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From Erick Erickson <erickerick...@gmail.com>
Subject Re: Difference between unloading of cores with LotsOfCores and unloading a core with CoreAdmin
Date Thu, 23 Oct 2014 13:51:10 GMT
Memory should eventually be returned when a core is unloaded. There's
a very small amount of overhead for keeping a list of all the cores
and their locations, but this shouldn't increase with time unless
you're adding more cores.

Do note that the transient cache size is fixed, but may be exceeded. A
core is held open when it gets reclaimed long enough to serve any
outstanding requests, but it _should_ have the memory reclaimed
eventually.

Of course there's always the possibility of some memory being kept
inadvertently, I'd consider that a  bug so if you can define how this
happens, perhaps with a test case that would be great. Dumping the
memory would help see what's kept if anything actually is.

Best,
Erick

On Wed, Oct 22, 2014 at 12:33 PM, Xiaolu Zhao <xiaolu.zhao@oracle.com> wrote:
> Hi Erick,
>
> Thanks a lot for your explanation.
>
> Last time, when I try out LotsOfCores, I find JVM memory usage will increase
> as the total number of cores grows, though the transient cache size is
> fixed. Finally, JVM will run out of memory when I have thousands of cores.
> Does it mean other currently unloaded cores will consume memory? Or swapping
> among loaded/unloaded cores will consume memory?
>
> Best,
> Xiaolu
>
> On 10/22/2014 12:23 PM, Erick Erickson wrote:
>>
>> The difference here is that the LotsOfCores is intended to cache open
>> cores and thus limit the number of currently loaded cores. However,
>> cores not currently loaded are available for use; the next request
>> that needs that core will cause it to be loaded (or reloaded).
>>
>> The admin/core/UNLOAD command, on the other hand, is designed to
>> _permanently_ remove the core from Solr. Or at least have it become
>> unavailable until another explicit admin/core command is executed to
>> bring it back. There is nothing automatic about this.
>>
>> Another way of looking at it is that LotsOfCores is used in a
>> situation where you don't know what requests are coming in, but you
>> _can_ predict that not many will be used at once. So if I have 500
>> cores, and my expectation is that only 20 of them are used at once,
>> there's no good in having the 480 other cores loaded all the time.
>> When a query comes in for one of the currently-unloaded cores (call it
>> core21), that core is loaded (perhaps displacing one of the
>> currently-loaded cores) and the request is served.
>>
>> If core21 above had been unloaded with the core/admin command, then a
>> request directed to it would return an error instead.
>>
>> Best,
>> Erick
>>
>> On Wed, Oct 22, 2014 at 12:11 PM, Xiaolu Zhao <xiaolu.zhao@oracle.com>
>> wrote:
>>>
>>> Hi All,
>>>
>>> I am confused about the difference between unloading of cores with
>>> LotsOfCores and unloading a core with CoreAdmin.
>>>
>>>  From my understanding of LotsOfCores, if one core is removed from
>>> transient
>>> cache, it is pending to close, it means close all resources allocated by
>>> the
>>> core if it is no longer in use, e.g. searcher, updateHandler... While for
>>> unloading a core with CoreAdmin, this core needs to be removed from the
>>> cores list, either ordinary cores list or transient cores list, and cores
>>> locator will delete it. If this core is loaded but not pending to close,
>>> it
>>> will be close.
>>>
>>> Also, one more interesting thing is if I unload a core with CoreAdmin,
>>> "core.properties" will be renamed "core.properties.unloaded". Then this
>>> core
>>> cannot be found in the Solr API, and STATUS url won't return its status
>>> as
>>> well. But with LotsOfCores, a core not in the transient cache will still
>>> have "core.properties" and could be found through STATUS url, though it
>>> is
>>> marked with "isLoaded=false".
>>>
>>> Could anyone tell me the underlying mechanism for these two cases? Why
>>> LotsOfCores could realize frequent unloading/loading of cores? Do cores
>>> not
>>> in the transient cores still consume JVM memory, while unloaded cores
>>> with
>>> CoreAdmin not?
>>>
>>> Thanks,
>>> Xiaolu
>
>

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