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List:       redhat-list
Subject:    Use nocache program to bypass page cache
From:       Yong Huang <yong321 () yahoo ! com>
Date:       2021-07-15 18:51:11
Message-ID: 1883652738.251328.1626375071861 () mail ! yahoo ! com
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Has anybody tested the nocache program?https://github.com/Feh/nocache
My test (on RHEL 8.3) shows it's doing what it claims:
$ grep Cache /proc/meminfo
Cached:         14119024 kB
SwapCached:          544 kB
$ grep Cache /proc/meminfo
Cached:         14119044 kB
SwapCached:          544 kB
$ ./nocache cp $ORACLE_HOME/bin/oracle .
$ grep Cache /proc/meminfo
Cached:         14119308 kB
SwapCached:          544 kB
$ cp $ORACLE_HOME/bin/oracle .
$ grep Cache /proc/meminfo
Cached:         14553412 kB
SwapCached:          544 kB
$ ll oracle
-rwxr-x--x 1 oracle oinstall 444390704 Jul 15 13:34 oracle
Basically, I copied a 400 MB file (19c Oracle server binary $ORACLE_HOME/bin/oracle). \
Using nocache, Page Cache size barely increased, probably due to fluctuations on this \
live server, and partly due to metadata usage of this cp command. But not using \
nocache, Page Cache usage went up by about the size of this binary file. I plan to \
use this program on many of our servers (just two files, nocache and nocache.so, are \
really needed on other servers once you make'd on one server running this version of \
OS). I think this is better than trying to mount a file system with direct I/O \
support (if you finally get it supported), because using this program you still \
generally benefit from Page Cache and consciously make a decision when running a \
specific program whether to cache the file content. I wish Red Hat officially \
supported this program, or one similar to this. What do you all think?

Yong
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