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SubjectRe: heavy file i/o on ext3 filesystem leads to huge ext3_inode_cache and dentry_cache that doesn't return to normal for hours
On 8/4/06, Dave Kleikamp <shaggy@austin.ibm.com> wrote:
> On Thu, 2006-08-03 at 18:27 +0200, Maarten Maathuis wrote:
> > I have a kernel specific problem and this seemed like a suitable place to ask.
> >
> > I would like responces to be CC'ed to me if possible.
> >
> > I use a 2.6.17-ck1 kernel on an amd64 system. I have observed this
> > problem on other/older kernels.
> >
> > Whenever there is serious hard drive activity (such as doing "slocate
> > -u") ext3_inode_cache and dentry_cache grow to a combined 400-500 MiB.
>
> The behavior of slocate (updatedb) is pretty well-known, but nobody has
> come up with a real solution.
>
> > The amount of objects is more than half a million.
> >
> > This will slowly decrease to normal, but will take many hours. It does
> > not result in any OOM, because i have 1 GiB of memory.
> >
> > As far as i understand hard drive cache should not be in the slab. Are
> > these just the inode's, because the amount of memory consumption seems
> > large for that?
>
> inodes and directory cache entries (dentries). In general, it's a good
> idea to cache inodes and dentries that have recently been read. slocate
> is a special case since it will traverse all of the directories and
> never look at them again (until the next time it runs). The kernel
> doesn't have any idea that it may be a good idea to free those objects.
>

Wasn't there something like a possible read only flag to prevent that?

> >
> > I have found a way to clear the memory (and unfortunately most of the cache):
> >
> > echo 100 > /proc/sys/vm/nr_hugepages
> > echo 0 > /proc/sys/vm/nr_hugepages
>
> A better way to clear just the inodes and dentries (that aren't in use)
> is:
>
> echo 2 > /proc/sys/vm/drop_caches
>
> This feature is relatively new.
>

Thank you, i tried echo'ing a 1 into that and it had no effect iirc.
Documentation on /proc/sys/vm seems pretty scarce.

> > This suggest the kernel can free this memory. It's not the caching
> > that bothers me, what bothers me is that it seems to reside in the
> > slab.
>
> I'm not sure why that bothers you. A more common complaint is that all
> the inodes and dentries being cached push out other pages to swap.
>

The reason i asked was because i expected cache related things to be
seperated from "normal" memory usage. Since free already seperates
normal and buffers/cache, it was very strange seeing 400 or 500 MiB
memory usage.

> Completely free memory doesn't do the system any good. The kernel
> attempts to keep as much as possible in cache in case something is
> needed again. These objects are easily reclaimable, so when more memory
> is needed, they can be freed with very little overhead.
>
> > I am not a developer, so please keep that in mind when replying.
> >
> > I hope someone can be of help.
> >
> > Sincerely,
> >
> > Maarten Maathuis.
>
> Shaggy
> --
> David Kleikamp
> IBM Linux Technology Center
>
>
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