RFR(XS) 8191369: NMT: Enhance thread stack tracking

[Severin Gehwolf]

On Thu, 2017-11-16 at 08:14 +0100, Thomas Stüfe wrote:
> On Thu, Nov 16, 2017 at 7:58 AM, David Holmes <david.holmes at oracle.com>
> wrote:
> 
> > Hi Zhengyu,
> > 
> > I can't review this in detail as I'm not familiar with NMT workings, but
> > have a couple of comments.
> > 
> > On 16/11/2017 8:25 AM, Zhengyu Gu wrote:
> > 
> > > This is Linux only enhancement for now, can be extended for other
> > > platforms. (See bug for details)
> > > 
> > 
> > I'm concerned about unnecessary platform skew. I added some comments to
> > the bug. I think the mincore trick can be used on Solaris and AIX as well -
> > but not on OS X / BSD. It may be that VirtualQuery can be used on Windows
> > to get the same information - but I'm unclear if the memory states support
> > what you're looking for. It would be good to extend this to other platforms
> > that will support it.
> > 
> 
> Just 5c:
> 
> I think this is definitely useful even with only the Linux implementation
> provided.

+1

While plaform skew is a valid concern, I'm doubtful it's "unnecessary".
I think there is value to get one platform fixed and then look at other
platforms one by one as follow-up bugs. For some platforms it might be
hard to do (or undesirable) as Thomas says.

Thanks,
Severin

> On AIX we have commit-on-touch and hence mincore may actually force commit
> the thread :P - would have to try it but have no high hopes. Well.
> 
> On Windows this is possible, we already use VirtualQuery in our fork to
> print out the commit-state of the current thread stack in case of an error.
> 
> ..Thomas
> 
> 
> > And on a minor note can you please correct the existing spelling error in
> > get_stack_commited_bottom. :)
> > 
> > Current implementation, thread stack is tracked when thread is created,
> > > its available stack space is tagged as reserved and committed.
> > > 
> > > However, this is not how stack works. Stack pages are committed on
> > > demand, so this approach overstates its memory usage.
> > > 
> > > This enhancement gathers thread stack usage at NMT query time, so that it
> > > can report more accurate usage.
> > > 
> > > Bug: https://bugs.openjdk.java.net/browse/JDK-8191369
> > > Webrev: http://cr.openjdk.java.net/~zgu/8191369/webrev.00/index.html
> > > 
> > > 
> > > Example:
> > > 
> > > Summary shows thread stacks only use small faction of reserved space.
> > > 
> > > -                    Thread (reserved=41216KB, committed=632KB)
> > >                              (thread #41)
> > >                              (stack: reserved=41032KB, committed=448KB)
> > >                              (malloc=137KB #222)
> > >                              (arena=47KB #76)
> > > 
> > > 
> > > Detail tracking shows stack guards and actually used/committed stack area.
> > > 
> > > [0x00007f66e95e7000 - 0x00007f66e96e8000] reserved 1028KB for Thread
> > > Stack from
> > >      [0x00007f67c0b57d5c] JavaThread::run()+0x2c
> > >      [0x00007f67c08bbea2] thread_native_entry(Thread*)+0x112
> > > 
> > >          [0x00007f66e95e7000 - 0x00007f66e95eb000] committed 16KB from
> > >              [0x00007f67c08b7319] os::pd_create_stack_guard_pages(char*,
> > > unsigned long)+0x29
> > >              [0x00007f67c0b56851] JavaThread::create_stack_guard_pages()
> > > [clone .part.125]+0xa1
> > >              [0x00007f67c0b57d6e] JavaThread::run()+0x3e
> > >              [0x00007f67c08bbea2] thread_native_entry(Thread*)+0x112
> > > 
> > >          [0x00007f66e96e7000 - 0x00007f66e96e8000] committed 4KB
> > > 
> > 
> > Can we capture this in a test so we can tell that the tracking has
> > improved?
> > 
> > Thanks,
> > David
> > -----
> > 
> > Thanks,
> > > 
> > > -Zhengyu
> > > 
> > >