[infinispan-dev] About size()

Fri Oct 10 10:23:35 EDT 2014

Exactly, in a monitoring application you wouldn't need the exact number of
key-value mappings in the cache.

The number of entries in memory and/or on disk should be much more
interesting, and we wouldn't have to worry about duplicated/missing/expired
entries to show that.

On Fri, Oct 10, 2014 at 5:06 PM, Tristan Tarrant <ttarrant at redhat.com>
wrote:

> What's wrong with sum(Datacontainer.size())/numOwners ?
>
> Tristan
>
> On 10/10/14 16:03, Radim Vansa wrote:
> > On 10/10/2014 02:38 PM, William Burns wrote:
> >> On Wed, Oct 8, 2014 at 11:19 AM, Radim Vansa <rvansa at redhat.com> wrote:
> >>> Users expect that size() will be constant-time (or linear to cluster
> >>> size), and generally fast operation. I'd prefer to keep it that way.
> >>> Though, even the MR way (used for HotRod size() now) needs to crawl
> >>> through all the entries locally.
> >> Many in memory collections require O(n) to do size such as
> >> ConcurrentLinkedQueue, so I wouldn't say size should always be
> >> expected to be constant time or O(c) where c is # of nodes.  Granted a
> >> user can expect anything they want.
> > OK, I stand corrected. Moreover, I was generalizing myself to all users,
> > a common mistake :)
> >
> > Anyway, monitoring tools love nice charts, and I can imagine monitoring
> > software polling every 1 second to update that cool chart with cache
> > size. Do we want a fast but imprecise variant of this operation in some
> > statistics class?
> >
> > Radim
> >
> >>> 'Heretic, not very well though of and changing too many things' idea:
> >>> what about having data container segment-aware? Then you'd just bcast
> >>> SizeCommand with given topologyId and sum up sizes of primary-owned
> >>> segments... It's not a complete solution, but at least that would
> enable
> >>> to get the number of locally owned entries quite fast. Though, you
> can't
> >>> do that easily with cache stores (without changing SPI).
> >>>
> >>> Regarding cache stores, IMO we're damned anyway: when calling
> >>> cacheStore.size(), it can report more entries as those haven't been
> >>> expired yet, it can report less entries as those can be expired due to
> >>> [1]. Or, we'll enumerate all the entries, and that's going to be slow
> >>> (btw., [1] reminded me that we should enumerate both datacontainer AND
> >>> cachestores even if passivation is not enabled).
> >> This is precisely what the distributed iterator does.  And also
> >> support for expired entries was recently integrated as I missed that
> >> in the original implementation [a]
> >>
> >> [a] https://issues.jboss.org/browse/ISPN-4643
> >>
> >>> Radim
> >>>
> >>> [1] https://issues.jboss.org/browse/ISPN-3202
> >>>
> >>> On 10/08/2014 04:42 PM, William Burns wrote:
> >>>> So it seems we would want to change this for 7.0 if possible since it
> >>>> would be a bigger change for something like 7.1 and 8.0 would be even
> >>>> further out.  I should be able to put this together for CR2.
> >>>>
> >>>> It seems that we want to implement keySet, values and entrySet methods
> >>>> using the entry iterator approach.
> >>>>
> >>>> It is however unclear for the size method if we want to use MR entry
> >>>> counting and not worry about the rehash and passivation issues since
> >>>> it is just an estimation anyways.  Or if we want to also use the entry
> >>>> iterator which should be closer approximation but will require more
> >>>> network overhead and memory usage.
> >>>>
> >>>> Also we didn't really talk about the fact that these methods would
> >>>> ignore ongoing transactions and if that is a concern or not.
> >>>>
> >>>>     - Will
> >>>>
> >>>> On Wed, Oct 8, 2014 at 10:13 AM, Mircea Markus <mmarkus at redhat.com>
> wrote:
> >>>>> On Oct 8, 2014, at 15:11, Dan Berindei <dan.berindei at gmail.com>
> wrote:
> >>>>>
> >>>>>> On Wed, Oct 8, 2014 at 5:03 PM, Mircea Markus <mmarkus at redhat.com>
> wrote:
> >>>>>> On Oct 3, 2014, at 9:30, Radim Vansa <rvansa at redhat.com> wrote:
> >>>>>>
> >>>>>>> Hi,
> >>>>>>>
> >>>>>>> recently we had a discussion about what size() returns, but I've
> >>>>>>> realized there are more things that users would like to know. My
> >>>>>>> question is whether you think that they would really appreciate
> it, or
> >>>>>>> whether it's just my QA point of view where I sometimes compute the
> >>>>>>> 'checksums' of cache to see if I didn't lost anything.
> >>>>>>>
> >>>>>>> There are those sizes:
> >>>>>>> A) number of owned entries
> >>>>>>> B) number of entries stored locally in memory
> >>>>>>> C) number of entries stored in each local cache store
> >>>>>>> D) number of entries stored in each shared cache store
> >>>>>>> E) total number of entries in cache
> >>>>>>>
> >>>>>>> So far, we can get
> >>>>>>> B via withFlags(SKIP_CACHE_LOAD).size()
> >>>>>>> (passivation ? B : 0) + firstNonZero(C, D) via size()
> >>>>>>> E via distributed iterators / MR
> >>>>>>> A via data container iteration + distribution manager query, but
> only
> >>>>>>> without cache store
> >>>>>>> C or D through
> >>>>>>>
> getComponentRegistry().getLocalComponent(PersistenceManager.class).getStores()
> >>>>>>>
> >>>>>>> I think that it would go along with users' expectations if size()
> >>>>>>> returned E and for the rest we should have special methods on
> >>>>>>> AdvancedCache. That would of course change the meaning of size(),
> but
> >>>>>>> I'd say that finally to something that has firm meaning.
> >>>>>>>
> >>>>>>> WDYT?
> >>>>>> There was a lot of arguments in past whether size() and other
> methods that operate over all the elements (keySet, values) are useful
> because:
> >>>>>> - they are approximate (data changes during iteration)
> >>>>>> - they are very resource consuming and might be miss-used (this is
> the reason we chosen to use size() with its current local semantic)
> >>>>>>
> >>>>>> These methods (size, keys, values) are useful for people and I
> think we were not wise to implement them only on top of the local data:
> this is like preferring efficiency over correctness. This also created a
> lot of confusion with our users, question like size() doesn't return the
> correct value being asked regularly. I totally agree that size() returns E
> (i.e. everything that is stored within the grid, including persistence) and
> it's performance implications to be documented accordingly. For keySet and
> values - we should stop implementing them (throw exception) and point users
> to Will's distributed iterator which is a nicer way to achieve the desired
> behavior.
> >>>>>>
> >>>>>> We can also implement keySet() and values() on top of the
> distributed entry iterator and document that using the iterator directly is
> better.
> >>>>> Yes, that's what I meant as well.
> >>>>>
> >>>>> Cheers,
> >>>>> --
> >>>>> Mircea Markus
> >>>>> Infinispan lead (www.infinispan.org)
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>> _______________________________________________
> >>>>> infinispan-dev mailing list
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> >>>>> https://lists.jboss.org/mailman/listinfo/infinispan-dev
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> >>> --
> >>> Radim Vansa <rvansa at redhat.com>
> >>> JBoss DataGrid QA
> >>>
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> >
>
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