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 <a...@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.
