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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 <> 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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