Suggested fix for JDK-4724038 (Add unmap method to MappedByteBuffer)
aph at redhat.com
Thu Sep 10 07:59:47 UTC 2015
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
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.
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