Re: heavy file i/o on ext3 filesystem leads to huge ext3_inode_cache and dentry_cache that doesn't return to normal for hours

["Jeffrey V. Merkey" <jmerkey@wolfmountaingroup.com>]

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.
>
>  
>
>>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.
>
>  
>
>>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.
>
>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
>  
>
dcache architecture is one of the big bottlenecks. 

Jeff