linux-kernel - Re: drop

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Date:	Wed, 4 Mar 2009 11:32:02 +0100
From:	Markus <M4rkusXXL@....de>
To:	linux-kernel@...r.kernel.org
Cc:	Wu Fengguang <fengguang.wu@...el.com>
Subject: Re: drop_caches ...

Hello Fengguang!

> Hi Markus,
> 
> On Wed, Mar 04, 2009 at 10:57:33AM +0100, Markus wrote:
> > Hello!
> > 
> > I have a small problem. Maybe its just a misunderstanding but I cant 
> > solve it.
> > 
> > I think that writing "3" to drop_caches should drop all buffers and 
> > caches which are already written. So its recommended to put a "sync" 
> > infront of it.
> > So I did "free -m ; sync ; echo 3 > /proc/sys/vm/drop_caches ; 
free -m"
> > And it gave me:
> >              total       used       free     shared    buffers     
> > cached
> > Mem:          3950       3922         28          0          1        
> > 879
> > -/+ buffers/cache:       3041        909
> > Swap:         5342        205       5136
> >              total       used       free     shared    buffers     
> > cached
> > Mem:          3950       3907         43          0          0        
> > 864
> > -/+ buffers/cache:       3041        908
> > Swap:         5341        206       5135
> > 
> > So the buffer was 1 and is 0 afterthat. But cached is at 879 MB 
before 
> > and is still 864 MB (!!!) after that!
> > 
> > I am at swappiness=0 and when I remove and readd one swap-partition 
> > after another (so there is always swap). It will keep the cached and 
> > put the swapped memory on other swaps?!
> > 
> > I _think_ thats not the way it should go?
> > 
> > It would be really kind if someone could explain that issue and 
> > what "cached" is at all!
> 
> The memory mapped pages won't be dropped in this way.
> "cat /proc/meminfo" will show you the number of mapped pages.

# sync ; echo 3 > /proc/sys/vm/drop_caches ; free -m ; cat /proc/meminfo
             total       used       free     shared    buffers     
cached
Mem:          3950       3262        688          0          0        
359
-/+ buffers/cache:       2902       1047
Swap:         5890       1509       4381
MemTotal:        4045500 kB
MemFree:          705180 kB
Buffers:             508 kB
Cached:           367748 kB
SwapCached:       880744 kB
Active:          1555032 kB
Inactive:        1634868 kB
Active(anon):    1527100 kB
Inactive(anon):  1607328 kB
Active(file):      27932 kB
Inactive(file):    27540 kB
Unevictable:         816 kB
Mlocked:               0 kB
SwapTotal:       6032344 kB
SwapFree:        4486496 kB
Dirty:                 0 kB
Writeback:             0 kB
AnonPages:       2378112 kB
Mapped:            52196 kB
Slab:              65640 kB
SReclaimable:      46192 kB
SUnreclaim:        19448 kB
PageTables:        28200 kB
NFS_Unstable:          0 kB
Bounce:                0 kB
WritebackTmp:          0 kB
CommitLimit:     8055092 kB
Committed_AS:    4915636 kB
VmallocTotal:   34359738367 kB
VmallocUsed:       44580 kB
VmallocChunk:   34359677239 kB
DirectMap4k:     3182528 kB
DirectMap2M:     1011712 kB

The cached reduced to 359 MB (after the dropping).
I dont know where to read the "number of mapped pages".
"Mapped" is about 51 MB.

thanks in advance

Markus
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