On 2014-04-29 09:31, Remi Forax wrote:
On 04/28/2014 05:43 PM, Claes Redestad wrote:
On 04/28/2014 08:57 AM, David Holmes wrote:
On 28/04/2014 1:05 PM, Otávio Gonçalves de Santana wrote:
In my opinion not, because Objects.requireNonNull is more readable
than
just string.toString. This way is more understandable which field is
required and doesn't impact on performance.
An invocation of requireNonNull is potentially more expensive than
the implicit null check that happens with foo.toString().
David
-----
My thought was that these two would be inlined to the exact same
thing, so I did a quick test to see what happens when you do
foo.toString() versus Objects.requireNonNull(foo) on a set of
randomly generated String[]'s with different amounts of null
elements(0p: no null entries, 1p: 1% null entries etc):
Benchmark Mode Samples Mean
Mean error Units
s.m.ThrowAwayBenchmark.nullToString0p thrpt 6 356653.044
3573.707 ops/ms
s.m.ThrowAwayBenchmark.nullToString1p thrpt 6 353128.903
2764.102 ops/ms
s.m.ThrowAwayBenchmark.nullToString10p thrpt 6 297956.571
9580.251 ops/ms
s.m.ThrowAwayBenchmark.nullToString50p thrpt 6 158172.036
1893.096 ops/ms
s.m.ThrowAwayBenchmark.nullToString100p thrpt 6
18194.614 472.091 ops/ms
s.m.ThrowAwayBenchmark.requireNonNull0p thrpt 6
357855.126 2979.090 ops/ms
s.m.ThrowAwayBenchmark.requireNonNull1p thrpt 6 67601.134
7004.689 ops/ms
s.m.ThrowAwayBenchmark.requireNonNull10p thrpt 6
8150.595 538.970 ops/ms
s.m.ThrowAwayBenchmark.requireNonNull50p thrpt 6
1604.919 220.903 ops/ms
s.m.ThrowAwayBenchmark.requireNonNull100p thrpt 6
820.626 60.752 ops/ms
Yikes! As long as the value is never null they're inlined nicely and
neither have the upper hand performance-wise, but as soon as you get
some null values, Objects.requireNonNull degenerates much faster than
its foo.toString counterpart. I think this is a JIT issue -
optimizing exception-paths might not be the highest priority, but
Objects.requireNonNull is used pretty extensively in the JDK and my
expectation would be that it shouldn't degrade performance when
things actually are null now and then.
/Claes
This is a know issue, I think it's not related to the way the JIT
handle exception path but what is called 'profile pollution'.
Hotspot JITs have two ways to do a null check, either do nothing (yes
nothing) and let the system do a fault and come back from dead using a
signal handler, this solution is named implicit null check or by doing
an explicit null check, i.e a conditional jump.
Implicit null check is faster but if the receiver is null, it cost you
an harm, so the VM profiles receiver to remember if the receiver of
each call can be null or not. The problem is that when you call
foo.toString(), the profile information is associated with the
instruction that does foo.toString() while if the call is
Objects.requireNonNull(foo), the profile associated with the nullcheck
is stored inside the method requireNonNull, so if one call to
requireNonNull in the entire program throw a NPE, the profile inside
requireNonNull now register that it may fail, so for the VM all calls
to requireNonNull may fail. So currently you should not use
requireNonNull is the value is not required to be null*
I guess I should have given more details. :-) I ran my micros on a
number of forked VMs to ensure I don't get excessive run-to-run
variations, which among other things avoids profile pollution. Also I
sloppily collected very few samples and only did minimal warmup after I
saw that the micros produced scores that were orders of magnitude apart
and the values stabilized after less than two seconds: Flags: -f 3 -wi 4
-i 2
When I run the micros sequentially in the same VM I see some added
degradation for the exceptional cases, but the 0p cases still inline to
the optimal(?) 360k ops/ms, so while it seems profile pollution might
play in when provoked, HotSpot seems to manage these simple micros well
enough.
So the problem is that Hotspot blindly trusts the recorded profile
compared to the profile that can come from the arguments of a call.
The good news is that recently some patches were included in the jdk9
tree to fix or at least mitigate that issue.
I've now run tests on JDK8 FCS and a recent internal build of JDK9 and
see the same characteristics. How recently are we talking here? :-)
cheers,
Rémi
* read that last sentence again, it seems very logical, no ?
While your reading of the method name and suggested approach makes sense
right now, the javadoc for Objects.requireNonNull states "this method is
designed primarily for doing parameter validation in methods and
constructors". To me, "parameter validation" suggests that we're dealing
with unknowns and that there's at least some calculated risk that the
input might be null, so the intent is really to encourage explicit
fail-fast designs, no? I thus think it makes sense to try and ensure
these methods have the same performance characteristics as when
provoking an implicit null check by dereferencing an object.
/Claes
