What does drop_caches do?

What does drop_caches do?

[Prateek Sharma]

Hello everyone,
 	I've been trying to understand the role of the pagecache, starting with 
drop_caches and observing what it does.
 	From my understanding of the code (fs/drop_caches.c) , it walks over all 
the open files/inodes, and invalidates all the mapped pages. Pages which are 
*not* dropped are either dirty,in-use,anonymous,mapped(to pagetable),or 
writeback) . Is my understanding correct?
 	But, when i run drop_caches, there are still some pages which show up as 
cached. Why arent all cache pages getting dropped ?
 	My confusion runs much deeper. What exactly constitutes the pagecache? 
All filebacked pages ? mmaped files ? If i copy a bunch of files, why does my 
cache get polluted with those pages?

Thanks for reading. I'd be grateful if someone can enlighten me about some 
pagecache internals .

(Please keep me CC'ed)

<begin experiment>
root@tripitz:/etc/apt# free -m
              total       used       free     shared    buffers     cached
Mem:          1995       1854        140          0         79        700
-/+ buffers/cache:       1074        920
Swap:         4767        762       4005
root@tripitz:/etc/apt# echo 3 > /proc/sys/vm/drop_caches
root@tripitz:/etc/apt# free -m
              total       used       free     shared    buffers     cached
Mem:          1995       1373        621          0          0        363
-/+ buffers/cache:       1009        985
Swap:         4767        762       4005

<end experiment>

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Re: What does drop_caches do?

[Greg Freemyer]

On Thu, Jul 28, 2011 at 2:58 AM, Prateek Sharma <prateeks@cse.iitb.ac.in> wrote:
> Hello everyone,
>        I've been trying to understand the role of the pagecache, starting with
> drop_caches and observing what it does.
>        From my understanding of the code (fs/drop_caches.c) , it walks over all
> the open files/inodes, and invalidates all the mapped pages.

Stepping up a little, I think you're missing
drop_caches_sysctl_handler() in that same file.  It is the entry point
I believe that implements the userspace ABI defined in
Documentation/sysctl/vm.txt.

==============================================================
 139
 140drop_caches
 141
 142Writing to this will cause the kernel to drop clean caches, dentries and
 143inodes from memory, causing that memory to become free.
 144
 145To free pagecache:
 146        echo 1 > /proc/sys/vm/drop_caches
 147To free dentries and inodes:
 148        echo 2 > /proc/sys/vm/drop_caches
 149To free pagecache, dentries and inodes:
 150        echo 3 > /proc/sys/vm/drop_caches
 151
 152As this is a non-destructive operation and dirty objects are not
freeable, the
 153user should run `sync' first.
 154
 155==============================================================

Note the corresponding logic in /fs/drop_caches.c that implements the
above binary logic.


  61              if (sysctl_drop_caches & 1)
  62                        iterate_supers(drop_pagecache_sb, NULL);
  63                if (sysctl_drop_caches & 2)
  64                        drop_slab();

Greg

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