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SubjectRe: vm-kswapd-incmin.patch problem
Hi Nick,

On Tue, Nov 22, 2005 at 03:48:18PM +0800, Wu Fengguang wrote:
> Hi, your vm-kswapd-incmin.patch looks nice, and I'd like to base my age
> balancing patch upon it. But while trying it, I ran into a problem.
>
> $ cp bigfile /dev/null
> $ free -m
> total used free shared buffers cached
> Mem: 501 495 6 0 2 321
> -/+ buffers/cache: 171 330
> Swap: 127 2 125
> $ sleep 10
> $ free -m
> total used free shared buffers cached
> Mem: 501 244 257 0 4 66
> -/+ buffers/cache: 173 328
> Swap: 127 2 125
>
> In a short time, the bigfile was totally evicted from page cache?
>
> Before/after the huge free pages:
>
> $ cat /proc/sys/fs/dentry-state
> 3393 28 45 0 0 0
> $ cat /proc/sys/fs/dentry-state
> 3626 260 45 0 0 0
>
>
> linux-2.6.15-rc1-mm2 without the patch seems ok.
> Any suggestions? Thanks.

Maybe I found the answer to it :)

I listed the dentry-state in several other normal machines, they all have
more than 10k unused dentries:

54215 42155 45 0 0 0
91704 81376 45 0 0 0
96304 88832 45 0 0 0
30057 26999 45 0 0 0

Then I disabled the shrinker by:
echo 0 > /proc/sys/vm/vfs_cache_pressure

That increased the number from
3393 28 45 0 0 0
to
6247 2672 45 0 0 0
And there is no sudden huge increases of free pages any more.

Maybe your patch is shrinking the slabs much more, though I cannot confirm this
from the source code. But one thing I'm sure is that there should be a lower
bound for the unused dentries, either absolutely or relatively, something like
this:

--- linux-2.6.15-rc1-mm2.orig/fs/dcache.c
+++ linux-2.6.15-rc1-mm2/fs/dcache.c
@@ -860,7 +860,7 @@ static int shrink_dcache_memory(int nr,
return -1;
prune_dcache(nr);
}
- return (dentry_stat.nr_unused / 100) * sysctl_vfs_cache_pressure;
+ return (dentry_stat.nr_unused / 1000) * 10 * sysctl_vfs_cache_pressure;
}

/**

The original 100 is way too small. It should be much more than the SHRINK_BATCH(128).

Thanks,
Wu
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