[infinispan-dev] Need help

[Sanne Grinovero]

On 6 October 2013 00:01, Pedro Ruivo <pedro at infinispan.org> wrote:
> Hi Sanne.
>
> Thanks for your comments. please see inline...
>
> Cheers,
> Pedro
>
>
> On 10/05/2013 09:15 PM, Sanne Grinovero wrote:
>>
>> Hi Pedro,
>> looks like you're diving in some good fun :-)
>> BTW please keep the dev discussions on the mailing list, adding it.
>>
>> inline :
>>
>> On 4 October 2013 22:01, Pedro Ruivo <pedro at infinispan.org> wrote:
>>>
>>> Hi,
>>>
>>> Sanne I need your expertise in here. I'm afraid that the problem is in
>>> FileListOperations :(
>>> I think the FileListOperations implementation needs a transactional cache
>>> with strong consistency...
>>>
>>> I'm 99% sure that it is originating the java.lang.AssertionError: file
>>> XPTO
>>> does not exist. I find out that we have multiple threads adding and
>>> removing
>>> files from the list. The scenario in [1] we see 2 threads loading the key
>>> from the cache loader and one thread adds a file and other removes. the
>>> thread that removes is the last one to commit and the file list is
>>> updated
>>> to an old state. When it tries to updat an index, I got the assertion
>>> error.
>>
>>
>> Nice, looks like you're on something.
>> I've never seen specifically an AssertionError, looks like you have a
>> new test. Could you share it?
>
>
> yes of course:
> https://github.com/pruivo/infinispan/blob/a4483d08b92d301350823c7fd42725c339a65c7b/query/src/test/java/org/infinispan/query/cacheloaders/CacheStoreTest.java
>
> so far, only the tests with eviction are failing...

Some of the failures you're seeing are caused by internal "assert"
keyword in Lucene's code, which have the purpose of verifying the
"filesystem" is going to be synched properly.
These assertions don't apply when using our storage: we don't need
this synch to happen: in fact if it weren't because of the assertions
the whole method would be a no-op as it finally delegates all logic to
a method in the Infinispan Directory which is a no-op.

In other words, these are misleading failures and we'd need to avoid
the TestNG "feature" of enabling assertions in this case.

Still, even if the stacktrace is misleading, I agree on your diagnosis below.

Could you reproduce the problem without involving also the Query framework?
I'd hope that such a test could be independent and live solely in the
lucene-directory module; in practice if you can only reproduce it with
the query module it makes me suspicious that we're actually debugging
a race condition in the initialization of the two services: a race
between the query initialization thread needing to check the index
state (so potentially triggering a load from cachestore), and the
thread performing the cachestore preload.
(I see your test also fails without preload, but just wondering if
that might be an additional complexity).

>>
>> Let's step back a second and consider the Cache usage from the point
>> of view of FileListOperations.
>> Note that even if you have two threads writing at the same time, as
>> long as they are on the same node they will be adding/removing
>> elements from the same instance of a ConcurrentHashMap.
>> Since it's the same instance, it doesn't matter which thread will do
>> the put operation as last: it will push the correct state.
>> (there is an assumptions here, but we can forget about those for the
>> sake of this debugging: same node -> fine as there is an external
>> lock, no other node is allowed to write at the same time)
>>
>
> 100% agreed with you but with cache store, we no longer ensure that 2 (or
> more) threads are pointing to the same instance of Concurrent Hash Set.
>
> With eviction, the entries are removed from in-memory container and
> persisted in the cache store. The scenario I've described, 2 threads are
> trying to add/remove a file and the file list does not exist in-memory. So,
> each thread will read from cache store and deserialize the byte array. In
> the end, each thread will have a pointer for different instances of
> ConcurrentHashSet but with the same elements. And when this happens, we lost
> one of the operation.

I'm seeing more than a couple of different smelly behaviors interacting here:

1## The single instance ConcurrentHashSet
I guess this could be re-thought as it's making some strong
assumptions, but considering this service can't be transactional I'd
rather explore other solutions first as I think the following changes
should be good enough.

2## Eviction not picking the right entry
This single key is literally read for each and every performed query,
and all writes as well. Each write, will write on this key.
Even with eviction being enabled on the cache, I would never expect
this key to be actually evicted!

 # Action 1: Open an issue to investigate the eviction choice: the
strategy seems to be making a very poor job (or maybe it's just that
maxEntries(10) is too low and makes LIRS degenerate into insane
choices).

 # Action 2: I think that for now we could disallow usage of eviction
on the metadata cache. I didn't have tests using it, as I wouldn't
recommended such a configuration as these entries are very hot and
very small: viable to make it an illegal option?

3## The CacheLoader loading the same entry multiple times in parallel
Kudos for finding out that there are situations in which we deal with
multiple different instances of ConcurrentHashSet! Still, I think that
Infinispan core is terribly wrong in this case:
from the client code POV a new CHS is created with a put-if-absent
atomic operation, and I will assume there that core will check/load
any enabled cachestore as well.
To handle multiple GET operations in parallel, or even in parallel
with preload or the client's put-if-absent operation, I would *not*
expect Infinispan core to storm the CacheStore implementation with
multiple load operations on the same put: a lock should be hold on the
potential key during such a load operation.

If your observation is right, this could also be one of the reasons
for so many complaints on the CacheStore performance: under these
circumstances - which I'd guesstimate are quite common - we're doing
lots of unnecessary IO, potentially stressing the slower storage
devices. This could even have dramatic effects if there are frequent
requests for entries for which the returned value is a null.

 # Action 3: Investigate and open a JIRA about the missing locking on
CacheStore load operations.

If this where resolves, we would still have a guarantee of single
instance, am I correct?

>
> Also, the problem is easily reproduce when you enable the storeAsBinary for
> values because each cache.get will deserialize the byte array and create
> different instances.

That's another option that I've never used on a cache meant to store a
Lucene index as it doesn't make sense for the purpose.
But you're right that's an assumption I haven't thought of:

  # Action 4: What do you think of promoting that as an illegal
configuration? created ISPN-3593

>
> That's why I think we would need a transaction.

-1 the Directory can not drive a transaction as we hope some day in
the future to include changes to the index in a user transaction.

Cheers,
Sanne