A configurable cache might be ok but what Robert is showing is a highly typical performance degradation pattern for any sufficiently large Java application. Tapestry's page cache is hardly the only place where soft references are used. When your memory budget is too small, most system engineers would argue that it's far better to slow down the application than OoM, but obviously that depends on the type of application and the traffic patterns you are facing. For the consumer facing application, it's not uncommon to see peak traffic 30-100 times over the averages at least with the applications I've been involved with and I would hate to to budget all resources based on peak consumption only. On the other hand, if the number of pages on the site is small and the site is evenly in use, then sure, it'd make sense to never purge.
Kalle On Wed, Apr 1, 2015 at 3:01 PM, Howard Lewis Ship <hls...@gmail.com> wrote: > I'm feeling that Robert is making a very good case here. I could imagine a > page-level annotation to either enable or disable evication of a page > instance after a period of time ... but that can come later. I do think > that hard-caching of pages will leading to more predictable response > performance. > > On Wed, Apr 1, 2015 at 7:31 AM, Robert Schmelzer <rob...@schmelzer.cc> > wrote: > > > Hi, > > > > I now found time to sum up a short report about that topic. > > > > I summarized my results in following pdf file: > > http://www.schmelzer.cc/Downloads/Files/Tapestry-Memory-Performance.pdf > > > > The main issue is, that you are able to bring a Tapestry based system > into > > a situation where it gets slower and slower and finally event not > > responding any more, just be decreasing memory on the JVM and you DO NOT > > get any error message or OutOfMemory warning or GC overhead warning. And > > that only because the PageImpl instances are held in SoftReferences. My > > opinion is still, that this does not work as it is supposed to do and I > > keep with my example about other infrastructure. You would not expect > e.g. > > Spring beans or a hibernate configuration to get thrown away under memory > > preasure - you would expect them to fail with OutOfMemory if they are not > > able to hold their necassary static information in memory. > > > > Regards > > Robert > > > > > > > > > > Am 19.03.2015 um 17:55 schrieb Kalle Korhonen: > > > >> On Thu, Mar 19, 2015 at 12:24 AM, Robert Schmelzer <rob...@schmelzer.cc > > > >> wrote: > >> > >> Sorry, I was unprecise - my example should have referenced to the > >>> EntityManagerFactory (SessionFactoryImpl in Hibernate). You would not > >>> expect them, to throw away its cached configuration on memory > preasure. I > >>> do not either expect that from Tapestry. > >>> I cannot make our results public because of regulatory issues. I will > try > >>> to setup a show case for that and will offer a patch. This will take > me a > >>> few days. > >>> > >>> I don't think we are going to simply do away with the SoftReferences > >> without any replacements so I wouldn't even attempt at offering such a > >> patch. I just don't agree that a memory cache should be permanent > >> construct. If your object is not in a cache, you'll simply incur a cache > >> miss and re-create the object on the fly. It is not typical that a cache > >> will grow indefinitely. If you are adamant on this approach, you could > >> probably convince us to add a symbol to control the cache behavior (i.e. > >> to > >> never purge objects from it). Guava has excellent, easily configurable > >> cache implementations. > >> > >> Kalle > >> > >> > >> Robert > >>> > >>> Am 18.03.2015 um 18:19 schrieb Kalle Korhonen: > >>> > >>> On Wed, Mar 18, 2015 at 12:44 AM, Robert Schmelzer > <rob...@schmelzer.cc > >>> > > >>> > >>>> wrote: > >>>> > >>>> I do not agree with you on that point. Tapestry is designed to > cache > >>>> the > >>>> > >>>>> page. When you do not have enough memory to hold your pages cached > >>>>> basically the system does not work as designed so you should fail > >>>>> early. > >>>>> Otherwise you possible defer the problem to production use. Fail > early > >>>>> means you should try to see the problem in the early stages on dev, > >>>>> where > >>>>> you try out all your pages. As I mentioned in my other post - you > would > >>>>> also not expect the EntityManager to work soft-refereences or spring > >>>>> application context to work soft referenced. > >>>>> > >>>>> > >>>>> That's the definition of a memory cache - it trades memory for > better > >>>>> > >>>> performance. The primary use case for soft refences is for caching so > >>>> seems > >>>> to me it works exactly as designed. Your comparison to the > EntityManager > >>>> is > >>>> flawed since it's created per request. An EntityManager is designed to > >>>> be > >>>> inexpensive to create. There are many areas that need improvements in > >>>> Tapestry but this is not one in my opinion. However, you seem to > >>>> strongly > >>>> think otherwise, so you probably