[infinispan-dev] About size()

Fri Oct 10 10:06:18 EDT 2014

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