Re: rpc.mountd reads /etc/mtab 17028 times, 100% CPU.

[Steve Dickson <SteveD@redhat.com>]

On 03/25/2015 05:37 AM, Sander Smeenk wrote:
> I'm running a server that has 1500+ mounted local filesystems.
> All of these local filesystems are exported through NFS by this server.
> 
> When an NFS-client tries to access one of these exported filesystems,
> for example doing a simple 'ls' in a mounted NFS, rpc.mountd on the
> server takes 100% CPU and starts to spin over reading /etc/mtab which is
> linked to /proc/mounts and totals to about 200KB in size. The client
> stalls all the while rpc.mountd is busy reading /etc/mtab for every
> filesystem mounted, and then some more.
> 
> This entire process of reading /etc/mtab 17028 times takes a lot of time
> during which the client stalls, but in the end 'it just works fine'.
> It just takes ages when you try to tab-complete on a client.
Taking a quick look... it appears the only way rpc.mountd
will read /etc/mtab is if the 'crossmnt' export flag is 
set. Is this the case?

> 
> What would be needed to debug and optimise this?
Maybe reorder you exports so the 'hot' file systems are on the
top of the list...

> Could someone point me to the code that is involved in doing this?
utils/mountd/cache.c:nfsd_fh():path = next_mnt(&mnt, exp->m_export.e_path);

steved.