Memory management

Jan Kara | 1 Oct 2010 01:27

Re: block cache replacement strategy?

  Hi,

On Tue 07-09-10 15:34:29, Johannes Stezenbach wrote:
> during some simple disk read throughput testing I observed
> caching behaviour that doesn't seem right.  The machine
> has 2G of RAM and AMD Athlon 4850e, x86_64 kernel but 32bit
> userspace, Linux 2.6.35.4.  It seems that contents of the
> block cache are not evicted to make room for other blocks.
> (Or something like that, I have no real clue about this.)
> 
> Since this is a rather artificial test I'm not too worried,
> but it looks strange to me so I thought I better report it.
> 
> 
> zzz:~# echo 3 >/proc/sys/vm/drop_caches 
> zzz:~# dd if=/dev/sda2 of=/dev/null bs=1M count=1000
> 1000+0 records in
> 1000+0 records out
> 1048576000 bytes (1.0 GB) copied, 13.9454 s, 75.2 MB/s
> zzz:~# dd if=/dev/sda2 of=/dev/null bs=1M count=1000
> 1000+0 records in
> 1000+0 records out
> 1048576000 bytes (1.0 GB) copied, 0.92799 s, 1.1 GB/s
> 
> OK, seems like the blocks are cached. But:
> 
> zzz:~# dd if=/dev/sda2 of=/dev/null bs=1M count=1000 skip=1000
> 1000+0 records in
> 1000+0 records out
> 1048576000 bytes (1.0 GB) copied, 13.8375 s, 75.8 MB/s

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Johannes Stezenbach | 1 Oct 2010 15:05

Re: block cache replacement strategy?

Hi,

On Fri, Oct 01, 2010 at 01:27:59AM +0200, Jan Kara wrote:
> On Tue 07-09-10 15:34:29, Johannes Stezenbach wrote:
> > 
> > zzz:~# echo 3 >/proc/sys/vm/drop_caches 
> > zzz:~# dd if=/dev/sda2 of=/dev/null bs=1M count=1000
> > 1000+0 records in
> > 1000+0 records out
> > 1048576000 bytes (1.0 GB) copied, 13.9454 s, 75.2 MB/s
> > zzz:~# dd if=/dev/sda2 of=/dev/null bs=1M count=1000
> > 1000+0 records in
> > 1000+0 records out
> > 1048576000 bytes (1.0 GB) copied, 0.92799 s, 1.1 GB/s
> > 
> > OK, seems like the blocks are cached. But:
> > 
> > zzz:~# dd if=/dev/sda2 of=/dev/null bs=1M count=1000 skip=1000
> > 1000+0 records in
> > 1000+0 records out
> > 1048576000 bytes (1.0 GB) copied, 13.8375 s, 75.8 MB/s
> > zzz:~# dd if=/dev/sda2 of=/dev/null bs=1M count=1000 skip=1000
> > 1000+0 records in
> > 1000+0 records out
> > 1048576000 bytes (1.0 GB) copied, 13.8429 s, 75.7 MB/s
>   I took a look at this because it looked strange at the first sight to me.
> After some code reading the result is that everything is working as
> designed.
>   The first dd fills up memory with 1GB of data. Pages with data just freshly
> read from disk are in "Inactive" state. When these pages are read again by

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