NMT does not seem to count malloc correctly

[Zhengyu Gu]

By commenting out ::free(), you created real memory leak, but not from 
NMT perspective, because it already adjusted internal counting to 
reflect os:free().

-Zhengyu



On 8/6/20 11:28 AM, Thomas Stüfe wrote:
> Hi,
> 
> maybe I am missing something fundamental, or there is something wrong with
> NMT.
> 
> As an experiment for an unrelated issue, I created a C-Heap leak by simply
> commenting out the ::free() call inside os::free():
> 
> --- a/src/hotspot/share/runtime/os.cpp  Thu Aug 06 17:08:02 2020 +0200
> +++ b/src/hotspot/share/runtime/os.cpp  Thu Aug 06 17:08:08 2020 +0200
> @@ -804,7 +804,7 @@
>     size_t size = guarded.get_user_size();
>     inc_stat_counter(&free_bytes, size);
>     membase = guarded.release_for_freeing();
> -  ::free(membase);
> +//  ::free(membase);
>   #else
>     void* membase = MemTracker::record_free(memblock,
> MemTracker::tracking_level());
>     ::free(membase);
> 
> As expected, this leads quickly to a very high memory usage. In my tiny
> example (which really does not matter) I quickly end up with ~4G RSS, and
> the glibc reports about 3G outstanding allocations:
> 
> Process Memory:
> Virtual Size: 11136852K (peak: 11136852K)
> Resident Set Size: 4057860K (peak: 4057860K) (anon: 4013076K, file: 44784K,
> shmem: 0K)
> Swapped out: 0K
> C-Heap outstanding allocations: 3070399K
> 
> 
> However, NMT reports just about 600M committed memory (into this number
> should fall all outstanding allocations done with os::malloc() AFAICS):
> 
> thomas at mainframe:~$ jjjcmd Interl VM.native_memory   scale=1
> 
> 19441:
> 
> Native Memory Tracking:
> 
> Total: reserved=6227876720, committed=626285424
> 
> ....
> 
> And the malloc information adds up to about only 68M:
> 
> thomas at mainframe:~$ jjjcmd Interl VM.native_memory   scale=1  | grep malloc
>                              (malloc=4044913 #55107)
>                              (malloc=38688 #146)
>                              (malloc=10812522 #258557)
>                              (malloc=18157899 #3629)
>                              (malloc=42496 #311)
>                              (malloc=216 #7)
>                              (malloc=594920 #1788)
>                              (malloc=27304 #2)
>                              (malloc=2781872 #60716)
>                              (malloc=543008 #5144)
>                              (malloc=200416)
>                              (malloc=152 #7)
>                              (malloc=6788 #214)
>                              (malloc=18977 #485)
>                              (malloc=994280 #2797)
>                              (malloc=153248 #744)
>                              (malloc=2152 #12)
> 
> I must be missing something obvious here. What is it?
> 
> Thank you,
> 
> Thomas
>