have some data to back this up. Do > you > >>>> have a memory dump and trending cpu/memory charts of a sufficiently > >>>> large > >>>> system you can share with us to demonstrate the problem? Jvisualvm > >>>> snapshots should work fine. And furthermore - how would you like this > >>>> changed? If it's just adding a Page as a threadlocal, perhaps you can > >>>> just > >>>> write a patch for it. > >>>> > >>>> Kalle > >>>> > >>>> > >>>> Am 18.03.2015 um 04:23 schrieb Kalle Korhonen: > >>>> > >>>>> In my opinion, soft referencing page objects is highly appropriate > >>>>> usage > >>>>> > >>>>> here. If there's pressure on the available memory, it makes sense to > >>>>>> trade > >>>>>> performance for memory instead of exiting with OoM. This is simple > >>>>>> condition to detect and should be visible with any reasonable > >>>>>> monitoring > >>>>>> tool. If you are hitting memory limits, you'll need to allocate more > >>>>>> memory > >>>>>> for the application for optimal performance. Soft references are > >>>>>> especially > >>>>>> useful here because you can optimize its behavior with the > >>>>>> -client/-server > >>>>>> setting depending on your preferences. > >>>>>> > >>>>>> Kalle > >>>>>> > >>>>>> On Tue, Mar 17, 2015 at 4:26 PM, Howard Lewis Ship < > hls...@gmail.com> > >>>>>> wrote: > >>>>>> > >>>>>> Possibly we need something more advanced; our own reference type > >>>>>> that > >>>>>> can > >>>>>> > >>>>>> react to memory pressure by discarding pages that haven't been used > >>>>>>> in > >>>>>>> configurable amount of time. > >>>>>>> > >>>>>>> Or perhaps we could just assume that any page that has been used > once > >>>>>>> need > >>>>>>> to be used in the future and get rid of the SoftReference entirely > >>>>>>> (or > >>>>>>> otherwise janitorize it in some way). > >>>>>>> > >>>>>>> On Tue, Mar 17, 2015 at 1:24 AM, Robert Schmelzer > >>>>>>> <rob...@schmelzer.cc > >>>>>>> wrote: > >>>>>>> > >>>>>>> Hello, > >>>>>>> > >>>>>>> I recently came accross the implementation of PageSourceImpl where > >>>>>>>> PageImpl instances are softly refereneced into the pageCache: > >>>>>>>> > >>>>>>>> private final Map<CachedPageKey, SoftReference<Page>> pageCache = > >>>>>>>> CollectionFactory.newConcurrentMap(); > >>>>>>>> > >>>>>>>> This implementation caused troubles, when you bring your system > into > >>>>>>>> memory preassure. The JVM will start to throw away the PageImpl to > >>>>>>>> free > >>>>>>>> > >>>>>>>> up > >>>>>>>> > >>>>>>> memory - but during request processing he needs the PageImpl > again > >>>>>>> and > >>>>>>> > >>>>>>>> starts creating it again. So basically you end up loosing your > >>>>>>>> pageCache > >>>>>>>> > >>>>>>>> at > >>>>>>>> > >>>>>>> all and start creating the PageImpl instances on every request, > >>>>>>> which > >>>>>>> > >>>>>>>> take > >>>>>>>> > >>>>>>> way to much time and takes load onto the CPU. So basically you > are > >>>>>>> > >>>>>>>> hiding a > >>>>>>>> > >>>>>>> memory problem by making the system slow and raise CPU load. > >>>>>>> > >>>>>>>> I would suggest to use "normal" references for the PageCache or at > >>>>>>>> least > >>>>>>>> only do SoftReferences only when not in production mode. Otherwise > >>>>>>>> we > >>>>>>>> are > >>>>>>>> going to cover memory problems for too long. > >>>>>>>> > >>>>>>>> What do you think about that? > >>>>>>>> > >>>>>>>> Robert > >>>>>>>> > >>>>>>>> ------------------------------------------------------------ > >>>>>>>> --------- > >>>>>>>> To unsubscribe, e-mail: users-unsubscr...@tapestry.apache.org > >>>>>>>> For additional commands, e-mail: users-h...@tapestry.apache.org > >>>>>>>> > >>>>>>>> > >>>>>>>> > >>>>>>>> -- > >>>>>>>> > >>>>>>> Howard M. Lewis Ship > >>>>>>> > >>>>>>> Creator of Apache Tapestry > >>>>>>> > >>>>>>> The source for Tapestry training, mentoring and support. Contact me > >>>>>>> to > >>>>>>> learn how I can get you up and productive in Tapestry fast! > >>>>>>> > >>>>>>> (971) 678-5210 > >>>>>>> http://howardlewisship.com > >>>>>>> @hlship > >>>>>>> > >>>>>>> > >>>>>>> ------------------------------------------------------------ > >>>>>>> --------- > >>>>>>> > >>>>>> To unsubscribe, e-mail: users-unsubscr...@tapestry.apache.org > >>>>> For additional commands, e-mail: users-h...@tapestry.apache.org > >>>>> > >>>>> > >>>>> > >>>>> > --------------------------------------------------------------------- > >>> To unsubscribe, e-mail: users-unsubscr...@tapestry.apache.org > >>> For additional commands, e-mail: users-h...@tapestry.apache.org > >>> > >>> > >>> > > > > --------------------------------------------------------------------- > > To unsubscribe, e-mail: users-unsubscr...@tapestry.apache.org > > For additional commands, e-mail: users-h...@tapestry.apache.org > > > > > > > -- > Howard M. Lewis Ship > > Creator of Apache Tapestry > > The source for Tapestry training, mentoring and support. Contact me to > learn how I can get you up and productive in Tapestry fast! > > (971) 678-5210 > http://howardlewisship.com > @hlship >
