Re: buffer cache question

["JaniD++" <djani22@dynamicweb.hu>]

----- Original Message ----- 
From: Nate Diller
To: JaniD++
Cc: Roger Heflin ; linux-kernel@vger.kernel.org
Sent: Friday, December 23, 2005 9:58 AM
Subject: Re: buffer cache question


On 11/21/05, JaniD++ <djani22@dynamicweb.hu> wrote:
Hi,

I have read back in the kernel-archives, and found this messages, about the
same theme, but there is one difference!

On the old messages:
>Nate Diller wrote:
> just found the culprit.  guess i should have read the code the first
> time.  get_dirty_limits() in drivers/block/page_writeback.c has a
> hard-coded upper limit to dirty_ratio.  it's capped to half of the
> unmapped pages, so maybe 30-40% of your system's memory.  so if you are
> brave, just remove the "/ 2" parts from the 'if (dirty_ratio >
> unmapped_ratio / 2) dirty_ratio = unmapped_ratio / 2;' check, and you
> can have all the OOM goodness you want.
...
>I changed that bit of code to:
>
> if (dirty_ratio > unmapped_ratio - 10)
>  dirty_ratio = unmapped_ratio - 10;
>
>and added a couple of sanity checks so that it couldn't get below 5 or
above 95.
>
>Then set /proc/sys/vm/dirty_ratio to 95 and dirty_background_ratio to 1.

In this case, this modification is only for the *dirty* memory buffer.
I want to use more buffer *cache*! :-)
The unwritten dirty memory ratio is good enough for me.

Anybody has an idea?

<snip>



> > [root@st-0001 root]# free
> >              total       used       free     shared
> > buffers     cached
> > Mem:       2073152     933188    1139964          0
> > 836776      43416
> > -/+ buffers/cache:      52996    2020156
> > Swap:            0          0          0
> > [root@st-0001 root]# cat /proc/meminfo
> > MemTotal:      2073152 kB
> > MemFree:       1139012 kB
> > Buffers:        835928 kB
> > Cached:          43448 kB
> > SwapCached:          0 kB
> > Active:          12872 kB
> > Inactive:       871424 kB
> > HighTotal:     1179584 kB
> > HighFree:      1129764 kB
> > LowTotal:       893568 kB
> > LowFree:          9248 kB
> > SwapTotal:           0 kB
> > SwapFree:            0 kB
> > Dirty:               0 kB
> > Writeback:           0 kB
> > Mapped:           9104 kB
> > Slab:            30248 kB
> > CommitLimit:   1036576 kB
> > Committed_AS:    15428 kB
> > PageTables:        408 kB
> > VmallocTotal:   114680 kB
> > VmallocUsed:       196 kB
> > VmallocChunk:   114476 kB
> > [root@st-0001 root]#

looks like you're barely using any of your high memory.  maybe NBD doesn't
have highmem support.  what file system are you using?

NATE


I cannot understant this.
NBD need to support highmem for buffering?
If know right, the kernel does buffering, not NBD!
But the kernel only use ~830MB for buffer cache instead of dinamically use
all free memory like page cache.

This is one raw disk node, independent from file system.

Cheers,
Janos

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