Proposal: Extend Native Memory Tracking across the whole process via interposition

Tue Dec 5 14:35:29 UTC 2023

> If it is just about using a standard replacement like jemalloc.

>From my experience, and what I believe Johan was asking as well, is indeed
that.

Deployment of workloads that need that, usually rely on "installing" an
allocator library that is configured via `LD_PRELOAD`. This usually gives
the option to change the allocator depending on multiple criteria : the
workload itself, the CPU architecture. Sometimes jemalloc is better,
sometimes tcmalloc is better, (not tried minimalloc), so the flexibility to
tweak that is important.
_All are better than glibc's malloc (arena "recycling" is quite bad in
containerized envs and with multiple threads, leading to many dirty pages
and higher RSS)._

So that's why I was envisioning a "standard" use of the preload ability of
the linker, e.g. `LD_PROLOAD=path/to/jdk/lib/libjnmt.so
/path/to/tcmalloc.so`.
...assuming it can work.

-- Brice

On Tue, Dec 5, 2023 at 1:50 PM Thomas Stuefe <tstuefe at redhat.com> wrote:

> Hi Brice,
>
> On Tue, Dec 5, 2023 at 12:49 AM Brice Dutheil <brice.dutheil at gmail.com>
> wrote:
>
>> Hi Joha,
>>
>> Thomas will correct me as he is proposed the idea and much more
>> experienced, also I'm a mere reader of this ML.
>>
>> So, I have not toyed with the code, but I believe this should work, at
>> least on linux if linker has no restrictions.
>>
>> Typically interception happens because there is a function with the right
>> signature preloaded (via `LD_PRELOAD`) that linker will look up. The
>> magic can work because in order to do real work and invoke the right
>> methods down the line using `dlsym(RTLD_NEXT, name)`. And that should be
>> the next library on the path or the system as the linker should process
>> from left to right this `LD_PRELOAD`.
>>
>> ```
>> void *malloc(size_t size) {
>>   void *(*p_malloc)(size_t) = dlsym(RTLD_NEXT, "malloc");
>>
>>   // report back mem operation
>>
>>   return p_malloc(size);
>> }
>> ```
>>
>> https://man7.org/linux/man-pages/man8/ld.so.8.html
>> https://www.man7.org/linux/man-pages/man3/dlsym.3.html
>>
>> That said this might be tricky to avoid loops, if one function calls
>> `malloc`.
>>
>
> I think a simpler way would be to just add a way for libjnmt.so to use
> custom allocators. If it is just about using a standard replacement like
> jemalloc, a custom-tailored solution for that would be a lot simpler. But,
> again, not sure about the use case.
>
> Cheers, Thomas
>
>
>>
>> Also I suppose this could work on macos via `DYLD_PRELOAD` but unsure
>> since macos has some restrictions.
>>
>> --
>> Brice
>>
>>
>> On Mon, Dec 4, 2023 at 13:14 Johan Sjölén <johan.sjolen at oracle.com>
>> wrote:
>>
>>> Hi Thomas,
>>>
>>> If a user would like to switch out the malloc which a JVM is using,
>>> would they be able to do that while simultaneously using your interception
>>> library?
>>>
>>> Thank you,
>>> Johan
>>>
>>> Hi, community,
>>>
>>> I experimented with extending Native Memory Tracking across the whole
>>> process. I want to share my findings and propose a new JDK feature to allow
>>> us to do that.
>>>
>>> TL;DR
>>>
>>> Proposed is a "native memory interposition library" shipped with the JDK
>>> that would intercept all native memory calls from everywhere and redirect
>>> them to NMT.
>>>
>>> Motivation:
>>>
>>> NMT is very useful but limited in its coverage. It only covers Hotspot
>>> and a select few sites from the JDK. Most of the JDK, third-party native
>>> code, and system libraries are not covered. This is a large hole in our
>>> observability. I have seen people do (and done myself! eg [1]) strange and
>>> weird things to hunt memory leaks in native code. This is especially tricky
>>> in locked-down customer scenarios.
>>>
>>> But NMT is a capable tracker. We could use it for much more than just
>>> tracking Hotspot.
>>>
>>> In the past, developers have attempted to extend NMT instrumentation
>>> over parts of the JDK (e.g. [2]), which met resistance from Oracle. This is
>>> understandable: a naive extension would require libraries to link against
>>> the libjvm and instrument their coding. That introduces new dependencies
>>> nobody wants.
>>>
>>> ---
>>>
>>> I propose a different way that works without instrumenting any caller
>>> code. I hope this proposal proves less controversial than brute-force NMT
>>> instrumentation of the JDK. And it would allow introspection of non-JDK
>>> parts too.
>>>
>>> We could ship an interception library (a "libjnmt.so") within the JDK.
>>> That library, if preloaded, would redirect native memory requests to NMT. A
>>> customer who wants to analyze the native memory footprint of its apps could
>>> start the JVM with LD_PRELOAD=libjnmt and then use NMT for
>>> introspection.
>>>
>>> Oracle and we continuously improve NMT; extending its reach across the
>>> whole process would leverage that investment nicely.
