Temporary hangs when using locking with apache+nfsv4

[Dennis Jacobfeuerborn <dennisml@conversis.de>]

Hi,
I'm experimenting with using NFSv4 as storage for web servers and while 
regular file access seems to work fine as soon as I bring flock() into 
play things become more problematic.
I've create a tiny test php script that basically opens a file, locks it 
using flock(), writes that fact into a log file (on a local filesystem), 
performs a usleep(1000), writes into the log that it is about to unlock 
the file and finally unlocks it.
I invoke that script using ab with a concurrency of 20 for a few 
thousand requests.

The result is that while 99% of the request respond quickly a few 
request seem to hang for up to 30 seconds. According to the log file 
they must eventually succeed since I see all expected entries and the 
locking seems to work as well since all entries are in the expected order.

Is it expected that these long delays happen? When I comment the locking 
function out these hangs disappear.
Are there some knobs to tune NFS and make it behave better in these 
situations?

Regards,
   Dennis