On 04/29/2014 11:20 AM, Claes Redestad wrote:
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.
Ok, so you have to take a look to the generated assembly code :)
https://wiki.openjdk.java.net/display/HotSpot/PrintAssembly
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? :-)
I don't know, I have seen the patches but not dive into the source yet,
it's on my todo list, maybe it's not even activated by default.
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?
The great Java Book is clear on that, you shall never try to recover
from an exception that inherits from RuntimeException :)
A runtime exception is an exception that developers throw at the head of
their fellow developers to indicate that they should read the fucking
manual.
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.
as I said, I think it's time to take a look to the generated assembly code.
/Claes
regards,
Rémi
