What is equivalent to Mac's purge in Linux?

Question

In Mac I use purge to free up some memory. What is equivalent to it in Linux(Ubuntu Server)? apt-get install purge gave me nothing. If you are no familiar with Mac's purge here is it's man page:

purge(8)                  BSD System Manager's Manual                 purge(8)

NAME
     purge -- force disk cache to be purged (flushed and emptied)

SYNOPSIS
     purge

DESCRIPTION
     Purge can be used to approximate initial boot conditions with a cold disk
     buffer cache for performance analysis. It does not affect anonymous mem-
     ory that has been allocated through malloc, vm_allocate, etc.

SEE ALSO
     sync(8), malloc(3)

                              September 20, 2005

What makes you think you need to? Linux has pretty good memory management, it should be able to do this on a need-to-do basis. — terdon, Jun 27 '14 at 18:05
Note that freeing up memory like this is used to make the machine go slower, not faster. Its if you want to benchmark something on a cold cache. — derobert, Jun 27 '14 at 18:14
I have a Java process eating all of my 24GB RAM. I can't terminate it. I don't know how to open a little space for other processes? — Mohsen, Jun 27 '14 at 18:37
@Mohsen, If the java process has already eaten up all of your ram, chances are that the kernel has already dropped all caches to give more memory to java. Purging, or dropping caches, won't give you any more free memory of a process has already demanded all of it. — daboross, Jun 27 '14 at 23:00

score 16 · Accepted Answer · edited Jun 27 '14 at 18:17

16

This can be do the same thing with purge:

sync && echo 3 > /proc/sys/vm/drop_caches

From man proc:

/proc/sys/vm/drop_caches (since Linux 2.6.16)
              Writing to this file causes the kernel  to  drop  clean  caches,
              dentries  and  inodes from memory, causing that memory to become
              free.

              To free pagecache, use echo  1  >  /proc/sys/vm/drop_caches;  to
              free dentries and inodes, use echo 2 > /proc/sys/vm/drop_caches;
              to  free  pagecache,  dentries  and  inodes,  use   echo   3   >
              /proc/sys/vm/drop_caches.

              Because this is a nondestructive operation and dirty objects are
              not freeable, the user should run sync(8) first.

And from man sync:

NAME
       sync - flush file system buffers

DESCRIPTION
       Force changed blocks to disk, update the super block.

edited Jun 27 '14 at 18:17

terdon

242,166

answered Jun 27 '14 at 18:06

cuonglm

153,898

7

My proc(5) manpage (from 2013-09-04) has this important information added: ... causing that memory to become free. This can be useful for memory management testing and performing reproducible filesystem benchmarks. Because writing to this file causes the benefits of caching to be lost, it can degrade overall system performance. – Dubu Jun 27 '14 at 18:28
It didn't free up any memory for me. All my memory is being used by a Java process. Does it impact Java memory cache too? – Mohsen Jun 27 '14 at 18:39
@Mohsen: May be your Java process does not cache anything. What is output of free -m? – cuonglm Jun 27 '14 at 18:42
@Gnouc output of free -m: http://pastebin.com/raw.php?i=XCBeLe4f – Mohsen Jun 27 '14 at 18:45
@Mohsen: It seems your system can not drop cache, see this:http://unix.stackexchange.com/questions/53930/drop-caches-doesnt-drop-caches – cuonglm Jun 27 '14 at 19:17
3

Totally relevant http://serverfault.com/q/597115/180142 – Braiam Jun 27 '14 at 21:37
1

@Mohsen this is a kernel feature. how is the kernel supposed to know what is a legitimate Java object and what is cache? (hint: it can't, therefore it doesn't free Java caches). – strugee Jun 27 '14 at 21:46

What is equivalent to Mac's purge in Linux?

1 Answers1