[CentOS] RSS usage on centos8.4 is higher for user-application compared to centos7.9

On 06.07.21 15:02, Anand Babu wrote:
> Hi Centos Community,
> 
> This is my first time here and I apologize in advance if I made a mistake
> here and hope you will correct me , if i made any.
> 
> 
> With that said,
> 
> This is a native example that was written to narrow down the higher RAM
> usage that we see when we are using Centos8 as opposed to Centos7.
> 
> The c-code is very small and looks like this :
> 
> #include<stdio.h>
> 
> #include<unistd.h>
> 
> int main()
> 
> {
> 
> pause();
> 
> return 0;
> 
> }
> 
> I compile and run the binary on centos7 and memory backed section of pmap
> output looks like below:
> 
> pmap -X $(pidof sleep)  | head -n -2 | awk '{ if (NR > 2 && $5 > 0  )
> printf "%12s %8s %8s %4s %s\n",  $1, $6, $7, $2,$13}'
>      00400000        4        4 r-xp sleep
>      00600000        4        4 r--p sleep
>      00601000        4        4 rw-p sleep*7f54514f9000     1808
> 204 r-xp libc-2.17.so <http://libc-2.17.so>
> *7f54516bd000     2044        0 ---p libc-2.17.so
> 7f54518bc000       16       16 r--p libc-2.17.so
> 7f54518c0000        8        8 rw-p libc-2.17.so
> 7f54518c7000      136      108 r-xp ld-2.17.so
> 7f5451ae8000        4        4 r--p ld-2.17.so
> 7f5451ae9000        4        4 rw-p ld-2.17.so
> 
> and ps says the following:
> 
> ** ps -o rss= pidof sleep 352**
> 
> Running the same binary on centos8 leads to
> 
> ** ps -o rss= pidof pause 784 **
> 
> and the pmap output looks like below:
> 
>      00400000        4        4 r-xp pause
>      00600000        4        4 r--p pause
>      00601000        4        4 rw-p pause* 7f24029a8000     1776
> 788 r-xp libc-2.28.so <http://libc-2.28.so> *
> 7f2402b64000     2044        0 ---p libc-2.28.so
> 7f2402d63000       16       16 r--p libc-2.28.so
> 7f2402d67000        8        8 rw-p libc-2.28.so
> 7f2402d6d000      176      176 r-xp ld-2.28.so
> 7f2402f99000        4        4 r--p ld-2.28.so
> 7f2402f9a000        8        8 rw-p ld-2.28.so
> 
> For running the same executable, the libc.so takes 788KB(204KB on centos7).
> 
> Note:
> 
>     1.
> 
>     This is not the only library that is showing this behavior, but we see
>     the same behavior for other shared-library as well as executables that were
>     compiled on centos7. Running the same executables/shared objects take a
>     higher amount of pages on centos8 than on centos7.
>     2.
> 
>     Since the glibc version on Centos8 was 2.28 , i have compiled 2.17
>     version and then used patchelf to patch the centos7 built binary to make
>     use of 2.17 glibc on centos8 host like patchelf --set-interpreter
> 
> /home/babu/RSS_measurement/2_17_downloaded/glibc_home/lib/ld-linux-x86-64.so.2
> --set-rpath /home/babu/RSS_measurement/2_17_downloaded/glibc_home/lib
> /tmp/pause
> 
> and then run it under LD_DEBUG=libs and verified the 2.17 version of libc
> libraries were used and still the memory usage on centos8 was higher than
> on centos7 (about the same numbers as above).
> 
> What could be the reason for the higher RAM usage on centos8 vs centos7? I
> can imagine some defaults have changed on centos8 and that has meant this
> impact and i could change this default and could reproduce the same RSS
> numbers on centos8 as well?
> 
> 

How does the results look like after doing:

echo never   > /sys/kernel/mm/transparent_hugepage/enabled

--
Leon