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3.16 kernel using available memory, 4.5 kernel not

PostPosted: 2016-08-04 11:34
by dotlj
I recently upgraded the memory in one machine using backports kernel and noticed that it's only using 2.6GB of the memory.

I'm used to kernel's using most of the memory for caching. Here's a comparison of two machines running Debian stable - one using 3.16 kernel and one using 4.5 kernel

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$ uname -vr
3.16.0-4-amd64 #1 SMP Debian 3.16.7-ckt25-2 (2016-04-08)
$ top
top - 21:21:34 up 108 days, 11:11, 17 users,  load average: 0.23, 0.21, 0.17
Tasks: 290 total,   1 running, 288 sleeping,   0 stopped,   1 zombie
%Cpu(s):  1.0 us,  0.4 sy,  0.0 ni, 98.6 id,  0.0 wa,  0.0 hi,  0.0 si,  0.0 st
KiB Mem:  16306084 total, 15959684 used,   346400 free,   400096 buffers
KiB Swap: 15622140 total,     1020 used, 15621120 free. 12279592 cached Mem
$ free -h
             total       used       free     shared    buffers     cached
Mem:           15G        15G       337M       643M       390M        11G
-/+ buffers/cache:       3.1G        12G


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$ uname -vr
4.5.0-0.bpo.2-amd64 #1 SMP Debian 4.5.4-1~bpo8+1 (2016-05-13)
$ top
top - 11:23:15 up  6:17,  5 users,  load average: 0.34, 0.18, 0.15
Tasks: 229 total,   2 running, 226 sleeping,   0 stopped,   1 zombie
%Cpu(s):  2.3 us,  0.7 sy,  0.0 ni, 96.9 id,  0.0 wa,  0.0 hi,  0.0 si,  0.0 st
KiB Mem:  32856480 total,  2690676 used, 30165804 free,    75472 buffers
KiB Swap:  3997692 total,        0 used,  3997692 free.  1141724 cached Mem
$ free -h
             total       used       free     shared    buffers     cached
Mem:           31G       2.6G        28G        50M        73M       1.1G
-/+ buffers/cache:       1.4G        29G
Swap:         3.8G         0B       3.8G


Is this a difference in the way the kernel reports memory use or the 4.5 kernel just not using available memory?

Re: 3.16 kernel using available memory, 4.5 kernel not

PostPosted: 2016-08-05 01:26
by sunrat
I would suggest the difference is that your old kernel was running 108 days and your new one only 6 hours. Run your new kernel for 108 days and then see how much is cached.

Re: 3.16 kernel using available memory, 4.5 kernel not

PostPosted: 2016-08-05 06:14
by dotlj
Many thanks. I was wondering about the uptime. I'll find out at the next reboot for the 3.16 kernel.

Re: 3.16 kernel using available memory, 4.5 kernel not

PostPosted: 2016-08-10 12:25
by dotlj
I've been watching some videos on an external usb drive using the 4.6.0 bpo kernel (upgraded from 4.5.0). After watching the videos,
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$ free -h
             total       used       free     shared    buffers     cached
Mem:           31G        28G       2.6G        30M        14G        13G
-/+ buffers/cache:       1.1G        30G
Swap:         3.8G         0B       3.8G


After removing the disk,
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$ free -h
             total       used       free     shared    buffers     cached
Mem:           31G       1.8G        29G        26M        70M       742M
-/+ buffers/cache:       1.0G        30G
Swap:         3.8G         0B       3.8G


Whatever I do with the older 3.16.7 kernel, the used is close to all and the free is very low.