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Re: free() does not physically trim/reclaim memory
- From: Shuxin Yang <shuxinyang dot oss at gmail dot com>
- To: kreijack at inwind dot it
- Cc: libc-help at sourceware dot org, "Yichun Zhang (agentzh)" <agentzh at gmail dot com>
- Date: Thu, 01 Sep 2016 15:23:10 -0700
- Subject: Re: free() does not physically trim/reclaim memory
- Authentication-results: sourceware.org; auth=none
- References: <57C8724B.3020707@gmail.com> <2eda01d8-9c3d-914e-73ea-a593016c263d@inwind.it>
Hi, Goffredo:
Thank you very much for the speedy response. Please see the
following interleaving comment.
On 09/01/2016 02:14 PM, Goffredo Baroncelli wrote:
On 2016-09-01 20:24, shuxin yang wrote:
Hi, There:
My environment is Linux with "Ubuntu GLIBC 2.21-0ubuntu4" (this is what ldd --version gives).
In my application, I need to call mmap() a block right after BSS in order to prevent heap from growing.
Then, the subsequent "malloc(not-very-big-size)" is to carve a block from a mmap()-ed block.
It seems to me that the corresponding free() does not physically reclaim the memory unless I
explicitly call malloc_trim().
Could you please shed some light on this issue?
I reproduce the problem with the following snippet, and observe RSS size using command
"smem -P "a\.out"
Disclaimer: I am not a libc expert; I have only played a bit with your code.
1) free(3) calls malloc_trim(3) if the block to free is greater than 128K. See mallopt(3), and the M_TRIM_THRESHOLD parameter. If you set M_TRIM_THRESHOLD to 126K or you allocate block greater 128k, you will observer the behavior that you expect.
It somehow does not work at my side: none of following approaches works:
- invoke the example by : LLOC_TRIM_THRESHOLD_=$((126 * 1024)) ./a.out
- call 'mallopt(M_TRIM_THRESHOLD, 126 * 1024)' right after main() is
entered.
Am I missing something here?
If I set MALLOC_MMAP_THRESHOLD_ to 126k, then the memory can be
reclaimed immediately.
(As far as I can understand the code, it is because each malloc(127k)
ends up mmap() a "chunk"
flagged as "mapped chunk", when free() is called, "mapped chunk" can
be easily deallocated)
2) this is a minor thing: it is not sufficient to allocate memory with malloc, but you have also to access it in order to make a real allocation.
If you add a memset(p[i], 0, 127*1024); after the malloc(), you can see that the RSS go from about 17MB to about 500MB. In fact 4096*127*1024 = 508MB.
Yes, you are absolutely right!
Thanks
Shuxin