Re: [C2] sitemap creation threading issue ?

[Ovidiu Predescu <ov...@cup.hp.com>]

On Mon, 20 Aug 2001 12:27:53 -0400, "Vadim Gritsenko" <va...@ionidea.com> wrote:

> According to http://www.javaworld.com/javaworld/jw-02-2001/jw-0209-toolbox.html
> this technique does not work, especially on SMP systems.

Very good article, thanks for posting a reference to it!

Greetings,
Ovidiu

> Vadim
> 
> > -----Original Message-----
> > From: Michael McKibben [mailto:mmckibben@ncube.com]
> > Sent: Monday, August 20, 2001 12:20 PM
> > To: 'cocoon-dev@xml.apache.org'
> > 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:John.Morrison@uk.experian.com]
> > Sent: Monday, August 20, 2001 5:47 AM
> > To: 'cocoon-dev@xml.apache.org'
> > 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:cziegeler@sundn.de]
> > > Sent: Monday, 20 August 2001 7:57 am
> > > To: cocoon-dev@xml.apache.org
> > > 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:crafterm@fztig938.bank.dresdner.net]
> > > > 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 : Marcus.Crafter@osa.de
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> > > >     &&&&&&&:
> > > > 
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