Re: Limit dentry cache entries

[Bob Peterson <rpeterso@redhat.com>]

----- Original Message -----
| Hello,
| 
| We have a bunch of servers that create a lot of temp files, or check
| for the existence of non-existent files. Every such operation creates
| a dentry object and soon most of the free memory is consumed for
| 'negative' dentry entries. This behavior was observed on both CentOS
| kernel v.2.6.32-358 and Amazon Linux kernel v.3.4.43-4.
| 
| There are also some processes running that occasionally allocate large
| chunks of memory, and when this happens the kernel clears out a bunch
| of stale dentry caches. This clearing takes some time. kswapd kicks
| in, and allocations and bzero() of 4GB that normally takes <1s, takes
| 20s or more.
| 
| Because the memory needs are non-continuous but negative dentry
| generation is fairly continuous, vfs_cache_pressure doesn't help much.
| 
| The thought I had was to have a sysctl that limits the number of
| dentries per super-block (sb-max-dentry). Everytime a new dentry is
| allocated in d_alloc(), check if dentry_stat.nr_dentry exceeds (number
| of super blocks * sb-max-dentry). If yes, queue up an asynchronous
| workqueue call to prune_dcache(). Also have a separate sysctl to
| indicate by what percentage to reduce the dentry entries when this
| happens.
| 
| Thanks for your input. If this sounds like a reasonable idea, I'll
| send out a patch.
| 
| Cheers,
| Keyur.

Hi Keyur,

I like the idea. I've had people bring up the same issue, relating
to GFS2. This is especially true for doing du and similar ops on a
very large file system. This wasn't on GFS2, was it?

Regards,

Bob Peterson
Red Hat File Systems