[infinispan-dev] CacheLoaders, Distribution mode and Interceptors

[Sanne Grinovero]

Hi Adrian,
let's forget about Lucene details and focus on DIST.
With numOwners=1 and having two nodes the entries should be stored
roughly 50% on each node, I see nothing wrong with that
considering you don't need data failover in a read-only use case
having all the index available in the shared CacheLoader.

In such a scenario, and having both nodes preloaded all data, in case
of a get() operation I would expect
either:
 A) to be the owner, hence retrieve the value from local in-JVM reference
 B) to not be the owner, so to forward the request to the other node
having roughly 50% chance per key to be in case A or B.

But when hitting case B) it seems that instead of loading from the
other node, it hits the CacheLoader to fetch the value.

I already had asked James to verify with 4 nodes and numOwners=2, the
result is the same so I suggested him to ask here;
BTW I think numOwners=1 is perfectly valid and should work as with
numOwners=1, the only reason I asked him to repeat
the test is that we don't have much tests on the numOwners=1 case and
I was assuming there might be some (wrong) assumptions
affecting this.

Note that this is not "just" a critical performance problem but I'm
also suspecting it could provide inconsistent reads, in two classes of
problems:

# non-shared CacheStore with stale entries
If for non-owned keys it will hit the local CacheStore first, where
you might expect to not find anything, so to forward the request to
the right node. What if this node has been the owner in the past? It
might have an old entry locally stored, which would be returned
instead of the correct value which is owned on a different node.

# shared CacheStore using write-behind
When using an async CacheStore by definition the content of the
CacheStore is not trustworthy if you don't check on the owner first
for entries in memory.

Both seem critical to me, but the performance impact is really bad too.

I hoped to make some more tests myself but couldn't look at this yet,
any help from the core team would be appreciated.

@Ray, thanks for mentioning the ClusterCacheLoader. Wasn't there
someone else with a CacheLoader issue recently who had worked around
the problem by using a ClusterCacheLoader ?
Do you remember what the scenario was?

Cheers,
Sanne

On 15 March 2013 15:44, Adrian Nistor <anistor at redhat.com> wrote:
> Hi James,
>
> I'm not an expert on InfinispanDirectory but I've noticed in [1] that
> the lucene-index cache is distributed with numOwners = 1. That means
> each cache entry is owned by just one cluster node and there's nowhere
> else to go in the cluster if the key is not available in local memory,
> thus it needs fetching from the cache store. This can be solved with
> numOwners > 1.
> Please let me know if this solves your problem.
>
> Cheers!
>
> On 03/15/2013 05:03 PM, James Aley wrote:
>> Hey all,
>>
>> <OT>
>> Seeing as this is my first post, I wanted to just quickly thank you
>> all for Infinispan. So far I'm really enjoying working with it - great
>> product!
>> </OT>
>>
>> I'm using the InfinispanDirectory for a Lucene project at the moment.
>> We use Lucene directly to build a search product, which has high read
>> requirements and likely very large indexes. I'm hoping to make use of
>> a distribution mode cache to keep the whole index in memory across a
>> cluster of machines (the index will be too big for one server).
>>
>> The problem I'm having is that after loading a filesystem-based Lucene
>> directory into InfinispanDirectory via LuceneCacheLoader, no nodes are
>> retrieving data from the cluster - they instead look up keys in their
>> local CacheLoaders, which involves lots of disk I/O and is very slow.
>> I was hoping to just use the CacheLoader to initialize the caches, but
>> from there on read only from RAM (and network, of course). Is this
>> supported? Maybe I've misunderstood the purpose of the CacheLoader?
>>
>> To explain my observations in a little more detail:
>> * I start a cluster of two servers, using [1] as the cache config.
>> Both have a local copy of the Lucene index that will be loaded into
>> the InfinispanDirectory via the loader. This is a test configuration,
>> where I've set numOwners=1 so that I only need two servers for
>> distribution to happen.
>> * Upon startup, things look good. I see the memory usage of the JVM
>> reflect a pretty near 50/50 split of the data across both servers.
>> Logging indicates both servers are in the cluster view, all seems
>> fine.
>> * When I send a search query to either one of the nodes, I notice the following:
>>    - iotop shows huge (~100MB/s) disk I/O on that node alone from the
>> JVM process.
>>    - no change in network activity between nodes (~300b/s, same as when idle)
>>    - memory usage on the node running the query increases dramatically,
>> and stays higher even after the query is finished.
>>
>> So it seemed to me like each node was favouring use of the CacheLoader
>> to retrieve keys that are not in memory, instead of using the cluster.
>> Does that seem reasonable? Is this the expected behaviour?
>>
>> I started to investigate this by turning on trace logging, in this
>> made me think perhaps the cause was that the CacheLoader's interceptor
>> is higher priority in the chain than the the distribution interceptor?
>> I'm not at all familiar with the design in any level of detail - just
>> what I picked up in the last 24 hours from browsing the code, so I
>> could easily be way off. I've attached the log snippets I thought
>> relevant in [2].
>>
>> Any advice offered much appreciated.
>> Thanks!
>>
>> James.
>>
>>
>> [1] https://www.refheap.com/paste/12531
>> [2] https://www.refheap.com/paste/12543
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