Extend NMT to JDK native libraries?

Zhengyu Gu zgu at redhat.com
Wed Nov 21 14:54:37 UTC 2018

FYI: There was a phase 2 RFE: Native Memory Tracking (Phase 2)


On 11/21/18 9:28 AM, Thomas Stüfe wrote:
> Hi all,
> (yet again not sure if this is serviceablity-dev or not - I start at
> hs-dev, feel free to move this mail around.)
> Do we have any plans to extend NMT to cover native JDK libaries too?
> That would be a really cool feature.
> --
> We at SAP have done a similar thing in the past:
> We have a monitoring facility in our port which tracks C-heap
> allocations, non-imaginatively called "malloc statistic". This feature
> predates NMT somewhat - had we had NMT at that time, we would not have
> bothered. Our Malloc statistic is less powerful than NMT and
> implementation-wise completely at odds with it, so I never felt the
> urge to bring it upstream. However, one thing we did do is we extended
> its coverage to the JDK native code.
> This has been quite helpful in the past to find leaks in JDK, see
> e.g.: https://bugs.openjdk.java.net/browse/JDK-8155211
> We did this by exposing os::malloc, os::free etc from libjvm.so
> ("JVM_malloc", "JVM_realloc", "JVM_free"). In the JDK native code, we
> then either manually replaced calls to raw ::malloc(), ::free() etc
> with JVM_malloc(), JVM_free(). Or, in places where this was possible,
> we did this replacement stuff wholesale by employing a header which
> re-defined malloc(), free() etc JVM_malloc, JVM_free etc. Of course,
> we also had to add a number of linkage dependencies to the libjvm.so.
> All this is pretty standard stuff.
> One detail stood out: malloc headers are evil. In our experience, JDK
> native code was more difficult to control and "unbalanced
> malloc/frees" kept creeping in - especially with the
> wholesale-redefinition technique. Unbalanced mallocs/frees means cases
> where malloc() is instrumented but ::free() stays raw, or the other
> way around. Both combinations are catastrophic since os::malloc uses
> malloc headers. We typically corrupted the C-Heap and crashed, often
> much later in completely unrelated places.
> These types of bugs were very hard to spot and hence very expensive.
> And they can creep in in many ways. One example, there exist a
> surprising number of system APIs which return results in C-heap and
> require the user to free that, which of course must happen with raw
> ::free(), not os::free().
> We fixed this by not using malloc headers. That means a pointer
> returned by os::malloc() is compatible with raw ::free() and vice
> versa. The only bad thing happening would be our statistic numbers
> being slightly off.
> Instead of malloc headers we use a hand-groomed hash table to track
> the malloced memory. It is actually quite fast, fast enough that this
> malloc statistic feature is on-by-default in our port.
> --
> Of course, if we extend NMT to JDK native code we also would want to
> extend it to mmap() etc - we never did this with our statistic, since
> it only tracked malloc.
> What do you think? Did anyone else play with similar ideas? Would it
> be worth the effort?
> Cheers, Thomas

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