7:20 a.m.
On 10/07/2013 11:43 AM, Sanne Grinovero wrote:
Proposal:
let's be more aggressive on validation.
- make preload mandatory (for the metadata only)
- eviction not allowed (for the metadata only)
Note that I don't think we're putting much of a restriction to users,
we're more likely helping
with good guidance on how these caches are supposed to work.
I think it's a good thing to narrow down the possible configuration options
and make it clear what we expect people to use for this purpose.
it's good enough for me. :)
>> 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.
>
>
> I didn't understand what you mean here ^^. Infinispan only tries to load
> from a cache store once per operation (get, put, remove, etc...).
>
> however, I think we have a big windows in which multiple operations can load
> the same key from the store. This happens because we only write to the data
> container after the operation end.
I think that's a very important consideration.
Imagine a REST cache is built on Infinispan, using a filesystem based
CacheStore, and we receive a million requests per second
for a key "X1", which doesn't exist.
Since each request is not going to find it in the data container, each
request is triggering a CacheStore load.
Do you want to enqueue a million IO operations to the disk?
I'd much rather have some locking in memory, that will prevent nut
just disks from struggling but also allow the CPUs to context switch
to perform more useful tasks: being blocked on a lock is a good thing
in this case, rather than allowing "parallel processing of pointless
requests". That's just parallel energy burning, and potentially DOSing
yourself regarding more interesting requests which could be handled
instead.
To translate it into an implementation design, you need to lock the
"X1" key before attempting to load it from the CacheStore, and when
the lock is acquired the data container needs to be re-checked as it
might have been loaded by a parallel thread.
Or alternatively, lock the key before attempting to read from the data
container: that's not too bad, it's a very brief local only lock.
you have good point here, but IMO this is a high risk operation because
we are getting close in the final release. and I don't want to make the
cache store unusable...
but, let's see what others think about it.
my 1 cent pseudo-code suggestion:
lock(key) {
value = dataContainer.get(key)
if value is not null; then return value
loaded = cacheLoader.load(key)
if loaded is null; then return null
existing = dataContainer.putIfAbsent(key, loaded)
if existing is null; then return loaded; otherwise existing
}
Worth a different thread? Not that I don't know how Infinispan core is
implementing this today, I'm just understanding from you that it's not
as good as I'd expected it.
This might also be the reason for some people to have recorded a very
bad performance when writing stress tests on the CacheStore, and of
course such a benchmark would highlight too much IO going on.. but
it's not how fast we do the IO which is the problem if we can just
avoid some of them.
>
>
>>
>> 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?
>>
>
> cache stores acquire the read lock to load a key. I'm not sure if exclusive
> locking a key would help in anything.
See above: allowing parallelism on loads is not a good idea as it will
storm the CacheStore with redundant requests.
Sanne