RFR: JDK-8322475: Extend printing for System.map

[Thomas Stuefe]

On Fri, 22 Dec 2023 13:08:40 GMT, Stefan Karlsson <stefank at openjdk.org> wrote:

>> This is an expansion on the new `System.map` command introduced with JDK-8318636.
>> 
>> We now print valuable information per memory region, such as:
>> 
>> - the actual resident set size
>> - the actual number of huge pages
>> - the actual used page size
>> - the THP state of the region (was advised, is eligible, uses THP, ...)
>> - whether the region is shared
>> - whether the region had been committed (backed by swap)
>> - whether the region has been swapped out.
>> 
>> Example output:
>> 
>> 
>> from                 to                       size        rss    hugetlb pgsz prot notes            vm info/file                                                                                                                                                                    
>> 0x00000000c0000000 - 0x00000000ffe00000 1071644672          0    4194304 2M   rw-p huge             JAVAHEAP /anon_hugepage                                                                                                                                                         
>> 0x00000000ffe00000 - 0x0000000100000000    2097152          0          0 2M   rw-p huge             JAVAHEAP /anon_hugepage                                                                                                                                                         
>> 0x0000558016b67000 - 0x0000558016b68000       4096       4096          0 4K   r--p                  /shared/projects/openjdk/jdk-jdk/output-fastdebug/images/jdk/bin/java
>> 0x0000558016b68000 - 0x0000558016b69000       4096       4096          0 4K   r-xp                  /shared/projects/openjdk/jdk-jdk/output-fastdebug/images/jdk/bin/java
>> 0x00007f3a749f2000 - 0x00007f3a74c62000    2555904    2555904          0 4K   rwxp                  CODE(CodeHeap 'profiled nmethods')                               
>> 0x00007f3a74c62000 - 0x00007f3a7be51000  119468032          0          0 4K   ---p nores            CODE(CodeHeap 'profiled nmethods')                               
>> 0x00007f3a7be51000 - 0x00007f3a7c1c1000    3604480    3604480          0 4K   rwxp                  CODE(CodeHeap 'profiled nmethods')                               
>> 0x00007f3a7c1c1000 - 0x00007f3a7c592000    4001792          0          0 4K   ---p nores            CODE(CodeHeap 'non-nmethods')                                    
>> 0x00007f3a7c592000 - 0x00007f3a7c802000    2555904    2555904          0 4K   rwxp                  CODE(CodeHeap 'non-profiled nmethods')                ...
>
> src/hotspot/os/linux/memMapPrinter_linux.cpp line 276:
> 
>> 274:   }
>> 275: 
>> 276:   FILE* f = os::fopen("/proc/self/smaps", "r");
> 
> We have seen that reading the smaps file can be extremely bad for the latency of the process. We've seen multi-seconds hangs because of external tools reading smaps. If we add a tool like this it would be good to add a big warning somewhere.

That is a good point, and I was afraid of that myself. 

I think I will revise this coding and give the command a "detail" mode, and read smaps (with a warning in the "cost" jcmd category) only in details mode. In normal mode, "maps" is sufficient. And in summary mode I can get probably away with reading stat.

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PR Review Comment: https://git.openjdk.org/jdk/pull/17158#discussion_r1435128384