Re: [RFC][PATCH] update /proc/sys/vm/drop_caches documentation

[Dave Hansen <dave@linux.vnet.ibm.com>]

On Thu, 2010-09-16 at 09:12 +0900, KAMEZAWA Hiroyuki wrote:
> I hear a customer's case. His server generates 3-80000+ new dentries per day
> and dentries will be piled up to 1000000+ in a month. This makes open()'s 
> performance very bad because Hash-lookup will be heavy. (He has very big memory.)
> 
> What we could ask him was
>   - rewrite your application. or
>   - reboot once in a month (and change hash size) or
>   - drop_cache once in a month
> 
> Because their servers cannot stop, he used drop_caches once in a month
> while his server is idle, at night. Changing HashSize cannot be a permanent
> fix because he may not stop the server for years.

That is a really interesting case.

They must have a *ton* of completely extra memory laying around.  Do
they not have much page cache activity?  It usually balances out the
dentry/inode caches.

Would this user be better off with a smaller dentry hash in general?  Is
it special hardware that should _have_ a lower default hash size?

> For rare users who have 10000000+ of files and tons of free memory, drop_cache
> can be an emergency help. 

In this case, though, would a WARN_ON() in an emergency be such a bad
thing?  They evidently know what they're doing, and shouldn't be put off
by it.

-- Dave