On 09/11/2015 12:42 AM, Vitaly Davidovich wrote:
There's still a race condition since someone may have passed the
guard, entered a safepoint, and then proceeded to access the memory.
Yeah, and I realized yesterday after posting that this wouldn't work for
views of the buffer (dulicate(), slice(), asXxxBuffer()) since they copy
over the address to their instance field, so multiple instances point to
the same block of memory with their own address fields...
Regards, Peter
sent from my phone
On Sep 10, 2015 6:29 PM, "Peter Levart" <peter.lev...@gmail.com
<mailto:peter.lev...@gmail.com>> wrote:
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 <mailto: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.
