We have Oracle database hosted on this server. We are in doubt that actual memory used is 25 GB or 4 GB. Performance team is telling that database is over utilizing the memory of 25 GB. But DB team is telling that actual memory utilization is 4 GB and remaining 20+ GB is used by buffers / cached.
I have googled it and noticed that 25 GB is summation of 4+5+15 (close to 25). I referred Linux material and it says that memory in buffers and cached will improve the performance.
1.Can somebody explain whether actual memory used is 4 GB or 25 GB?
2.What is the use of buffers and cached?
3.Sometimes used value is peaking close to 31 GB from 25 GB. Mostly occupied by buffers and cached.
4.At a particular moment, -/+ buffers/cache: will be same. But Mem used will be more in node 1 and less in node 2. After particular hours, the Mem used will be shifting from node 1 to node 2. i.e., in node 1, Mem used will be 6 GB and Mem used in node 2 will be 25 GB approximately. Any body have any idea about this behaviour?
Hi user7509,
1. On node 1 the actual usage is 25 GiB. But only a small part is in use by Oracle and large parts by probably almost useless filesystem caching.
2. buffers and cached is part of free memory that is being used for buffering an caching. It remains however free memory. The moment it is needed, the most of the buffers and file system cache gets discarded. This cache is not the cache that Oracle uses.
3. See 2.
4. Usually the memory use of the oracle instances are largely determined by the SGA, which is used almost entirely for the database cache. This is shared memory allocation. Do you have single instance databases that sometimes run on one node and sometimes on the other? You can check the allocated chunks of shared memory by running.
as root or by checking the oracle init file.. The owner of the chunks is the oradba user that runs the database.
Filesystem cache will be of almost no use on a RAC cluster. You would probably be better off by enlarging the SGA of your databases so the memory is put to practical use, but no more than is useful of course.
S.
Last edited by Scrutinizer; 10-14-2009 at 07:40 PM..
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LEARN ABOUT PHP
free
FREE(1) User Commands FREE(1)NAME
free - Display amount of free and used memory in the system
SYNOPSIS
free [options]
DESCRIPTION
free displays the total amount of free and used physical and swap memory in the system, as well as the buffers and caches used by the ker-
nel. The information is gathered by parsing /proc/meminfo. The displayed columns are:
total Total installed memory (MemTotal and SwapTotal in /proc/meminfo)
used Used memory (calculated as total - free - buffers - cache)
free Unused memory (MemFree and SwapFree in /proc/meminfo)
shared Memory used (mostly) by tmpfs (Shmem in /proc/meminfo)
buffers
Memory used by kernel buffers (Buffers in /proc/meminfo)
cache Memory used by the page cache and slabs (Cached and SReclaimable in /proc/meminfo)
buff/cache
Sum of buffers and cache
available
Estimation of how much memory is available for starting new applications, without swapping. Unlike the data provided by the cache or
free fields, this field takes into account page cache and also that not all reclaimable memory slabs will be reclaimed due to items
being in use (MemAvailable in /proc/meminfo, available on kernels 3.14, emulated on kernels 2.6.27+, otherwise the same as free)
OPTIONS -b, --bytes
Display the amount of memory in bytes.
-k, --kibi
Display the amount of memory in kibibytes. This is the default.
-m, --mebi
Display the amount of memory in mebibytes.
-g, --gibi
Display the amount of memory in gibibytes.
--tebi Display the amount of memory in tebibytes.
--pebi Display the amount of memory in pebibytes.
--kilo Display the amount of memory in kilobytes. Implies --si.
--mega Display the amount of memory in megabytes. Implies --si.
--giga Display the amount of memory in gigabytes. Implies --si.
--tera Display the amount of memory in terabytes. Implies --si.
--peta Display the amount of memory in petabytes. Implies --si.
-h, --human
Show all output fields automatically scaled to shortest three digit unit and display the units of print out. Following units are
used.
B = bytes
K = kibibyte
M = mebibyte
G = gibibyte
T = tebibyte
P = pebibyte
If unit is missing, and you have exbibyte of RAM or swap, the number is in tebibytes and columns might not be aligned with header.
-w, --wide
Switch to the wide mode. The wide mode produces lines longer than 80 characters. In this mode buffers and cache are reported in two
separate columns.
-c, --count count
Display the result count times. Requires the -s option.
-l, --lohi
Show detailed low and high memory statistics.
-s, --seconds delay
Continuously display the result delay seconds apart. You may actually specify any floating point number for delay using either .
or , for decimal point. usleep(3) is used for microsecond resolution delay times.
--si Use kilo, mega, giga etc (power of 1000) instead of kibi, mebi, gibi (power of 1024).
-t, --total
Display a line showing the column totals.
--help Print help.
-V, --version
Display version information.
FILES
/proc/meminfo
memory information
BUGS
The value for the shared column is not available from kernels before 2.6.32 and is displayed as zero.
Please send bug reports to
<procps@freelists.org>
SEE ALSO ps(1), slabtop(1), top(1), vmstat(8).
procps-ng 2016-06-03 FREE(1)