RFR: 8207851 JEP Draft: Support ByteBuffer mapped over non-volatile memory (original) (raw)

Andrew Dinn adinn at redhat.com
Wed Aug 15 11:08:00 UTC 2018

Hi Alan,

Just a quick follow-up regarding the BufferPoolMXBean.

On 14/08/18 12:34, Andrew Dinn wrote:

On 06/08/18 10:29, Alan Bateman wrote:

Are you familiar with BufferPoolMXBean which can be used by management tools to monitor pool of buffers? I'm wondering if we should introduce a new pool for NVM so that it doesn't show up in the "mapped" pool.

Ok, so I have worked through the code and can see that MappedByteBuffers for file device-based mappings (as opposed to direct memory mappings) are currently counted by class Unmapper local to FileChannelImpl and the counts are made visible via a BufferPoolMXBean with name "mapped" which is present in the list exposed by method getBufferPoolMXBeans() of class ManagementFactoryHelper.

So, it seems what you are asking for is to keep separate tallies for persistent MappedByteBuffers and expose them in a new BufferPoolMXBean also inserted in the list exposed by getBufferPoolMXBeans. That sounds like quite a good idea. It will leave any current code that wants to see file mappings counting the 'same' thing yet still makes it possible to count persistent mappings on their own and also tally all mappings by iterating over all BufferPoolMXBeans in the list. I suggest giving the bean the name "mapped_persistent".

I would happily update the JEP to include this as an 'API change' of sorts but I'm not quite sure how to document it. Is this ok

the JEP requires exposing a new java.lang.management.BufferPoolMXBean with name "mapped_persistent" as a platform managed bean which can be accessed using the existing ManagementFactory.getPlatformMXBeans() API

the "mapped_persistent" bean records the buffer count, total bytes and total allocation count stats for persistent MappedByteBuffers (i.e. those mapped using MapMode READ_ONLY_PERSISTENT or READ_WRITE_PERSISTENT)

these stats are collected separately from those currently collected in the "mapped" BufferPoolMXBean which records the equivalent counts for non-persistent MappedByteBuffers (i.e. those mapped using MapMode READ_ONLY, READ_WRITE or PRIVATE)

It seems straightforward to implement this behaviour. If you like I can add it to the next webrev?

regards,

Andrew Dinn

Senior Principal Software Engineer Red Hat UK Ltd Registered in England and Wales under Company Registration No. 03798903 Directors: Michael Cunningham, Michael ("Mike") O'Neill, Eric Shander

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