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

Tue Dec 5 15:12:24 UTC 2023

On Tue, Dec 5, 2023 at 3:36 PM Brice Dutheil <brice.dutheil at gmail.com>
wrote:

> > 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)._
>
>
I always wondered how much of that is urban legend. I measured myself a
while ago (maybe I can dig up the results somewhere), and IIRC, I could
produce artificial scenarios with way more overhead for the glibc case, but
in the practical cases, it seemed not to matter. I even saw cases where
glibc was better.

In any case, let us not derail this discussion. If jemalloc compatibility
is required, I don't think it would be a show-stopper.

> 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
>>>>
>>>>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://mail.openjdk.org/pipermail/jdk-dev/attachments/20231205/c26de83b/attachment-0001.htm>