>>>
>>> It also meshes well with other improvements. For example, we report NMT
>>> numbers via JFR since [4] - with interposition, we could now expose
>>> third-party native allocations via JFR. The new jcmd "System.map" would
>>> automatically show memory mappings from outside Hotspot. There is a
>>> precedent (libjsig), so shipping interposition libraries is not that
>>> strange.
>>>
>>> ---
>>>
>>> I have a Linux-based POC that works and looks promising [3]. With that
>>> prototype, I can see:
>>>
>>> - allocations from the JDK - e.g., now I finally see mapped byte buffers.
>>> - allocations from third-party user code
>>> - most allocations from system libraries, e.g., from the system zlib
>>> - allocations via the new FFI interface
>>>
>>> The prototype tracks both mmap and malloc. Technically, the tricky part
>>> was to handle the initialization window: being able to correctly handle
>>> allocations starting at the process C++ initialization while dynamically
>>> handing over allocations to the libjvm once it is loaded and NMT is
>>> initialized. Another tricky problem was to prevent circularities stemming
>>> from call intercepting. The prototype solves these problems and is already
>>> stable enough to be used.
>>>
>>> Note that the patch is not complex or large. Some small interaction with
>>> the JVM is needed, though, so this cannot be done just with an outside
>>> library.
>>>
>>> The prototype was developed and tested on Linux x64 and with glibc 2.31.
>>> It seems stable so far, but of course, the work is in an early stage, and
>>> bugs may exist. If you want to play with the prototype, build it [3] and
>>> then call:
>>>
>>> LD_PRELOAD=${JDK_DIR}/lib/server/libjnmt.so ${JDK_DIR}/bin/java
>>> -XX:NativeMemoryTracking=detail <program> <args>
>>>
>>> Example: quarkus with "third-party code" injected that leaks
>>> periodically [5]:
>>>
>>> LEAK_MALLOC=1 LEAK_MMAP=1 LD_PRELOAD=${JDK_DIR}/lib/server/libjnmt.so
>>> ${JDK_DIR}/bin/java -agentpath:/shared/projects/jvmti-leak/leaker.so
>>> -XX:NativeMemoryTracking=detail -jar ./quarkus-profiling-workshop/
>>> target/quarkus-app/quarkus-run.jar
>>>
>>> In Summary mode, we see the slowly growing leaks:
>>>
>>> -External (via interposition) (reserved=82216KB, committed=82216KB)
>>>                             (malloc=81588KB #585) (at peak)
>>>                             (mmap: reserved=628KB, committed=628KB, at
>>> peak)
>>>
>>>
>>> and in Detail mode, their call stacks:
>>>
>>> [0x00007ff067ee7000 - 0x00007ff067ee8000] reserved and committed 4KB for
>>> External (via interposition) from
>>>     [0x00007ff067ef5056]the_mmap(void*, unsigned long, int, int, int,
>>> long)+0x66 in libjnmt.so
>>>     [0x00007ff067ef5781]mmap+0x71 in libjnmt.so
>>>     [0x00007ff067ee955a]leak_mmap+0x3f in leaker.so
>>>     [0x00007ff067ee95b1]leakleak+0x1c in leaker.so
>>>     [0x00007ff067ee95c6]leakleakleak+0x12 in leaker.so
>>>     [0x00007ff067ee95db]leakabit+0x12 in leaker.so
>>>     [0x00007ff067ee95f8]leaky_thread+0x1a in leaker.so
>>>
>>>
>>> [0x00007ff067ef5166]the_malloc(unsigned long)+0x106 in libjnmt.so
>>> [0x00007ff067ee94ae]do_malloc+0xb8 in leaker.so
>>> [0x00007ff067ee9518]leak_malloc+0x20 in leaker.so
>>> [0x00007ff067ee95a7]leakleak+0x12 in leaker.so
>>> [0x00007ff067ee95c6]leakleakleak+0x12 in leaker.so
>>> [0x00007ff067ee95db]leakabit+0x12 in leaker.so
>>> [0x00007ff067ee95f8]leaky_thread+0x1a in leaker.so
>>>                              (malloc=17679KB type=External (via
>>> interposition) #34) (at peak)
>>>
>>> ---
>>>
>>> What about MEMFLAGS?
>>>
>>> The prototype does not extend MEMFLAGS apart from introducing a new
>>> "External" category that tracks allocations done via interposition. The
>>> question of MEMFLAGS - in particular, opening it up to outside extension -
>>> has been contentious. It is orthogonal to this proposal - nice but not
>>> required.
>>>
>>> This proposal makes external allocations visible under the new
>>> "External" tag:
>>> - in NMT summary mode, we only have the "External" total, which is
>>> already useful even as a lump sum: it shows the footprint non-hotspot
>>> libraries contribute to RSS. An RSS increase that is reflected neither by
>>> hotspot allocations nor by "External" can only stem from a select few
>>> places, e.g. from libc malloc retention.
>>> - In NMT detail mode, this proposal shows us the call stacks to foreign
>>> call sites, pinpointing at least the libraries involved.
>>>
>>> --
>>>
>>> What do you think, does this make sense?
>>>
>>> Thanks, Thomas
>>>
>>>
>>> [1] https://github.com/SAP/SapMachine/wiki/SapMachine-MallocTracer
>>> [2]
>>> https://mail.openjdk.org/pipermail/core-libs-dev/2022-November/096197.html
>>> [3] https://github.com/tstuefe/jdk/tree/libjnmt
>>> [4] https://bugs.openjdk.org/browse/JDK-8157023
>>> [5] https://github.com/tstuefe/jvmti_leak
>>>
>>>
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