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

[Andrew Haley]

Ha
On 09/10/2015 02:17 AM, David Holmes wrote:
> On 10/09/2015 12:01 AM, Andrew Haley wrote:
>> On 09/09/2015 02:55 PM, David M. Lloyd wrote:
>>> On 09/09/2015 08:17 AM, Andrew Haley wrote:
>>>> On 09/09/2015 02:13 PM, David M. Lloyd wrote:
>>>>> How would you hook into the safepoint to perform the unmapping?  You'd
>>>>> have to wait for all threads to arrive at safepoints, wouldn't you?
>>>>
>>>> Yes.  That happens in the VM already, e.g. when we need to revoke the
>>>> bias of a lock.  It's a well-established mechanism.
>>>
>>> Could you extend the technique to other native resources as well?
>>
>> Yes, but of course there will be some back-pressure from HotSpot
>> developers: it requires changes to the VM, and you have to be
>> extremely careful not to cause a deadlock.
> 
> Adding a new JVM_* function that requests a safepoint VM operation be 
> executed is not a major concern in my opinion**. Of more of a concern is 
> the cost of doing a stop-the-world safepoint in the kinds of 
> environments being described. How frequently are these unmap operations 
> needed?

That's an interesting point.  I wonder if it might make sense to piggyback
on a safepoint which would have happened anyway.  But having said that, we
seem to be quite happy to safepoint for biased lock revocation and all
manner of other things.

Andrew.