from the same sitemap, the first line
> > > above will return null multiple times causing the same
> > sitemap to be
> > > compiled several times, each by individual Handler objects.
> > >
> > > This happens because sitemaps.put() executes after each
> > > sitemap handler
> > > object is created (which can take time for large sitemaps),
> > > and cannot
> > > prevent other incoming threads from waiting until it
> > adds the newly
> > > created handler object into the 'sitemaps' hashmap.
> > >
> > > When we synchronized the getHandler method to protect the
> > > getting/setting of the sitemaps hashmap, we saw that the sitemap
> > > handler object was created only once, and that the application
> > > performed much better under load. Previously the same
> > > sitemap handler
> > > object was created as many times as we had simultaneous threads
> > > make requests.
> > >
> > > Attached is a diff of the change we made. There might
> > be a better
> > > solution as the Handler class seems to be built to
> > handle this, it's
> > > just that the allocation of a new Handler objects per
> > > sitemap, defeats
> > > it's internal multi-thread logic.
> > >
> > > Any comments/thoughts/suggestions ?
> > >
> > > Cheers,
> > >
> > > Marcus
> > >
> > > --
> > > .....
> > > ,,$$$$$$$$$, Marcus Crafter
> > > ;$' '$$$$: Computer Systems Engineer
> > > $: $$$$: Open Software Associates GmbH
> > > $ o_)$$$: 82-84 Mainzer Landstrasse
> > > ;$, _/\ &&:' 60327 Frankfurt Germany
> > > ' /( &&&
> > > \_&&&&' Email : [EMAIL PROTECTED]
> > > &&&&. Business Hours : +49 69 9757 200
> > > &&&&&&&:
> > >
> >
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> >
>
>
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