[hibernate-dev] Pooled Optimiser Improvements
Scott Marlow
smarlow at redhat.com
Tue Dec 15 17:58:17 EST 2015
On 12/15/2015 05:40 PM, Scott Marlow wrote:
> I changed the new test methods a bit. [2] seems to be passed the tests
> but I am not understanding how PooledThreadLocalLoOptimizer should
> coordinate with the AccessCallback to allocate the next chunk of
> sequence numbers.
>
> We seem to be able to call AccessCallback.getNextValue() to get the next
> available sequence number but how do we reserve a block of 5000 sequence
> ids? Am I supposed to call callback.getNextValue() an extra time to get
> a range of values? Is there a separate database transaction that is
> used by the AccessCallback.getNextValue() calls? I'm missing something.
Thinking more about this, I assume that AccessCallback.getNextValue() is
operating under a database transaction that we are probably ending
before AccessCallback.getNextValue() returns. It also sounds like the
database table is tracking the "lo" value, as mentioned in the
PooledLoOptimizer. This implies that only the application layer knows
what the range is. This seems like an important dependency to understand.
Make sense?
>
> Note that [2] also includes a test change to comment out a few lines in
> SchemaUpdateDelimiterTest, due to the compiler error that I am seeing in
> intellij. Will need to remember to remove that change.
>
> [2]
> https://github.com/scottmarlow/hibernate-orm/commits/pooled-optimiser-hack-2
>
> On 12/15/2015 12:36 PM, Steve Ebersole wrote:
>> Those tests tend to assert the increments. We seem to agree that this
>> ThreadLocal one can skip gaps of values. I'd look there first.
>>
>>
>> On Tue, Dec 15, 2015 at 11:34 AM Scott Marlow <smarlow at redhat.com
>> <mailto:smarlow at redhat.com>> wrote:
>>
>> I'm trying to move the optimizer to PooledThreadLocalLoOptimizer [1].
>> We are currently failing some new unit tests, which are cloned from
>> existing PooledLoOptimizer tests which might be part of the problem.
>>
>> Scott
>>
>> [1]
>> https://github.com/scottmarlow/hibernate-orm/tree/pooled-optimiser-hack
>>
>> On 12/14/2015 10:12 PM, Scott Marlow wrote:
>> >
>> >
>> > On 12/11/2015 09:30 AM, Steve Ebersole wrote:
>> >> It's hard to say without understanding the scenario where you
>> are seeing
>> >> this as a problem. I have some guesses as to what may be the
>> problem,
>> >> but without understanding more about why you see this as a
>> problem in
>> >> the first place it is hard to give you an answer. For example,
>> I wonder
>> >> if for environments not using multi-tenancy whether the recent
>> changes
>> >> for the generators to support multi-tenancy might be the
>> culprit. If
>> >> that is the case, and those changes are in fact the underlying
>> cause of
>> >> the perf issues you see then I think there is actually a better
>> >> solution. But again, its hard to say unless we understand the
>> reason
>> >> this "shows up" as a perf problem for you.
>> >
>> > As best as I can tell from looking at the current PooledLoOptimizer,
>> > versus the proposed change (to have a chunk of ids per thread),
>> we went
>> > from accessing a contented lock, to instead using per thread memory
>> > (eliminating the contended lock on PooledLoOptimizer.generate()).
>> >
>> >>
>> >> Until we hear more I think at this stage I'd vote for a separate
>> >> optimizer. And maybe even not one that is upstream.
>> >>
>> >> Also I agree with Scott that I am VERY leery of not cleaning up a
>> >> ThreadLocal.
>> >
>> > My mistake, as Stuart pointed out, the TL is not static, so we
>> shouldn't
>> > introduce any leaks.
>> >
>> >>
>> >> On Fri, Dec 11, 2015 at 7:55 AM Scott Marlow <smarlow at redhat.com
>> <mailto:smarlow at redhat.com>
>> >> <mailto:smarlow at redhat.com <mailto:smarlow at redhat.com>>> wrote:
>> >>
>> >> Should this be a specialized pooled optimizer that is only
>> used in
>> >> environments that do not suffer from leaving the
>> ThreadLocal around
>> >> after the application is undeployed? In other words, the
>> expectation is
>> >> that classloader leaks with this pooled optimizer are
>> expected (e.g.
>> >> user must restart the jvm to really undeploy the application
>> >> completely).
>> >>
>> >> I am thinking that there are at least three typical situations:
>> >>
>> >> 1. Applications are deployed in Java standalone edition.
>> Generally,
>> >> when the app undeploys the jvm is shutting down.
>> >>
>> >> 2. Applications are deployed as part of some container
>> (e.g. an EE
>> >> server) and the Hibernate jars are on the global
>> classloader path (or
>> >> something like that). On each shared container thread,
>> there would be
>> >> one Optimizer for all deployed applications. I wonder if
>> instead, we
>> >> would want one Optimizer instance per Hibernate SessionFactory
>> >> associated with the many container threads?
>> >>
>> >> 3. Applications are deployed as part of some container
>> (e.g. an EE
>> >> server) and the Hibernate jars are deployed with the
>> application. The
>> >> ThreadLocals are associated with threads that are shared by
>> different
>> >> deployed applications. The application classloader contains the
>> >> Hibernate classes. Each deployed application has its own
>> Optimizer
>> >> threadlocal. On each shared container thread, there would
>> be one
>> >> Optimizer per application (since each application has its
>> Optimizer TL).
>> >> Like (2), there would be sharing of the same Optimizer
>> with the many
>> >> application session factories. Should we instead have an
>> optimizer per
>> >> session factory?
>> >>
>> >> Scott
>> >>
>> >> On 12/10/2015 11:31 PM, Stuart Douglas wrote:
>> >> > Hello,
>> >> >
>> >> > I have been working on a change to the pooled optimizer
>> that we
>> >> have been seeing good performance results with. Basically
>> it hands
>> >> out blocks of ID's to a thread local, rather than having every
>> >> thread contend on the lock every time an ID is required.
>> >> >
>> >> >
>> >>
>> https://github.com/hibernate/hibernate-orm/compare/master...stuartwdouglas:pooled-optimiser-hack
>> >> >
>> >> > What would I need to do to get a change like this in? In
>> particular:
>> >> >
>> >> > - Does it need to be a new type of optimizer, or is
>> modifying the
>> >> existing one like I have done OK?
>> >> > - How should it be configured?
>> >> >
>> >> > I am happy to do up a PR for this, but I am just not
>> really sure
>> >> what would be required to get it to a point where it would be
>> >> acceptable for inclusion.
>> >> >
>> >> > Stuart
>> >> > _______________________________________________
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