[infinispan-dev] About size()

Fri Oct 10 10:03:16 EDT 2014

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