[infinispan-dev] About size()

Wed Oct 8 12:41:42 EDT 2014

On Wed, Oct 8, 2014 at 6:19 PM, 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.
>

They might expect that, but there is nothing in the Map API suggesting it.

>
> '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).
>

We could create a separate DataContainer for each segment. But would it
really be worth the trouble? I don't know of anyone using size() for
something other than checking that their data was properly loaded into the
cache, and they don't need a super-fast size() for that.

>
> 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).
>

Exactly, we need to iterate all the entries from the stores if we want
something remotely accurate (although I had forgotten about expiration also
being a problem). Otherwise we could just leave size() as it is now, it's
pretty fast :)

>
> 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)
> >>
> >>
> >>
> >>
> >>
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>
> --
> Radim Vansa <rvansa at redhat.com>
> JBoss DataGrid QA
>
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