Re: vm-kswapd-incmin.patch problem

[Nick Piggin]

Wu Fengguang wrote:
> Hi Nick,

Hi Wu,

> 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.

Yes, the inode shrinker can discard all pagecache from a file if
it is under a lot of pressure to free inodes. This is what you
are seeing I guess.

> 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;
>  }
>  

I don't think that kswapd-incmin puts much more pressure on the slab
(unless there is a bug), but I'll take a look. It could just be a
"weird" interaction in the reclaim code, or possibly a rounding issue.

Changing the pressure calculation here is probably not the right way
to do this, but rather in vmscan.c. I know Andrea has recently been
looking at a problem with slab shrinking too.

Thanks,
Nick

--
SUSE Labs, Novell Inc.

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