Hello,
Am Wed, 02 Jul 2014 00:45:01 +0200
schrieb Peter Levart <peter.lev...@gmail.com>:
> > L782: is it better to use putIfAbsent unconditionally, instead of
> > get/putIfAbsent in NameServicdeAddr?
>
> I want to keep the semantics of original code that guarantees that
> there will only be a single look-up to the name service per hostname
> in a given caching period. If several threads want to look-up the
> same hostname at the same time, only one will do it, others will wait
> and then use the result of that thread. That's why I re-check the
> cached value after entering the synchronized block and act upon what
> I find there...
Yes, I was talking about the get inside the synchronized with same
semantic, but I havent completely tested my thinking through (idea was
to re-use putIfAbsent for the first get):
synchronized (this) {
// re-check that we are still us
addresses = cache.putIfAbsent(host, this);
// address is null if we had added us
// if address did not change it is this
>
> > L732: I am unsure about the id field, isnt it enough to have the
> > identity equality check for the replacement check and otherwise
> > depend on equals()?
>
> The ConcurrentSkipListSet (based on ConcurrentSkipListMap) that is
> used as an ordered concurrent (but not blocking or locking) queue of
> items to be expired is not using hashCode/equals, but compareTo
> instead. So two instances of CachedAddresses must never compareTo
> each other as being "equal" if one wants to keep them all in the
> ConcurrentSkipListSet...
Yes, I mean not using id in compareTo but idendity if (other==this)
return 0; But maybe not worth it as you still need to order identical
entries reliable (maybe by system hash).
> > BTW1: might be the wrong RFR, but considering your good performance
> > numbers for an active cache, would having 100ms or similiar default
> > negative cache time make sense without impacting (visible) the
> > semantic.
>
> It depends on the "application". If some app is doing more than 10
> look-ups of same non-existent hostname per second, it will have
> effect. Otherwise not. It seems that negative answers are not so
> frequent in practice that default policy would require caching them.
> One can always choose the policy manually.
Actually I think an application who retries lookups without rate limit
might not be too uncommon as they typically fail only after timeouts.
However if you have for example a dropped default route nameserver
answer can return the error immediatelly and you get into a busy loop.
But yes most likely most apps only try it a small time.
Gruss
Bernd
