RFR JDK-7186258: InetAddress$Cache should replace currentTimeMillis with nanoTime (+more)

Tue Jul 1 22:45:01 UTC 2014

Hi Bernd,

Thanks for reviewing.

On 07/01/2014 10:06 PM, Bernd Eckenfels wrote:
> Looks good, like it, Peter.
>
> some nits: instead of adding createTime and cacheNanos, only have a
> expireAfter?

I could, yes. I just have to be careful not co compare two expireAfter 
values with '<' or '>'.

The javadoc for System.nanoTime() states:

"
To compare two nanoTime values

  long t0 = System.nanoTime();
  ...
  long t1 = System.nanoTime();

one should use t1 - t0 < 0, not t1 < t0, because of the possibility of 
numerical overflow.
"

The thing is that one never knows with System.nanoTime() when its start 
(value 0) is. It's not defined. So a later time instant can be negative 
while an earlier one can be positive. But I could compare two instants 
which are no more than 290 years apart with no problem using (t1 - t2) 
<=> 0 . I'll improve this.

> 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...

> 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...

> L1223: What about moving the cache expiry inside the if (useCache)

That's a possibility. InetAddress.getLocalHost() calls will not expire 
cache entries then. There are also other "optimizations" possible. For 
example if there is a "storm" of distinct host names to be looked-up, 
they enter the cache at about the same time, so they expire at about the 
same time too. It could happen that one thread must remove a lot of 
expired entries at one time. Perhaps an upper bound on the number of 
entries that get removed in one getByName() call could be specified.

> 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.

>
>
> Gruss
> Bernd

Regards, Peter

>
> Am Tue, 01 Jul 2014 20:35:57 +0200
> schrieb Peter Levart <peter.levart at gmail.com>:
>
>> Hi,
>>
>> I propose a patch for this issue:
>>
>> https://bugs.openjdk.java.net/browse/JDK-7186258
>>
>> The motivation to re-design caching of InetAddress-es was not this
>> issue though, but a desire to attack synchronization bottlenecks in
>> methods like URL.equals and URL.hashCode which use host name to IP
>> address mapping. I plan to tackle the synchronization in URL in a
>> follow-up proposal, but I wanted to 1st iron-out the "leaves" of the
>> call-tree. Here's the proposed patch:
>>
>> http://cr.openjdk.java.net/~plevart/jdk9-dev/InetAddress.Cache/webrev.01/
>>
>> sun.net.InetAddressCachePolicy:
>>
>> - two static methods (get() and getNegative()) were synchronized.
>> Removed synchronization and made underlying fields volatile.
>> - also added a normalization of negative policy in
>> setNegativeIfNotSet(). The logic in InetAddress doesn't cope with
>> negative values distinct from InetAddressCachePolicy.FOREVER (-1), so
>> this was a straight bug. The setIfNotSet() doesn't need this
>> normalization, because checkValue() throws exception if passed-in
>> value < InetAddressCachePolicy.FOREVER.
>>
>> java.net.InetAddress:
>>
>> - complete redesign of caching. Instead of distinct Positive/Negative
>> caches, there's only one cache - a ConcurrentHashMap. The value in
>> the map knows if it contains positive or negative answer.
>> - the design of this cache is similar but much simpler than
>> java.lang.reflect.WeakCache, since it doesn't have to deal with
>> WeakReferences and keys are simpler (just strings - hostnames).
>> Similarity is in how concurrent requests for the same key (hostname)
>> are synchronized when the entry is not cached yet, but still avoid
>> synchronization when entry is cached. This preserves the behaviour of
>> original InetAddress caching code but simplifies it greatly (100+
>> lines removed).
>> - I tried to preserve the interaction between
>> InetAddress.getLocalHost() and InetAddress.getByName(). The
>> getLocalHost() caches the local host address for 5 seconds privately.
>> When it expires it performs new name service look-up and "refreshes"
>> the entry in the InetAddress.getByName() cache although it has not
>> expired yet. I think this is meant to prevent surprises when
>> getLocalHost() returns newer address than getByName() which is called
>> after that.
>> - I also fixed the JDK-7186258 as a by-product (but don't know yet
>> how to write a test for this issue - any ideas?)
>>
>> I created a JMH benchmark that tests the following methods:
>>
>> - InetAddress.getLocalHost()
>> - InetAddress.getByName() (with positive and negative answer)
>>
>> Here're the results of running on my 4-core (8-threads) i7/Linux:
>>
>> http://cr.openjdk.java.net/~plevart/jdk9-dev/InetAddress.Cache/InetAddress.Cache_bench_results.01.pdf
>>
>>
>> The getByNameNegative() test does not show much improvement in
>> patched vs. original code. That's because by default the policy is to
>> NOT cache negative answers. Requests for same hostname to the
>> NameService(s) are synchronized. If
>> "networkaddress.cache.negative.ttl" system property is set to some
>> positive value, results are similar to those of getByNamePositive()
>> test (the default policy for positive caching is 30 seconds).
>>
>> I ran the jtreg tests in test/java/net and have the same score as
>> with original unpatched code. I have 3 failing tests from original
>> and patched runs:
>>
>> JT Harness : Tests that failed
>> java/net/MulticastSocket/Promiscuous.java: Test for interference when
>> two sockets are bound to the same port but joined to different
>> multicast groups
>> java/net/MulticastSocket/SetLoopbackMode.java: Test
>> MulticastSocket.setLoopbackMode
>> java/net/MulticastSocket/Test.java: IPv4 and IPv6 multicasting broken
>> on Linux
>>
>> And 1 test that had error trying to be run:
>>
>> JT Harness : Tests that had errors
>> java/net/URLPermission/nstest/lookup.sh:
>>
>> Because of:
>>
>> test result: Error. Can't find source file: jdk/testlibrary/*.java in
>> directory-list:
>> /home/peter/work/hg/jdk9-dev/jdk/test/java/net/URLPermission/nstest
>> /home/peter/work/hg/jdk9-dev/jdk/test/lib/testlibrary
>>
>> All other 258 java/net tests pass.
>>
>>
>>
>> So what do you think?
>>
>>
>> Regards, Peter
>>
>>

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