Suggested fix for JDK-4724038 (Add unmap method to MappedByteBuffer)

[Peter Levart]

Hi guys,

Perhaps there's no need for this protection/trap dance. If the situation 
is never tripped in correct programs (that unmap only after noone is 
using the buffers any more), then checking for address and throwing in 
case it is equal to some guard value is a never taken branch that is 
predicted perfectly. I wrote this little benchmark to test this claim:

@BenchmarkMode(Mode.AverageTime)
@Fork(value = 1, warmups = 0)
@Warmup(iterations = 5)
@Measurement(iterations = 10)
@OutputTimeUnit(TimeUnit.NANOSECONDS)
@State(Scope.Benchmark)
public class MappedBufferBench {

     private ByteBuffer bb;

     @Setup(Level.Trial)
     public void setup() {
         bb = ByteBuffer.allocateDirect(64);
     }

     @Benchmark
     public int directBufferGet() {
         int sum = 0;
         for (int i = 0; i < 64; i++) {
             sum += bb.get(i);
         }
         return sum;
     }
}


The results are:

Original:

Benchmark                                 Mode   Samples Score  Score 
error    Units
j.t.MappedBufferBench.directBufferGet     avgt        10 17.740        
0.247    ns/op

Patched:

Benchmark                                 Mode   Samples Score  Score 
error    Units
j.t.MappedBufferBench.directBufferGet     avgt        10 17.796        
0.220    ns/op



What did I patch? There's a private method in DirectByteBuffer to 
convert index to address:

Original:

     private long ix(int i) {
         return address + (i << 0);
     }

Patched:

     private long ix(int i) {
         long a = address;
         if (a == 0L) throw new IllegalStateException();
         return a + (i << 0);
     }



That's not all that has to be done of course. There would still have to 
be a wait for safe-point to return before unmapping. This is just a 
demonstration that maybe guarding mapping with protection is not needed.


Regards, Peter

On 09/10/2015 04:37 PM, David M. Lloyd wrote:
> Or, the Java methods which wrap this access can just catch NPE and 
> throw the new exception type.
>
> On 09/10/2015 09:35 AM, Vitaly Davidovich wrote:
>> Well, you'd probably want something other than NPE here -- perhaps a new
>> dedicated exception to signal this condition.  And this means the 
>> segfault
>> handling now needs to know about this type of situation as well, rather
>> than just NPEs.
>>
>> On Thu, Sep 10, 2015 at 10:32 AM, Andrew Haley <aph at redhat.com> wrote:
>>
>>> On 09/10/2015 03:26 PM, Vitaly Davidovich wrote:
>>>> Yes, so what happens when that guard page is accessed by a thread 
>>>> after
>>> safepoint?
>>>
>>> A segfault and a null pointer exception.
>>>
>>> Andrew.
>>>
>>>
>