Re: drop_caches ...

[Wu Fengguang <fengguang.wu@intel.com>]

On Wed, Mar 04, 2009 at 12:32:02PM +0200, Markus wrote:
> 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.

Does your tmpfs store lots of files?

Thanks,
Fengguang