long delay in getting lock on 2.4.16

[Deniss]

hello !

I have an issue with cyrus-imapd-2.4.16 and locks.
cmdtimer reports long time execution like this:

Nov 14 08:47:15 brat5 imap[16897]: cmdtimer: 'ega9024' 'status' '<none>' 
'24.577343' '0.000000'
Nov 14 10:18:02 brat5 imap[26631]: cmdtimer: 'evijanagle' 'select' 
'user.evijanagle.Photos' '24.136381' '0.000000'
Nov 14 10:18:02 brat5 imap[26622]: cmdtimer: 'evijanagle' 'select' 
'user.evijanagle.Photos' '31.103348' '0.000000'
Nov 14 10:58:32 brat5 imap[30646]: cmdtimer: 'egil^k' 'status' '<none>' 
'22.480196' '0.000000'
Nov 14 10:58:32 brat5 imap[30708]: cmdtimer: 'egil^k' 'status' '<none>' 
'10.285937' '0.000000'

i traced down to func mailbox_lock_index() where following code produces 
delays:

     if (locktype == LOCK_EXCLUSIVE)
         r = lock_blocking(mailbox->index_fd);
     else
         r = lock_shared(mailbox->index_fd);

lock files are stored in tmpfs


what is the problem may be here ?

Thanx in advance,
   Deniss