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

[Stefan Karlsson]

On Tue, 19 Dec 2023 15:48:58 GMT, Thomas Stuefe <stuefe 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')                           
> 0x00007f3a7c802000 - 0x00007f3a839f200...

Just some initial comments after reading through the patch.

src/hotspot/os/linux/memMapPrinter_linux.cpp line 31:

> 29: #include "nmt/memMapPrinter.hpp"
> 30: #include "utilities/align.hpp"
> 31: #include "utilities/powerOfTwo.hpp"

This include section is not sorted correctly. It would also be good to add a blank line between the hotspot includes and the system includes.

src/hotspot/os/linux/memMapPrinter_linux.cpp line 206:

> 204:               _swapped_out(0), _hugetlb(0), _thp(0) {}
> 205: 
> 206:   void add_mapping(ProcMapsInfo& info) {

`info` could be const ref.

src/hotspot/os/linux/memMapPrinter_linux.cpp line 207:

> 205: 
> 206:   void add_mapping(ProcMapsInfo& info) {
> 207:     _num_mappings ++;

We tend to not put a space before `++` in HotSpot code.

src/hotspot/os/linux/memMapPrinter_linux.cpp line 250:

> 248:   st->print_cr("                     thpel: mapping is eligible for THP");
> 249:   st->print_cr("                    thpadv: mapping is THP-madvised");
> 250:   st->print_cr("                     nothp: mapping will not THP");

"mapping will not THP" sounds a bit weird.

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.

src/hotspot/share/nmt/memMapPrinter.cpp line 30:

> 28: #ifdef LINUX
> 29: 
> 30: #include "code/codeCache.hpp"

The include lines below are weirdly sorted. Could you fix it while you are changing this file?

-------------

PR Review: https://git.openjdk.org/jdk/pull/17158#pullrequestreview-1794452228
PR Review Comment: https://git.openjdk.org/jdk/pull/17158#discussion_r1435045106
PR Review Comment: https://git.openjdk.org/jdk/pull/17158#discussion_r1435037709
PR Review Comment: https://git.openjdk.org/jdk/pull/17158#discussion_r1435037943
PR Review Comment: https://git.openjdk.org/jdk/pull/17158#discussion_r1435040208
PR Review Comment: https://git.openjdk.org/jdk/pull/17158#discussion_r1435041926
PR Review Comment: https://git.openjdk.org/jdk/pull/17158#discussion_r1435042952