[Bug malloc/30769] malloc_trim is not working correctly for arenas other than arena 0

["shubhankargambhir013 at gmail dot com" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=30769

Shubhankar Gambhir <shubhankargambhir013 at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |shubhankargambhir013@gmail.
                   |                            |com

--- Comment #2 from Shubhankar Gambhir <shubhankargambhir013 at gmail dot com> ---
In the original example (mem.cpp) given, the MMAP_THRESHOLD is 128KB (by
default) in starting and can increase upto 64MB. In first iteration the blocks
are mmaped because they are above threshold, now When you free the first 18MB
block the MMAP_THRESHOLD is adjusted, so now from 2nd iteration the blocks will
be allocated in arenas, and when they are freed they will be added to malloc
cache and not given back to os. 

When you set M_TOP_PAD (or M_TRIM_THRESHOLD, M_MMAP_THRESHOLD, M_MMAP_MAX)
MMAP_THRESHOLD is disabled, so the blocks from 2nd iterations also get mmaped
and memory is given back to os when they are freed.

Further malloc_trim calls madvise(DONT_NEED) on free pages in arenas, this
causes pages to be on demand loaded on next access, and the way your program
calculates rss is through proc/self/stat. I can see following description in
man documentation.

```
number of pages the process has in real memory.  This is just  the  pages
which  count  toward text, data, or stack space.  This does not include pages
which have not been  demand-loaded  in,  or  which  are  swapped  out.   This 
value  is  inaccurate;
```

-- 
You are receiving this mail because:
You are on the CC list for the bug.