RFR JDK-7186258: InetAddress$Cache should replace currentTimeMillis with nanoTime (+more)
peter.levart at gmail.com
Tue Jul 1 15:16:08 UTC 2014
I propose a patch for this issue:
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:
- 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
- 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
- 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.getByName() (with positive and negative answer)
Here're the results of running on my 4-core (8-threads) i7/Linux:
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
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
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
java/net/MulticastSocket/Test.java: IPv4 and IPv6 multicasting broken on
And 1 test that had error trying to be run:
JT Harness : Tests that had errors
test result: Error. Can't find source file: jdk/testlibrary/*.java in
All other 258 java/net tests pass.
So what do you think?
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