linux-kernel - Re: drop

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Date:	Thu, 5 Mar 2009 22:52:53 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Markus <M4rkusXXL@....de>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Zdenek Kabelac <zkabelac@...hat.com>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Lukas Hejtmanek <xhejtman@....muni.cz>
Subject: Re: drop_caches ...

On Thu, Mar 05, 2009 at 04:43:22PM +0200, Markus wrote:
> > > > Could you please try the attached patch which will also show the
> > > > user and process that opened these files? It adds three more 
> fields
> > > > when CONFIG_PROC_FILECACHE_EXTRAS is selected.
> > > > 
> > > > Thanks,
> > > > Fengguang
> > > >  
> > > > On Thu, Mar 05, 2009 at 01:55:35PM +0200, Markus wrote:
> > > > > 
> > > > > # sort -n -k 3 filecache-2009-03-05 | tail -n 5
> > > > >      15886       7112     7112     100      1    d- 00:08
> > > > > (tmpfs)        /dev/zero\040(deleted)
> > > > >      16209      35708    35708     100      1    d- 00:08
> > > > > (tmpfs)        /dev/zero\040(deleted)
> > > > >      16212      82128    82128     100      1    d- 00:08
> > > > > (tmpfs)        /dev/zero\040(deleted)
> > > > >      15887     340024   340024     100      1    d- 00:08
> > > > > (tmpfs)        /dev/zero\040(deleted)
> > > > >      15884     455008   455008     100      1    d- 00:08
> > > > > (tmpfs)        /dev/zero\040(deleted)
> > > > > 
> > > > > The sum of the third column is 1013 MB.
> > > > > To note the biggest ones (or do you want the whole file?)... and 
> > > thats 
> > > > > after a sync and a drop_caches! (Can be seen in the commands 
> given.)
> > > 
> > > I could, but I know where these things belong to. Its from sphinx (a 
> > > mysql indexer) searchd. It loads parts of the index into memory.
> > > The sizes looked well-known and killing the searchd will 
> reduce "cached" 
> > > to a normal amount ;)
> > 
> > And it's weird about the file name: /dev/zero.  I wonder how it
> > managed to create that file, and then delete it, inside a tmpfs!
> 
> I dont know exactly. But in the source its just a:
> ... mmap ( NULL, m_iLength, PROT_READ | PROT_WRITE, MAP_SHARED | 
> MAP_ANON, -1, 0 );
> Perhaps thats the way shared anonymous memory is handled?!

Good to know this. The corresponding kernel function is:

        /**     
         * shmem_zero_setup - setup a shared anonymous mapping
         * @vma: the vma to be mmapped is prepared by do_mmap_pgoff
         */
        int shmem_zero_setup(struct vm_area_struct *vma)
        {       
                struct file *file;
                loff_t size = vma->vm_end - vma->vm_start;
                
                file = shmem_file_setup("dev/zero", size, vma->vm_flags);

Here goes the /dev/zero ^_^

> > Just out of curiosity, are they shm objects? Can you show us the
> > output of 'df'? In your convenient time.
> 
> Thats all:
> # df -h
> Filesystem            Size  Used Avail Use% Mounted on
> /dev/md6               14G  7.9G  5.9G  58% /
> udev                   10M  304K  9.8M   3% /dev
> cachedir              4.0M  100K  4.0M   3% /lib64/splash/cache
> /dev/md4               19G   15G  3.2G  82% /home
> /dev/md3              8.3G  4.5G  3.8G  55% /usr/portage

> shm                   2.0G     0  2.0G   0% /dev/shm

shm objects will be accounted here. It's clean.

Thanks,
Fengguang

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